Thermodynamic Data for Mn O ,MnO and MnO - …ugcdskpdf.unipune.ac.in/Journal/uploads/PH/PH080039-A-5.pdf · thermodynamic data for the ... evidence in preliminary experiments of

High Temp. Mater. Proc., Vol. 30 (2011), pp. 459–472 Copyright © 2011 De Gruyter. DOI 10.1515/HTMP.2011.069

Thermodynamic Data for Mn3O4, Mn2O3 and MnO2

K. T. Jacob,1;� A. Kumar,1 G. Rajitha1 andY. Waseda2

1 Department of Materials Engineering, Indian Institute ofScience, Bangalore, India

2 Institute of Multidisciplinary Research for AdvancedMaterials, Tohoku University, Sendai, Japan

Abstract. Thermodynamic properties of Mn3O4, Mn2O3and MnO2 are reassessed based on new measurements andselected data from the literature. Data for these oxides areavailable in most thermodynamics compilations based onolder calorimetric measurements on heat capacity and en-thalpy of formation, and high-temperature decompositionstudies. The older heat capacity measurements did notextend below 50 K. Recent measurements have extendedthe low temperature limit to 5 K. A reassessment of ther-modynamic data was therefore undertaken, supplementedby new measurements on high temperature heat capacityof Mn3O4 and oxygen chemical potential for the oxida-tion of MnO1�x , Mn3O4, and Mn2O3 to their respectivehigher oxides using an advanced version of solid-state elec-trochemical cell incorporating a buffer electrode. Becauseof the high accuracy now achievable with solid-state elec-trochemical cells, phase-equilibrium calorimetry involvingthe “third-law” analysis has emerged as a competing toolto solution and combustion calorimetry for determining thestandard enthalpy of formation at 298.15 K. The refinedthermodynamic data for the oxides are presented in tabu-lar form at regular intervals of temperature.

Keywords. Heat capacity, entropy, enthalpy of formation,gibbs energy of formation, solid-state electrochemical cell,buffer electrode, assessment.

PACS®(2010). 82.60.-s, 82.60.He, 82.60.Cx, 82.60.Fa.

1 Introduction

A great deal of attention has been given to the effectiveutilization of manganese and improved recovery from itsores, which are principally oxides. In manganese oxides,the cations exist in three oxidation states (Mn2C, Mn3C and

Corresponding author: K. T. Jacob, Department of MaterialsEngineering, Indian Institute of Science, Bangalore 560012, India;E-mail: [email protected].

Received: March 4, 2011. Accepted: April 15, 2011.

Mn4C/. The oxides MnO1�x , Mn3O4, Mn2O3 and MnO2can be converted from one to another by adjustment of tem-perature (T / and oxygen partial pressure (PO2 ). Manganesein oxidation states of 2C, 3C, 4C has interesting appli-cations which encompass systems such as LaMnO3-basedperovskites and metastable manganese oxides and oxyhy-droxides, which are phases encountered in fuel cell, battery,electronic and environmental applications. Thermodynamicproperties of binary oxides provide the foundation for mea-surements and assessments on ternary and higher order ox-ides.

Thermodynamic properties of manganese oxides and thephase diagram for the system Mn-O were recently assessedby Grundy et al. [1]. An exhaustive compilation of all pub-lished data on phase diagram and thermodynamic proper-ties is presented by Grundy et al. They concluded that theGibbs energy of formation of MnO is known only withlarge uncertainty (˙5 kJ mol�1/ and this leads to uncer-tainties in the Gibbs energies of formation of all other ox-ides since Gibbs energies of higher oxides are based on datafor MnO. Subsequently, Gibbs energy of formation of MnOwas measured accurately (˙0:25 kJ mol�1/ by Jacob etal. [2], consistent with earlier measurements of Alcock andZador [3]. In this communication, we combine the accuratedata for MnO with new measurements on oxygen chem-ical potentials of three biphasic regions in the phase dia-gram of the system Mn-O and high-temperature heat capac-ity of Mn3O4 to derive accurate data for Mn3O4, Mn2O3and MnO2, taking cognizance of other reliable measure-ments available in the literature including new low temper-ature heat capacity measurements. For new measurementson oxygen chemical potential, an advanced version of thesolid-state cell incorporating a buffer electrode is utilized.Of the four oxides of manganese, only MnO exhibits signif-icant nonstoichiometry at temperatures above 900 K. Newexperimental data obtained in this study is first presented,followed by reassessment of data for the three higher ox-ides.

2 Experimental Work

2.1 Materials

Pure MnO2 (99.999% pure) was obtained from JohnsonMatthey Inc. Mn2O3 was prepared by heating MnO2 in airat 973 K for � 20 ks, and Mn3O4 by heating at 1323 K for� 20 ks. Formation of the various phases was confirmed bypowder XRD.

AUTHOR’S COPY | AUTORENEXEMPLAR


460 K. T. Jacob, A. Kumar, G. Rajitha and Y. Waseda

2.2 Differential Scanning Calorimetry (DSC)

Since the high-temperature heat capacity of ˛-Mn3O4 hasnot been reported in the literature, DSC was used to mea-sure this property of under pure argon gas in the tempera-ture range from 350 to 1100 K. The DSC instrument wasoperated in the step-heating mode to increase accuracy; ˛-Al2O3 was used as the reference material. The aluminapowder was dehydrated via vacuum treatment at 1200 K.The difference in the heat flux into the sample and the ref-erence material was integrated during heating at a constantrate (2 K=min) over small temperature steps (10 K) withan isothermal dwell time of 15 min. The accuracy of themeasured heat capacity is estimated to be 1% (2� ). (Thestandard error estimate is denoted as � ). XRD analysis ofthe oxide after the DSC experiments did not indicate anychange in structure or lattice parameter.

2.3 Oxygen Potential Measurements

The reversible emfs of three solid-state cells were measuredas a function of temperature:

Cell 1:

Pt, Mn1�xOCMn3O4 // (Y2O3/ZrO2 // O2(0.1 MPa), Pt

Cell 2:

Pt, Mn3O4 CMn2O3 // (Y2O3/ ZrO2// O2(0.1 MPa), Pt

Cell 3:

Pt, Mn2O3 CMnO2 // (Y2O3/ ZrO2 // O2(0.1 MPa), Pt

The cells are written such that the right-hand electrodesare positive. Two experiments were conducted on each cell.Yttria-stabilized zirconia, (Y2O3) ZrO2, is an oxygen ionconductor with ionic transport number greater than 0.99 atthe temperatures and oxygen partial pressures encounteredat the electrodes of the three cells [4]. However, the pres-ence of trace hole conduction in the electrolyte gives rise toa small electrochemical flux of oxygen from the referenceelectrode on the right side to the working electrodes on theleft side of each cell [5]. The electrochemical permeabilityis caused by the coupled transport of oxygen ions and holesin the solid electrolyte under the oxygen potential gradient.

