Accepted Manuscript

Thermodynamic characteristics of a new phase Bi12.5Ho1.5ReO24.5 by solutioncalorimetry

Nata I. Matskevich, Thomas Wolf, Peter Adelmann, Anna N. Bryzgalova

PII: S0925-8388(14)00731-2DOI: http://dx.doi.org/10.1016/j.jallcom.2014.03.132Reference: JALCOM 30903

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Received Date: 16 December 2013Revised Date: 17 March 2014Accepted Date: 22 March 2014

Please cite this article as: N.I. Matskevich, T. Wolf, P. Adelmann, A.N. Bryzgalova, Thermodynamic characteristicsof a new phase Bi12.5Ho1.5ReO24.5 by solution calorimetry, (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.03.132

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Thermodynamic characteristics of a new phase Bi12.5Ho1.5ReO24.5

by solution calorimetry

Nata I. Matskevicha*

, Thomas Wolfb, Peter Adelmann

b, Anna N. Bryzgalova

a

aNikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of

Sciences, Novosibirsk, 630090, Russia

bKarlsruhe Institute of Technology, Institute of Solid State Physics, Karlsruhe,

D-76344, Germany

Keywords: Bismuth oxide doped by holmium and rhenium; Synthesis; Formation Enthalpy;

Thermodynamic Stability

Abstract

Bi12.5Ho1.5ReO24.5 phase has been prepared from Bi2O3, Ho2O3 and NH4ReO4. X-ray

measurements have showed that Bi12.5Ho1.5ReO24.5 has a cubic structure (Fm3m space group).

The standard formation enthalpy of Bi12.5Ho1.5ReO24.5 has been determined by solution

calorimetry combining solution enthalpies of Bi12.5Ho1.5ReO24.5,

6.25Bi2O3+0.75Ho2O3+0.5Re2O7 mixture in 2 M HCl and literature data. The thermodynamic

stability of Re, Ho -doped bismuth oxide with respect to binary oxide mixture has been

determined. It has been established that the above-mentioned mixed oxide is

thermodynamically stable with respect to its decomposition into binary oxides at room

temperatures.

_______________________________________________

*Corresponding author. Tel.: +7-383-3306449; fax: +7-383-3309489; E-mail:

nata@niic.nsc.ru

1. Introduction

As is known, Bi2O3 shows the highest oxide ion conductivity of any known solid, with

an ionic conductivity of about 1 S /cm at temperatures above 730 C [1-9]. Bi2O3 has four

different polymorphs, denoted as monoclinic , tetragonal , body-centered cubic , and face-

centered cubic , among which the low-temperature phase and the high-temperature

phase are stable; while the other two phases are metastable. Unfortunately, the -phase is only

stable above 730 ºC in the pure Bi2O3 system, but it readily transforms to other lower

symmetry polymorphs below this temperature. The structures closely related to that of -

Bi2O3 can be stabilized partially substituting Bi by aliovalent or isovalent cations at room

temperatures. The synthesis of new phases in which Bi2O3 is doped with both rare earth-and

rhenium cations has been recently reported [1-3]. These materials have significantly higher

low-temperature oxide ion conductivities than Bi2O3.

The synthesis of Bi12.5Ho1.5ReO24.5 new material and its examination by solution

calorimetry to investigate the thermodynamic stability are presented here.

2. Experimental

Bi12.5Ho1.5ReO24.5 sample was synthesized for the first time.

Polycrystalline samples of Bi12.5Ho1.5ReO24.5 were prepared by solid-state reaction.

The synthesis was performed according to the reaction: 6.25Bi2O3 + NH4ReO4 + 0.75Ho2O3 =

Bi12.5Ho1.5ReO24.5 + NH3 + 0.5H2O. A starting reagent was annealed before synthesis at 800

ºC (Ho2O3) up to constant weight. A stoichiometric mixture of Bi2O3 (99.999%, ABCR),

NH4ReO4(>99%, Alfa Aesar, Johnson Matthey Company) and Ho2O3 (99.99%, Ventron) was

mixed in an agate mortar and ground for about 70 h with 10 intermediate regrounds in a

planetary ball mill (FRITSCH pulverisette 5). The rate was changed from 50 up to 200 rpm.

Then the mixture was pressed (pellet 14 mm, press Herzog (5.5t)), placed in a furnace

(Carbolite, 1100 ºC) and heat treated in air at 800 ºC for 70 h. The phase purity was analyzed

by X-ray diffraction (STADI-P, Stoe diffractometer, Germany, Cu Kα1 radiation). The

samples were found to be phase-pure ceramics with a cubic structure (space group is Fm3m).

The refined cell parameters obtained for Bi12.5Ho1.5ReO24.5 are: a = 5.57235 (22) A, Cell

volume 173.028 (12) A3. The powder X-ray diffraction pattern of bismuth oxide doped by

rhenium and holmium oxides is presented in Figure 1.

Recently [2, 3, 7, 10, 11] the thermodynamic properties of the following compounds:

Bi12.5La1.5ReO24.5, Bi12.5Nd1.5ReO24.5, Bi12.5Gd1.5ReO24.5, Bi12.5Dy1.5ReO24.5,

Bi12.5Lu1.5ReO24.5, and Bi12.5Sm1.5ReO24.5 have been studied. Based on the data the

dependency of the formation enthalpy from binary oxides as a function of the ionic radii was

derived. Using this dependence the formation enthalpy of Bi12.5Ho1.5ReO24.5 from binary

oxides was estimated as -152 kJ/mol. The ionic radius values were taken from Shannon’s

paper [12].

Our estimation has been checked experimentally. For the purpose the formation

enthalpy of Bi12.5Ho1.5ReO24.5 is obtained by solution calorimetry.