The electrochemical flux of oxygen can polarize the two-phase solid electrodes. The chemical potential of oxygen inthe micro system near the solid electrode/electrolyte inter-face can be altered because of the semi-permeability of theelectrolyte to oxygen. A buffer electrode, introduced be-tween reference and working electrodes can act as a sinkfor the oxygen flux and prevent the flux from reaching theworking electrode. To be effective, the buffer electrodeshould be maintained at an oxygen chemical potential closeto that of the working electrode. Since there is no significantdifference between the chemical potentials of buffer andworking electrodes, driving force for transport of oxygenthrough the zirconia tube separating these electrodes does

Figure 1. Schematic diagram of the apparatus used for oxy-gen chemical potential measurements in three biphasicregions of the system Mn-O. The solid-state electrochemi-cal cell has a buffer electrode that prevents polarization ofthe measuring electrode.

not exist. The working electrode therefore remains unpo-larized. Pure oxygen gas at a pressure of 0.1 MPa, flowingover a platinized surface of zirconia, constitutes the primaryreference standard for oxygen chemical potential and formsa non-polarizable electrode. Thus, the three-electrode de-sign of the cell prevents error in emf caused by polarizationof the working electrode. Measuring separately the emf be-tween the three electrodes, two at a time, can assess themagnitude of the polarization effect. Transport of oxygenbetween the electrodes through the gas phase is preventedby physical isolation of the gas phase over the three elec-trodes.

The cell design used for high-temperature emf measure-ments on cells 1 and 2 is shown schematically in Figure 1.It consisted of three distinct compartments, separated bytwo impervious yttria-stabilized zirconia (YSZ) tubes anda YSZ crucible. An ionic bridge was provided through thebuffer electrode. The cell can be represented schematicallyas follows:

(Y2O3)ZrO2

Working

Electrode

O2 ( '2OP ),

Pt

(Y2O3)ZrO2

No flux

Buffer Electrode

O2 ( ''2OP ), Pt/Pd

''2OP '

2OP

(Y2O3)ZrO2

O2-

Reference

Electrode

O2 (0.1 MPa),

Pt



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Higher Oxides of Manganese – Thermodynamic Data 461

The working and reference electrodes were contained in-side separate zirconia tubes. The cell emf measured be-tween the working and reference electrodes was determinedonly by the oxygen chemical potential at these electrodesand was not affected by the gradient of chemical potentialthrough the connecting chain consisting of the solid elec-trolyte segments including the ionic bridge across the bufferelectrode. Construction of the high-temperature galvaniccell was rendered more difficult by the introduction of thebuffer electrode. Moreover, in some of the cells used in thisstudy, the partial pressure of oxygen at the working elec-trode was quite appreciable, especially at the higher temper-atures. Therefore, the static sealed design used by Charetteand Flengas [6] was found more appropriate than other de-signs that employ either dynamic vacuum or inert gas flowover the electrodes [4, 7].

The working electrodes consisted of a mixture of twoadjacent manganese oxides in equimolar ratio. The averageparticle size of the powders used to prepare the working andbuffer electrodes was in the range from 2 to 10 μm. Thecompositions of the buffer electrodes were identical to thatof the corresponding working electrodes.

The details of cell assembly and operational proceduresused in this study for cells 1 and 2 were identical with thosereported elsewhere [8, 9]. The working and buffer elec-trodes are sealed under vacuum in separate compartmentswhere the equilibrium oxygen partial pressures are estab-lished by dissociation of the higher oxide at high tempera-ture. Because of the high dissociation pressure of MnO2,cell 3 can be operated only at relatively low temperatures.The high internal resistance associated with the cell designshown in Figure 1 does not permit measurements on cell 3at relatively lower temperatures. Further, there was someevidence in preliminary experiments of formation of PtO2as a dilute solution in MnO2. Hence, a different type of ap-paratus [10, 11], in which there was no contact between theelectrode containing Mn2O3 CMnO2 and Pt, was used formeasurements on cell 3.

A mixture of Mn2O3 and MnO2 in the molar ratio 1:2was taken in a zirconia crucible, which was placed inside anarrow quartz tube closed at one end. A long tube of yttria-stabilized zirconia, with RuO2/Pt electrodes attached to theinside and outside surfaces of the closed end, was used tomeasure the oxygen potential in the gas phase above thesample. The gap between the outer silica tube and the zirco-nia tube at the cold end was sealed by Dekhotinsky cement.The silica tube housing the cell assembly was evacuatedthrough a side arm and flame sealed under vacuum. Pureoxygen gas was passed through the inside of the zirconiatube. The cell was suspended inside a vertical furnace. Theoxygen molecules generated by the dissociation of MnO2 toMn2O3 established the equilibrium oxygen pressure insidethe silica tube housing the cell. The time required to estab-lish equilibrium oxygen pressure and hence steady emf wasof the order of 1.5 Ms.

3 Results

3.1 High-temperature Heat Capacity of Mn3O4

The high-temperature heat capacity of Mn3O4 obtained inthis study using a differential scanning calorimeter (DSC) isshown in Figure 2 as a function of temperature. The resultscan be expressed by the equation:

C 0P =J K�1 mol�1 D 145:322C 49:411 � 10�3 .T=K/

� 1:574 � 106 .T=K/�2 (1)

in the temperature range from 350 to 1100 K. The authorsare not aware of any prior high-temperature heat capacitymeasurement for comparison with the data obtained in thisstudy.

3.2 Oxygen Potentials for Mn1�xO-Mn3O4,Mn3O4-Mn2O3 and Mn2O3-MnO2 equilibria

The reversibility of the cells was tested by microcoulomet-ric titration in forward and reverse directions. Since theemf returned to the same value after essentially infinitesi-mal successive displacements from equilibrium in oppositedirections, reversibility was confirmed. The emf was foundto be reproducible on temperature cycling. At the end ofeach experiment, the electrodes were cooled to room tem-perature and examined by XRD. There was no detectablechange in the phase composition of the electrodes duringemf measurement.

The reversible emfs of cells 1 and 2 are plotted as a func-tion of temperture in Figures 3 and 4, repectively. The emfof cell 1 was positive and could be measured in the tempera-ture range from 850 to 1400 K. The magnitude of the polar-ization effect, assessed by measuring the emf between the

Figure 2. Heat capacity of Mn3O4 measured using DSC.The curve represents the fitted equation.



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Figure 3. Temperature dependence of the emf of cell 1.

Figure 4. Temperature dependence of the emf of cell 2.

buffer electrode and the other two electrodes, varied from 4to 6 mV for cell 1 depending on temperature. If the bufferelectrode was not used the measured emf would have beenlower by this amount. The emf of cell 1 exhibited a lineardependence on temperature over the extended temperaturerange, probably because the expected non-linearity due to�CP for the cell reaction is compensated by the variation ofnonstoichiometry of MnO1�x with temperature.

The emf of cell 2 exhibited irreversibility below 900 Kand became negative at temperatures over 1247 K, indicat-ing that the oxygen pressure over the measuring electrodeexceeded atmospheric pressure. Hence, meaurements oncell were restricted in the range from 900 to 1250 K. Thepolarization effect was of the order on 1 mV for cell 2. Bystatistical analysis a mild non-linearity was detected in thevariation of emf of cell 2 with temperature, although this isnot apparent in the scale of Figure 4. The expressions for

the emf obtained by least-squares regression analysis are:

E1 .˙0:96/=mV D 1154:6 � 0:57445 .T=K/; (2)

E2 .˙0:2/=mV D 610:0 � 0:80402 .T=K/

C 0:04415 .T=K/ ln.T=K/: (3)

The reversible emf of cell 3 could be measured only atone temperature, 773 K. At lower temperatures the emfwas not reversible on micro-coulometric titration of oxygenthrough the cell in opposite directions. At 773 K the cellemf was reversible but significantly negative (�20:4 mV)indicating that the decomposition pressure of MnO2 toMn2O3 is significantly above atmospheric. Beacuse of thedanger of explosion, experiments were not attempted athigher temperatures.

From the emf of cell 1, the oxygen chemical potentialcorresponding to the biphasic equilibrium between nonstoi-chiometric MnO and essentially stoichiometric Mn3O4 canbe calculated in the temperature range from 850 to 1400 K.For the reaction:

¹6=.1 � 4x/ºMn1�xOC O2

D ¹2.1 � x/=.1 � 4x/º˛ �Mn3O4; (4)

��O2 .˙370/=J mol�1 D �G04

D �4FE1 D �445 606C 221:70 .T=K/: (5)

From the emf of cell 2, the oxygen chemical potential forthe biphasic equilibrium involving Mn3O4 and Mn2O3 andstandard Gibbs energy change for the reaction,

4Mn3O4 C O2 D 6Mn2O3; (6)

��O2 .˙77/=J mol�1 D �G06 D �4FE2

D �235 421C 310:3 .T=K/ � 17:04 .T=K/ ln .T=K/(7)

are obtained in the temperature range from 900 to 1250 K.The emf of cell 3 was obtained only at 773 K. From thevalue of emf, �20:4 .˙0:6/ mV, the oxygen potential forthe oxidation of Mn2O3 to MnO2 and the standard Gibbsenergy change for the reaction,

2Mn2O3 C O2 D 4MnO2; (8)

��O2 .˙232/=J mol�1 D �G08 D �4FE3 D 7873: (9)

The measured oxygen chemical potentials and Gibbs en-ergy changes for oxidation reactions will be discussed incomparison with some of the values reported in the liter-ature in the following section on assessment of thermody-namic data.










4 Assessment of Thermodynamic Data

4.1 Mn3O4 (Hausmannite)

Mn3O4 undergoes a first order phase transition from ˛

phase, with Jahn–Teller distorted spinel structure, to ˇ

phase with cubic spinel structure at 1445 K. There is a eu-tectic between MnO and Mn3O4 at 1835 K. It has a verynarrow homogeneity range, 0.5710 < XO < 0.5717.

4.1.1 Low-temperature Heat Capacity and Entropyof ˛-Mn3O4 at 298.15 K

Millar [12] measured the heat capacity of ˛-Mn3O4 inthe temperature range from 72 to 305 K. In order to cal-culate the entropy at 298.15 K, it is necessary to inte-grate C 0P =T as a function of temperature from 0 to 298.15K. This requires extrapolation of experimental heat ca-pacity data to 0 K, which is associated with some uncer-tainty (˙3:4 J K�1 mol�1/. Millar [12] reported a value of149.5 J K�1mol�1 for entropy of ˛-Mn3O4 at 298.15 K. Ro-bie and Hemingway [13] have measured low-temperatureheat capacities of Mn3O4 between 5 and 380 K using a fullyautomated adiabatically shielded calorimeter. The transi-tion from ferri-magnetic to paramagnetic was found to oc-cur at 43.12 K. Robie and Hemingway [13] reported a valueof 164:1 .˙0:2/ J K�1mol�1 for the standard entropy of ˛-Mn3O4 at 298.15 K. Chhor et al. [14] measured the heat ca-pacity of Mn3O4 in the temperature range from 10 to 310 Kusing an automated adiabatic calorimeter. Magnetic transi-tion was found at 43.15 K. Chhor et al. [14] reported a valueof 167.1 J K�1mol�1 for entropy of ˛-Mn3O4 at 298.15 K.

Compared in Figure 5, are the low temperature heat ca-pacity reported by Robie and Hemingway [13] and Chhoret al. [14] in the transition region. Above 50 K there isgood general agreement between these two sets of mea-surement. However, minor discrepancies can be observedat lower temperature. The shoulder on the ascending sideof the heat capacity peak at 40.1 K found by Robie andHemingway [13] was not confirmed by Chhor et al. [14].The maximum measured C 0P values are 121.4 J K�1 mol�1

at 43.12 K according to Robie and Hemingway [13] and188.1 J K�1 mol�1 at 43.15 K according to Chhor etal. [14]. Robie and Hemingway [13] did not assess the en-tropy associated with the transition. The entropy was esti-mated by Chhor et al. [14] by interpolating the base line C 0Pvariation between 10 and 50 K and integrating .C Ex

P =T /dT

in this range. The molar entropy of Mn3O4 associated withmagnetic transition was assessed as 11.5 J K�1 mol�1 [14].The data of Robie and Hemingway [13] was analyzed byChhor et al. [14] to generate the entropy of magnetic transi-tion as 8.5 J K�1 mol�1.