The hydrochloric acid solution (2 M) was chosen as a solvent. It was prepared by

diluting concentrated HCl of high purity with distilled water.

Reaction enthalpies of Bi12.5Ho1.5ReO24.5 and Ho2O3 with 2 M HCl were determined

experimentally using an automatic calorimeter with an isothermal jacket. The calorimeter

consists of a thin-walled brass vessel with a reaction Dewar vessel (200 ml). The construction

of the solution calorimeter and the experimental procedure were described elsewhere [13-17].

The thermometric sensitivity was 10-4

K. The calorimetric vessel was maintained at 298.15 K.

The temperature stability was better than 10-3

K.

Bi12.5Ho1.5ReO24.5 or Ho2O3 samples were placed in a thin-walled glass ampoule. The

ampoule was placed in the reaction vessel and broken by a special device.

The calorimeter was tested by the dissolution of KCl in water several times. The final

solution molarity was 0.028 mol kg-1

. The dissolution heat of KCl obtained was 17.41 0.08

kJ mol1

. Literature data are: 17.42 0.02 kJ mol1

[18] and 17.47 0.07 kJ mol1

[19]. As can

be seen, our data are in a good agreement with literature. It means that the calorimeter was

working correctly.

The calorimetric scheme to determine the formation enthalpy of Bi12.5Ho1.5ReO24.5 is

described by equations 1-4, which have been written so as to sum up equation 5,

corresponding to the formation of Bi12.5Ho1.5ReO24.5 from binary oxides. “Solution” refers to 2

M HCl in all of these equations. The reactions for solH2o, solH4

o are measured in this

research.

6.25 Bi2O3 + solution 1 = solution 2 + 6.25 solH1o (1)

0.75 Ho2O3 + solution 2 = solution 3 + 0.75solH2o (2)

0.5 Re2O7 + solution 3 = solution 4 + 0.5solH3o (3)

Bi12.5Ho1.5ReO24.5 + solution 1 = solution 4 + solH4o (4)

net: 6.25Bi2O3 + 0.75 Ho2O3 + 0.5 Re2O7 = Bi12.5Ho1.5ReO24.5 + rH5o (5)

The dissolution enthalpies of solH1o, solH3

o were taken from our earlier paper [7].

The measured solution enthalpies of Ho2O3 and Bi12.5Ho1.5ReO24.5 were:

solHo2(298.15 K) = .1 .2 kJ/mol (n = 5), solH

o4(298.15 K) = .5 6.7 kJ/mol

(n = 6). The errors were calculated for 95% confidence interval using the standard procedure

of processing experimental data.

The measured standard molar enthalpies of dissolution were used for calculating the

reaction enthalpy

6.25 Bi2O3 + 0.75 Ho2O3 + 0.5 Re2O7 = Bi12.5Ho1.5ReO24.5 + rH5o

according to the equation

rH5o = oxH

o solH1

o + solH2

o + solH3

o solH4

o

Here, rH5o = oxH

oBi12.5Ho1.5ReO24.5, s, 298.15 K) = 156.9 ± 8.5 kJ/mol is the

standard molar formation enthalpy of bismuth oxide doped by rhenium and holmium from

binary oxides.

To calculate this value our experimental data and our earlier data for solution molar

enthalpies of Re2O7 and Bi2O3 taken from Ref. [7] were used.

Then the experimental value of oxHoBi12.5Ho1.5ReO24.5, s, 298.15 K) = 156.9 ± 8.5

kJ/mol was compared with the same value estimated earlier (152 kJ/mol). As can be seen

there is a good agreement between these values.

The standard molar formation enthalpies of Bi2O3, Ho2O3, Re2O7 taken from Ref. [18]

were used to calculate the standard molar formation enthalpy of Bi12.5Ho1.5ReO24.5 as follows:

fHo = 5815.0 ± 9.1 kJ/mol.

To understand whether Bi12.5Ho1.5ReO24.5 phase is stable or unstable with respect to its

decomposition into 6.25 Bi2O3 + 0.75 Ho2O3 + 0.5 Re2O7 mixture it is necessary to know

Gibbs energies (G = H - TS). There is no entropy data of Bi12.5Ho1.5ReO24.5 phase in

literature. This value was estimated using entropies of Bi2O3, Ho2O3, Re2O7 taken from

literature [18]. Gibbs energy for the process (5) was the same as the formation enthalpy from

binary oxides.

As can be seen, the above-mentioned mixed oxide is thermodynamically stable with

respect to its decomposition into binary oxides at room temperatures.

Conclusions

The synthesis of the compound Bi12.5Ho1.5ReO24.5 by solid-state reaction performed for

the first time has been presented in this paper. The compound has a cubic structure (space

group Fm3m). Also, the standard molar formation enthalpy of Bi12.5Ho1.5ReO24.5 has been

measured by solution calorimetry in 2 M HCl. The stability of Re, Ho-doped bismuth oxide

with respect to mixtures of binary oxides has been determined. It has been established on the

basis of these data that the above-mentioned mixed oxide is thermodynamically stable with

respect to its decomposition into binary oxides at room temperatures.

Acknowledgments

This work is supported by Karlsruhe Institute of Technology (Germany), Russian

Fund of Basic Research (Project No 13-08-00169) and Program of Fundamental Investigation

of Siberian Branch of the Russian Academy of Sciences.

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Figure 1. X – ray pattern of Bi12.5Ho1.5ReO24.5

We synthesized new phase Bi12.5Ho1.5ReO24.5.

We measured formation enthalpy of Bi12.5Ho1.5ReO24.5 by solution calorimetry.

We established that Bi12.5Ho1.5ReO24.5 is the most thermodynamically stable with respect to their

decomposition into binary oxides at room temperature.