The two recent measurements of low-temperature heatcapacity of ˛-Mn3O4 [13, 14] are of equal caliber and itis difficult to differentiate between the two. Hence, an av-

20 30 40 50 600

50

100

150

200

Robie & Hemingway /13/ Chhor et al. /14/

Co p

/ J K

-1 m

ol-1

T / K

-Mn3O4

Figure 5. Comparison of low-temperature heat capacitiesof ˛-Mn3O4 reported by Robie and Hemingway [13] andChhor et al. [14].

erage value, 165:6 .˙1:0/ J K�1 mol�1, is selected for thestandard entropy of ˛-Mn3O4 at 298.15 K.

4.1.2 High-temperature Heat Capacity and Heat Content.H 0T�H 0

298:15/ of Mn3O4

The equation for heat capacity of ˛-Mn3O4 obtained in thisstudy (Eq. 1) can be integrated to give .H 0

T � H0298:15/ as

a function of temperature. Southard and Moore [15] mea-sured high-temperature heat content or enthalpy increment.H 0

T � H0298:15/ of Mn3O4 in the temperature range from

498 to 1769 K by drop calorimetry. Samples of Mn3O4were dropped from a platinum-rhodium high temperaturefurnace into a copper block calorimeter. Compared in Fig-ure 6, are values of (H 0

T �H0298:15/ measured by Southard

and Moore [15] with those calculated from heat capacitydata obtained in this study for ˛-Mn3O4. There is goodagreement between both sets of data. The heat content dataof Southard and Moore [15] for ˇ-Mn3O4 in the tempera-ture range from 1445 to 1800 K can be adequately fitted bylinear least-squares analysis to give the expression;

H 0T �H

0298:15=J mol�1 D �75 135:6C 210:9 .T=K/:

(10)

The expression can be differentiated to obtain heat capac-ity for Mn3O4 in this range The tetragonal to cubic phasetransition of spinel Mn3O4 at 1445 K is associated withan enthalpy change of 18.83 kJ mol�1. Based on thesedata thermodynamic functions, H 0

T � H0298:15, S0T and

�.G0T �H0298:15/=T for Mn3O4 can be constructed as func-




200 400 600 800 1000 1200 1400 1600 1800

0

50

100

150

200

250

300

Mn3O4

Southard and Moore /15/ This study (from Cp) Linear fit ( -Mn3O4)

Ho T

-H

o 298.

15 /

kJ

mol

-1

T / K

Figure 6. Comparison of the enthalpy increment.H 0

T � H0298:15/ of ˛-Mn3O4 reported by Southard and

Moore [15] with values calculated from C 0P measured inthis study as a function of temperature. Experimental datafor ˇ-Mn3O4 can be represented by a linear relation.

tions of temperature. These values are listed in Table 4along with other data assessed in this study.

4.1.3 Third-law Analysis

The third-law of thermodynamics provides a better methodfor deriving the enthalpy of formation from measurementof the corresponding Gibbs energy of formation measuredat high temperatures than the second-law method. It allowsthe calculation of the formation enthalpy from each datapoint. If the same value is obtained for enthalpy of forma-tion at a reference temperature from Gibbs energy of forma-tion at different temperatures, then internal consistency ofall the data used in the analysis is confirmed. The third-lawmethod is based on knowledge of the absolute entropy ofthe reactants and products based on low-temperature heatcapacity measurements and the third law. The third-lawanalysis of the high-temperature data on Gibbs energy offormation is based on the equation,

�rH0298:15

D �rG0.T / ��.H 0

T �H0298:15/

C T ¹�rS0298:15 C�.S

0T � S

0298:15/º (11)

where .H 0T � H

0298:15/ for each phase can be calculated

asR T298:15 C

0p dT and .S0T � S

0298:15/ can be calculated asR T

298:15.C 0p =T /dT . The quantity �.H 0

T � H0298:15/ and

�.S0T �S0298:15/ represent the difference between the sums

of these quantities for products and reactants multiplied bythe appropriate stoichiometric coeffients (i/;

Pi.H 0

T �

H 0298:15/ and

Pi.S0T � S

0298:15/, respectively. �rH

0298:15

is the derived standard enthalpy change for the specifiedreaction at 298.15 K and �rG

0.T / is the measured stan-dard Gibbs free energy change at temperature T . Values of�rH

0298:15

can be derived from each value of free energychange. If the derived values and independent of the tem-perature of the free energy measurement, then all the dataused for analysis are internally consistent and accurate.

4.1.4 Enthalpy of formation of Mn3O4 at 298.15 K

For assessing the enthalpy of formation of ˛-Mn3O4 at298.15 K, consider the oxidation of stoichiometric MnO ac-cording to the reaction,

6MnOC O2 D 2˛-Mn3O4: (12)

Measured value for the Gibbs energy change for this reac-tion is available only below 900 K when the nonstoichiom-etry of MnO becomes negligible. At higher temperatures,MnO contains excess of oxygen because of cation vacan-cies, and the oxidation reaction is best represented by reac-tion (4). Since auxiliary thermodynamic functions for non-stoichiometric Mn1�xO are not available, the best optionis to compute standard free energy change for the hypo-thetical reaction (12) for temperatures higher than 900 Kusing measured oxygen potentials for the biphasic equilib-rium and evaluated activity of MnO in Mn1�xO in equilib-rium with Mn3O4.

For the evaluation of activity of MnO in Mn1�xO, infor-mation on the variation of oxygen potential with nonstoi-chiometric parameter, x, is required. Using Gibbs–Duhemequation, activity of Mn .aMn/ at the oxygen-rich phaseboundary of Mn1�xO (X s

O/ can be evaluated:

log aMn D

Z XsO

XOD0:5

�1=2.XO=XMn/d logPO2 : (13)

Since stoichiometric MnO (XO D 0:5/ is in equilibriumwith pure Mn metal, aMn D 1 at this boundary. The molefraction of oxygen in Mn1�xO is related to the nonstoichio-metric parameter: XO D 1=.2 � x/. The oxygen-rich com-positions of Mn1�xO, defined by the phase diagram, arelisted in Table 1. Keller and Dieckmann [16] measured val-ues of x as a function of oxygen partial pressure (PO2/ atregular intervals of temperature from 1173 to 1673 K. Thevalues of the activities of Mn at oxygen-rich compositionsat different temperatures are presented in Table 2. Fromthe activity of Mn and oxygen potential at the oxygen-richboundary, activity of MnO can be computed using accu-rately known value of Gibbs energy of formation of MnO[2] For the reaction,

MnC 1=2O2 D MnO; (14)

�G0MnO D �RT ln aMnO=aMn C 0:5��O2 : (15)




T / K Metal saturationboundaryXO D 1=.2 � x/

Mn3O4 saturationboundaryXO D 1=.2 � x/

1173 0.5 0.50168

1273 0.5 0.50279

1373 0.5 0.50581

Table 1. Stability field of Mn1�xO.

T=K ��O2(Mn1�xOCMn3O4/

ln aMn ln aMnO �rG0=

J mol�1

1173 �185 552 �21:20840 �0:01538 �186 446

1273 �163 382 �19:88873 �0:02512 �164 972

1373 �141 212 �18:76980 �0:04214 �144 093

Table 2. The standard Gibbs energy change for the reaction(12), 6 MnO C O2 ! 2 Mn3O4, computed from oxygenpotential measured in this study with correction for activ-ity of MnO.

The activities of MnO (aMnO/ in the oxygen-rich nonstoi-chiometric phase thus calculated at different temperaturesare displayed in Table 2. Using this activity and the mea-sured oxygen potentials for the biphasic equilibrium be-tween Mn1�xO and Mn3O4, the standard Gibbs energychange for reaction (12) is computed at higher tempera-tures:

�G012 D 6RT ln aMnO C��O2 : (16)

Since Mn3O4 is essentially stoichiometric, its activity istaken as unity. The calculated Gibbs energy changes forreaction (12) are also listed in Table 2.

For third-law analysis, auxiliary data {S0298:15, �.H 0T �

H 0298:15/ and �.S0T � S

0298:15/}for O2 gas and solid MnO

are needed. High temperature heat content.H 0T �H

0298:15/,

entropy increment .S0T � S0298:15/ and standard entropy

{S0298:15=J K�1 mol�1 D 59:02 .˙0:4/} of MnO weretaken from a recent publication [2] and the correspond-ing quantities for O2 gas from JANAF thermochemicaltables [17] {S0298:15=J K�1 mol�1 D 205:147 .˙0:035/}.Using the selected standard entropy of Mn3O4 at298.15 {165:6 .˙1:5/ J K�1 mol�1}, the standard en-tropy change for reaction (12) at 298.15 K, (�rS

0298:15/ is

�228:067 J K�1 mol�1.Third-law analysis of the results obtained in this study

for the biphasic equilibrium between Mn1�xO and Mn3O4was carried out with and without the correction for the ac-tivity of MnO. The correction is applied only at three tem-peratures, 1173, 1273 and 1373 K; the magnitude of thecorrection increases with temperature and nonstoichiome-try. The results are shown in Figure 7. It is seen that withoutcorrection (filled symbol) the derived values of �rH

0298.15

800 900 1000 1100 1200 1300 1400-451

-450

-449

-448

-447

-446

-445

-444 This Study Charette and Flengas /6/ Schaefer /18/

6MnO + O2 = 2Mn3O4

Ho 29

8.15

/ k

J m

ol-1

T / K

Figure 7. Results of “third-law” analysis of Gibbs energyof formation of Mn3O4 from MnO derived from oxygenpotential measurements, with and without correction forthe activity of MnO. The filled symbols represent valueswithout correction and open symbols values with correc-tion.

for reaction (12) show significant increase with tempera-ture. However, when correction is applied �rH

0298.15 is al-

most independent of temperature (open symbol) and yieldthe correct value for the enthalpy change.

There are numerous measurements on the oxygen poten-tial corresponding to the oxidation of Mn1�xO to Mn3O4using a variety of techniques with lesser accuracy. Sincemost of these have been discussed and analyzed at lengthby Grundy et al. [1], the discussion will not be repeatedhere. However, two among the better data sets are analyzedto show trends: data from Charette and Flengas [6], andSchaefer [18]. The results of the third-law analysis of theirresults are also displayed in Figure 6. The trend in theirresults is similar to that seen with the data obtained in thisstudy. Without correction, all the values of�rH

0298.15 for re-

action (12) increase significantly with temperature and non-stoichiometry. The results of Charette and Flengas [6] yielda value for �rH

0298.15, which is 1.63 kJ mol�1 more pos-

itive than the result obtained in this study. The results ofSchaefer [18] show a temperature dependent error in mea-surement since the value of �rH

0298.15 appear to vary with

temperature even after correction for the activity of MnO.Because of the use of buffer electrode, the result obtainedin this study is considered more accurate than the valueevaluated from the measurements and Charette and Flengas[6]. Hence, the value �r(12)H

0298.15 D �450:562 kJ mol�1

is selected for reaction (12). Using transposed temperaturedrop calorimetry and drop solution calorimetry in lead bo-rate melt at 977 K, Fritsch and Navrotsky [19] determined a




Source�fH

0298:15= kJ mol�1 S0298:15= J K�1 mol�1

Value Method Value Method

Siemonsen[20] �1407:92 .˙0:8/ CombustionCalorimetry

– –

Shomate [21] �1387:62 .˙1:1/ Solution calorimetry – –

Millar [12] – – 149.5 Heat Capacity70 to 300 K

Robie andHemingway [13]

�1384:5 .˙1:4/ Assessment 164:1 .˙0:2/ Heat Capacity5 to 380 K

Chhor et al. [14] – – 167:1 .˙0:5/ Heat Capacity10 to 310 K

Grundy et al. [1] �1382:74 Assessment 168.34 Assessment

Wagman et al. [22] �1387:8 Compilation 155.7 Compilation

Knacke et al. [23] �1386:1 Compilation 154.4 Compilation

Pankratz [24] �1387:8 Compilation 155.7 Compilation

This study �1386:185 .˙1:2/ Third-law Analysis 165:6 .˙1:0/ Selected value

Table 3. Comparison of basic thermodynamic data for Mn3O4 selected in this study with values reported in the literature.

value of �441:4.˙5:8/ kJ mol�1 for reaction (12) throughan indirect route. Since complete oxidation or reduction ofthe oxide samples dropped into the calorimeter at 997 K inair was assumed but not ascertained, the result is of doubtfulaccuracy

Enthalpy of formation of Mn3O4 can be calculated fromthis value and enthalpy of formation of MnO [2]:

�r.12/H0298:15 D 2�f(Mn3O4)H

0298:15 � 6�f(MnO)H

0298:15:

(17)

The standard enthalpy of formation of Mn3O4from elements thus obtained is �f(Mn3O4)H

0298.15 D

�1386:185 kJ mol�1, in fair agreement with solutioncalorimetric value of Shomate [19]. Comparison of se-lected data �fH

0298:15 and S0298:15 for Mn3O4 with values

available in the literature is presented in Table 3. Theliterature values are taken from calorimetric measurements[20, 21], assessments [1] or compilations [22–24]. As-sessment of Grundy et al. [1] uses both thermodynamicand phase diagram inputs based on Thermo-Calc databaseand PARROT computational software. A heavy relianceon phase diagram information and the use of a physicallyunrealistic model for the liquid solution, slightly distortsthe assessment of thermodynamic data. Combustion calori-metric value of Siemonsen [20] for enthalpy of formation issignificantly more negative than information from all othersources. This old value can now be discarded. Becauseof the high accuracy now achievable with solid-state elec-trochemical cells, phase-equilibrium calorimetry involvingthe “third-law” analysis has emerged as a competing toolto solution and combustion calorimetry for determiningthe standard enthalpy of formation of oxides at 298.15 K,

when heat capacities of both products and reactants areknown over the full range of temperature. The new resultssuggest a major revision in the value of standard entropy ofMn3O4 at 298.15 K given in standard compilations. Basedon the basic thermodynamic data for Mn3O4 selected inthis study, a table of refined data for Mn3O4 is prepared(Table 4). Thermodynamic properties are listed at regularintervals of temperature and at temperatures of all phasetransitions for Mn and Mn3O4. Values given in Table 4supersede those available in earlier compilations [22–24]and a more recent assessment [1]. There are no data formanganese oxides in JANAF thermochemical tables [17].

4.2 Mn2O3 (Bixbyite)

Orthorhombic ˛-Mn2O3 transforms to cubic ˇ-Mn2O3 at307.5 K. This oxide is essentially stoichiometric.

4.2.1 Low-temperature Heat Capacity and Entropyof Mn2O3 at 298.15 K

King [25] measured the heat capacity of Mn2O3 in the tem-perature range from 51 to 298.15 K. Anti-ferromagneticto paramagnetic transition occurs at 79.4 K which makesthe extrapolation of heat capacity to 0 K rather difficult.King [25] reported a value of 110.5 J K�1 mol�1 for en-tropy of Mn2O3 at 298.15 K, with a probable uncertaintyof ˙2:4 J K�1 mol�1. Robie and Hemingway [13] havemeasured heat capacity of Mn2O3 between 5 and 349 Kusing a fully automated adiabatically shielded calorime-ter. Their values are systematically greater than that ofKing [25] by 0.5%. A sharp �-point corresponding to of




T=K C 0pJ K�1 mol�1

H 0T � H

0298:15

J mol�1S0TJ K�1 mol�1

�.G0T �H0298:15/=T

J K�1 mol�1�fH

0

kJ mol�1�fG

0

kJ mol�1

298.1530040050060070080090098098010001100120013001360136014001411141114451445150015191519160017001800

142.36142.67155.26163.75170.61176.72182.42187.88192.14192.14193.19198.41203.56208.66211.71211.71213.74214.29214.29216.01210.90210.90210.90210.90210.90210.90210.90

026415 21531 18547 91165 28283 24110 175711 695811 695812 081214 0392160 490181 102193 713193 713202 222204 576204 576211 891229 615241 214245 222245 222262 304283 394304 484

165.60166.48209.41245.01275.48302.25326.22348.03364.20364.20368.10386.75404.24420.73430.22430.22436.38438.06438.06443.18455.43463.31465.96465.96476.92489.71501.76

165.60165.60171.37182.64195.63208.99222.17234.96244.86244.86247.28259.12270.50281.42287.78287.78291.94293.07293.07296.54296.53302.50304.53304.53312.98323.00332.60

�1386:185

�1386:176

�1385:372

�1384:332

�1383:264

�1382:210

�1381:159

�1380:081

�1379:150

�1385:825

�1385:648

�1384:449

�1382:945

�1381:121

�1379:872

�1386:232

�1385:785

�1385:713

�1391:353

�1391:195

�1373:471

�1373:337

�1373:442

�1406:442

�1406:393

�1406:499

�1406:651

�1284:409

�1283:779

�1249:751

�1215:966

�1182:394

�1149:001

�1115:755

�1082:647

�1056:369

�1056:370

�1049:525

�1015:969

�982:538

�949:236

�929:462

�929:462

�915:903

�912:238

�912:240

�900:813

�900:788

�882:651

�876:614

�876:612

�848:179

�813:289

�778:391

Table 4. Reassessed thermodynamic data for Mn3O4.

anti-ferromagnetic to paramagnetic transition was seen at79.45 K. A broad change in heat capacity was also observedat 307.5 K, which was attributed to orthorhombic to cu-bic transition. The transition was continuous, with no en-thalpy of transition. Robie and Hemingway [13] reported avalue of 113:7 .˙0:8/ J K�1 mol�1 for entropy of Mn2O3 at298.15 K. Uncertainty in the extrapolation of heat capacityfrom 0 to 51 K by King [25] accounts for a large part of thedifference of 3.2 J K�1 mol�1between values of standardentropy reported by King [25] and Robie and Hemingway[13].

4.2.2 High-temperature Heat Content (H 0T�H 0

298:15/ and

heat capacity of Mn2O3

Orr [26] reported high-temperature heat content (H 0T �

H 0298:15/ of Mn2O3 in the temperature range from 397 to

1351 K by drop calorimetry. Heat capacity at 298.15 Kderived from this high temperature data do not agree withthat obtained from low-temperature measurements, proba-bly because of the broad hump in heat capacity at 307.5 K.To overcome this difficulty, Robie and Hemingway [13]

integrated their C 0P between 298.15 and 325 K to obtain(H 0

325 � H0298:15 D 2872 J mol�1/, then subtracted this

quantity from each of the measurements of Orr [26] to ob-tain H 0

T � H0325, which was fitted to a polynomial. Dif-

ferentiation of the polynomial gave an expression for heatcapacity in the temperature range from 325 to 1400 K:

C 0P =J mol�1 K�1 D 162:36C 0:01211 .T=K/

C 1:046 � 106 .T=K/�2 C 3:462 � 10�6 .T=K/2

� 1317:3 .T=K/�1=2: (18)

This equation joins smoothly with the heat capacity mea-surements of Robie and Hemingway [13]. Graphically in-tegrating C 0P =T as a function of temperature one obtainsS0325 � S

0298:15 D 9:24 J K�1 mol�1.

4.2.3 Enthalpy of formation of Mn2O3 at 298.15 K

The third-law analysis of the oxygen potential (Eq. 7) cor-responding to the biphasic equilibrium between Mn3O4




and Mn2O3 can be used to derive enthalpy of forma-tion of Mn2O3. For this analysis, auxiliary data on O2gas and Mn3O4 are also needed. Data for O2 gas fromJANAF thermochemical tables [17] and for Mn3O4 fromTable 4 are used. Using the selected standard entropy ofMn2O3 at 298.15 {113:7.˙0:2/ J K�1 mol�1}, the entropyof change for reaction (6) is obtained: �r.6/S

0298:15 D

�185:35 J K�1 mol�1.Results of third-law analysis of free energy data for re-

action (6) is presented in Figure 8. The results from threesets of high temperature data are reasonably concordant.However, data of Charette and Flengas [6] and Schae-fer [18] shows systematic increase in the derived value of�r.6/H

0298:15 with temperature, suggesting possible temper-

ature dependent errors in their measurements. Neverthe-less, the mean values from their study connect well withthe constant value of �r.6/H

0298:15 obtained in this study.

In view of this, �r.6/H0298:15 D �224:477 kJ mol�1 is

selected. Transposed temperature and drop calorimetricstudies of Fritsch and Navrotsky [19] suggest a value of�201:8 .˙8:7/ kJ mol�1. As discussed earlier there is un-certainty regarding the completion of oxidation/reductionreactions during the course of their calorimetric measure-ment. Enthalpy of formation of Mn2O3 from elements canbe derived from the value and enthalpy of formation ofMn3O4 assessed in the study:

�r(6)H0298:15 D 6�f(Mn2O3/H

0298.15 � 4�f.Mn3O4/H

0298.15:

(19)

Thus, the enthalpy of formation of Mn2O3,�f(Mn2O3/H

0298.15 D �961:536 .˙1:0/ kJ mol�1.

Comparison of selected data �fH0298:15 and S0298:15 for

Mn2O3 with values available in the literature is presentedin Table 5. The literature values are taken from calori-

900 1000 1100 1200 1300-230

-228

-226

-224

-222

-220

-218

Charette and Flengas /6/ Schaefer /18/ This study

4Mn3O4 + O2 = 6Mn2O3

Ho 29

8.15

/ k

J m

ol-1

T / K

Figure 8. Results of “third-law” analysis of Gibbs energyof formation of Mn2O3 from Mn3O4.

metric measurements [20], assessment [1] or compilations[22–24]. As in the case of Mn3O4 combustion calorimet-ric value of Siemonsen [20] for enthalpy of formation issignificantly more negative than that obtained in this study.Based on the basic thermodynamic data for Mn2O3 selectedin this study, a table of refined data for Mn2O3 is prepared(Table 6). Thermodynamic properties are listed at regularintervals of temperature and at temperatures of all phasetransitions. Values given in Table 6 supersede those avail-able in earlier compilations [22–24] and more recent assess-ment [1].

4.3 MnO2 (Pyrolusite)

There are several modifications of MnO2. However, theyare all metastable relative to the rutile form (space groupP 42/mnm).

4.3.1 Low-temperature Heat Capacity and Entropyof MnO2 at 298.15 K

Millar [12] measured the heat capacity of MnO2 in thetemperature range from 72 to 294 K and calculated avalue of 58.28(±1.4) J K�1mol�1 for entropy of MnO2 at298.15 K by extrapolation to 0 K. Kelly and Moore [27]also measured the heat capacity of MnO2 using an adi-abatic calorimeter in the temperature range from 55 to295 K and calculated entropy of MnO2 at 298.15 K as53:14 .˙0:4/ J K�1 mol�1. In view of uncertainty in ex-trapolation of heat capacity to 0 K, the real uncertaintyis probably ˙1 J K�1mol�1. Robie and Hemingway [13]measured the heat capacity of MnO2 in the temperaturerange from 6 to 377 K using a fully automated adiabaticallyshielded calorimeter. Robie and Hemingway [13] reporteda value of 52:75 .˙0:07/ J K�1 mol�1 for entropy of MnO2at 298.15 K. Compared in Figure 9, are the low tempera-ture heat capacity reported by Millar [12], Kelly and Moore[27] and Robie and Hemingway [13]. There is good gen-eral agreement between the different measurements. In thetemperature range of 107 to 294 K heat capacity reportedby Millar [12] is slightly higher than those of Kelly andMoore [27] and Robie and Hemingway [13]. The transi-tion from anti-ferromagnetic to paramagnetic state occursat Neel temperature TN= 92.2 K.

4.3.2 High-temperature Heat Content (H 0T�H 0

298:15/ and

heat capacity of MnO2

Moore [28] reported high-temperature heat content (H 0T �

H 0298:15/ of MnO2in the temperature range from 406 to

778 K by drop calorimetry. Robie and Hemingway [13]integrated their C 0P between 298.15 and 380 K to obtain en-thalpy increment and combined with (H 0

T �H0298:15/ values




Source�fH

0298:15=kJ mol�1 S0298:15= J K�1 mol�1


Siemonsen [20] �973:6 .˙1:3/ CombustionCalorimetry

– –

King [25] – – 110:46 .˙2:1/ Heat Capacity51 to 298 K

Robie andHemingway [13]






This study �961:536 .˙1:0/ Third-law Analysis 113:70 .˙0:4/ Selected

Table 5. Comparison of basic thermodynamic data for Mn2O3 selected in this study with values reported in the literature.


H 0T � H

0298:15


�.G0T �H0298:15/=T


0

kJ mol�1�fG

0

kJ mol�1

298.153004005006007008009009809801000110012001300136013601400

109.4109.8108.4114.6120.0124.9129.3133.4136.6136.6137.3141.0144.6148.0150.1150.1151.4

020310 82321 97833 71145 95958 67271 81382 61482 61485 35399 271113 552128 183137 127137 127143 157

113.70114.38144.93169.79191.17210.04227.01242.48253.98253.98256.75270.01282.43294.14300.87300.87305.24

113.70113.70117.87125.84134.98144.38153.67162.69169.68169.68171.39179.76187.80195.54200.04200.04202.98

�961:536

�961:512

�960:818

�960:126

�959:359

�958:531

�957:646

�956:698

�955:869

�960:319

�960:149

�959:071

�957:778

�956:269

�955:260

�959:500

�959:086

�884:475

�883:998

�858:275

�832:719

�807:309

�782:033

�756:877

�731:839

�711:977

�711:978

�706:820

�681:537

�656:365

�631:302

�616:425

�616:424

�606:242

Table 6. Reassessed thermodynamic data for Mn2O3.

for MnO2 between 406 and 778 K from Moore [28]. Theyfitted the combined data with a polynomial and then dif-ferentiated the expression to obtain the heat capacity in thetemperature range from 298.15 to 850 K.

C 0P =J mol�1 K�1 D 290:41 � 0:14424.T=K/

C 2:0119 � 106.T=K/�2 C 4:541 � 10�5.T=K/2

� 3786:7.T=K/�1=2: (20)

4.3.3 Enthalpy of Formation of MnO2 at 298.15 K

Compared in Figure 10, are values of the Gibbs energychange for reaction (8) reported by Klingsberg and Roy [29]and Otto [30] with that obtained in this study at 773 K Thereis inconsistency in the data presented in Figure 1 and Ta-ble VI of Klingsberg and Roy [29] at lower temperatures.Since the data in the figure are more extensive, they areplotted in Figure 10 At low temperatures, Gibbs energiesfor the reaction reported by two groups [29, 30] differ sig-




0 50 100 150 200 250 300 350 400

0

10

20

30

40

50

60

70C

o p / J

K-1 m

ol-1

T / K

Robie and Hemingway /13/ Kelly and Moore /27/ Millar /12/

Figure 9. Comparison of low-temperature heat capacity ofMnO2 reported by Millar [12], Kelly and Moore [27], andRobie and Hemingway [13].

700 750 800 850 900 950-10

0

10

20

30

40

2 Mn2O3 + O2 = 4 MnO2

Klingsberg and Roy /29/ Otto /30/ Assessment, This study Experiment, This study

Go /

kJ m

ol-1

T / K

Figure 10. Comparison of Gibbs energy change for theoxidation of Mn2O3 to MnO2 reported by Klingsberg andRoy [29], and Otto [30] with the result obtained this study.

nificantly but at higher temperatures they converge. In thisstudy Gibbs energy change for the reaction was measuredat 773 K, which is significantly more negative than the dataof Klingsberg and Roy [29], but close to that of Otto [30].The enthalpy change for reaction (8) is calculated fromthe measured Gibbs free energy change at 773 K (Eq. 9),by the third-law method. Using the selected standard en-tropy of MnO2 at 298.15 {52:75 .˙0:2/ J K�1 mol�1},the standard entropy change for reaction (8),�r.8/S

0298:15

is �110:77 J K�1 mol�1. Auxiliary data on O2 gas andMn2O3 are from JANAF thermochemical tables [17] andTable 6, respectively.

The enthalpy change for the reaction, �r.8/H0298:15, is

obtained as�162:724 .˙3:0/ kJ mol�1 in reasonable agree-ment with the value of �162:1 .˙7:2/ kJ mol�1 suggestedby Fritsch and Navrotsky [19]. In view of the large uncer-tainties associated with their values and concerns regardingthe completion of reduction/oxidation reaction, the calori-metric measurements of Fritsch and Navrotsky [19] are notvery helpful in refining thermodynamic data for the higheroxides of manganese. Enthalpy of formation of MnO2 fromelements can be calculated as:

�r.8/H0298:15 D 4�f(MnO2/H

0298.15 � 2�f(Mn2O3)H

0298.15:

(21)Enthalpy of formation of MnO2, �f(MnO2/H

0298.15, is

�521:449 .˙0:9/ kJ mol�1.Comparison of selected data �fH

0298:15 and S0298:15 for

MnO2 with values available in the literature is presentedin Table 7. The literature values are taken from calori-metric measurements [20, 21], assessment [1] or compila-tions [22–24]. The value from combustion calorimetry ofSiemonsen [20] is again more negative than values obtainedby others. The result computed in this study by the “third-law” method agrees well with solution calorimetric valueof Shomate [21]. Based on the basic thermodynamic datafor Mn2O3 evaluated in this study, a table of refined datafor MnO2 is prepared Table 8. Thermodynamic propertiesare listed at regular intervals of temperature and at temper-atures of all phase transitions. As shown in Figure 10, thereassessed Gibbs free energy change for reaction (8) basedon Tables 6 and 8 agrees well the measurements of Otto[30]. Values for MnO2 given in this Table 8 supersede thoseavailable in earlier compilations [22–24] and a more recentassessment [1].

5 Conclusions

Since accurate thermodynamic data on higher oxides ofmanganese are required in metallurgy, ceramics and ge-ology, they have been reassessed based on new measure-ments and critically selected information from the litera-ture. The availability of accurate low-temperature heat ca-pacity data [13,14] allows precise definition of entropy. Theentropy values are combined with new high-precision mea-surements on the chemical potential of oxygen in differentbiphasic regions of the system Mn-O using a sophisticatedversion of solid-state electrochemical cell, to generate accu-rate information on enthalpies of formation by applying the“third-law” method. This procedure, which may be called“phase-equilibrium calorimetry”, now competes with directcalorimetry for determination of enthalpies of formation ofoxides. Refined thermodynamic data for the three higheroxides of manganese, Mn3O4, Mn2O3 and MnO2 are pre-sented in tabular form at regular intervals of temperature




Source�fH

0298:15=kJ mol�1 S0298:15= J K�1mol�1


Siemonsen [20] �524:67 .˙4:2/ CombustionCalorimetry

– –

Shomate [21] �521:49 .˙0:8/ Solution Calorimetry – –

Millar [12] – – 58.28 Heat Capacity97 to 294 K

Kelly and Moore[27]

– – 53:14 .˙0:4/ Heat Capacity54 to 295 K

Robie and Heming-way [13]






This study �521:449 .˙0:9/ Third-law Analysis 52:75 .˙0:2/ Selected

Table 7. Comparison of data for MnO2 selected in this study with values reported in the literature.


H 0T � H 0

298:15


�.G0T �H0298:15/=T


0

kJ mol�1�fG

0

kJ mol�1

298.15300400500600700800

54.7754.9563.2268.3471.2172.6873.34

0102603812 63919 63226 83534 140

52.7553.0970.1184.8297.56108.66118.42

52.7552.7555.0259.5464.8470.3375.74

�521:449

�521:450

�521:220

�520:615

�519:895

�519:216

�518:659

�466:405

�466:064

�447:620

�429:286

�411:088

�393:009

�375:018

Table 8. Reassessed thermodynamic data for MnO2.

and temperatures of phase transitions. The data now avail-able in thermodynamic compilations [22–24] require revi-sion, especially for Mn3O4 and Mn2O3. The new assesseddata for the higher oxides of manganese supplement im-proved information on MnO published earlier [2].

Acknowledgments

G. Rajitha thanks the University Grants Commission of In-dia for the award of Dr. D. S. Kothari Postdoctoral Fel-lowship and K. T. Jacob acknowledges the Indian NationalAcademy of Engineering for the award of their Distin-guished Professorship.

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