lhe~~ochimica Acta, 92 (1985) 619-622 Elsevier Science Publishers B.V., Amsterdam

Na2CrO4,(1) SCLID SOLUTIONS: CRYSTAL CHEMISTRY, DEFECTS AND IONIC CONDUCTIVITY

J. Philipp and W. Eysel*, Universitat Heidelberg, Germany, and

H. Bohm, Universitat Miinster, Germany

ABSTRACT

In the system Na2CrO4-CaCrO4 a field of solid solutions Nai_xCa2+tI] Cr04 was found extending up to 40% CaCrO4, i.e. 20% cation Xvacakies (0). These defects are the reason for a remark- able ionic conductivity with a maximum of a = 10-j (Q.cm)-r at 7:5% a and at 500°C. In the chromate and the corresponding selenate system four new phases were found: Na6Ca(Cr04)4, Na4Ca(CrO4)3, Na4Ca(Se04)3 and high-CaSe04.

METHODS

CaCrO4 was precipitated from aqueous solutions of Na2Cr04 and

CaC12. Then mixtures of CaCr04 and Na2Cr04 were molten for 3 min.

in Pt crucibles at 950°C and quenched in air or liquid N2 (weight

losses l/2%) or slowly cooled in the furnace (weight loss 2%).

The resulting phases were identified by X-ray powder diffrac-

tion (Norelco Diffractometer) or Guinier Camera fAEG) with silicon

NBS-SRM 640 as internal standard. High temperature Debye Scher'rer

patterns were calibrated with a-Al203 (a = 4.775 and c = 13.036 A

at 5OO'C; a = 4.759 and c = 12.992 A at 2O'C).

For thermal analysis a Du Pont Thermal Analyzer 1090/910 'with

microcomputer, DSC-cell (< 700°C) and 12OO'C cell was employed.

(For temperature calibration c.f. ref .l). In addition a hot stage

(Leitz Company, < 135O'C) was used with a polarizing microscope.

Ionic conductivities could be determined by impedance meas-

urements (2) since the Na2Cr04(I)ss exhibit no significant elec-

tronic conductivity fc.f. ref.3).

THE SYSTEM Na2Cr04-CaCr04

Various methods were used to determine the Na-rich part of the

phase diagram Na2Cr04-CaCr04. DTA, DSC and high temperature X-ray

(Fig.11 results are included in Fig.2. In addition the phases

found by X-ray powder diffraction after quenching or slow cooling

were identified. The two polymorphs of Na2Cr04 are called

Na2Cr04(III) and Na2Cr04(I) in analogy to the isostructural

Proceedings of ICTA 85, Bratislava

- 620 -

modification of Na2S04,(111) and (I). The latter has a strongly

disordered structure (4).

A

alAl

5.67Y .

565 \ a \

563 l ._t’_

-ih c.fAl 0. 758'

c /

-_-

256 “J l *

750

74

/

viA31' _ '. 0 2.11

-*+,_--

0 10 20 30 LO 50 Na2Cr04 MO/% CaCrOL

0 5 10 15 20 25 %O Fig.1 Lattice parameters of

Na2Cr04(I)ss at 5OO'C

I ss I 0 Is + CaCrOL

u s+Iss

u be4&8dd+&d - -3-xy _ _ __

I

I -Lu, 2:l

$,&+3:1 $1 I I 2:1+ CaCrO4

3.11 il loo0 . . -

20 LO 60 80

Fig.2

The system Na2Cr04-CaCr04

0 High temperature X-ray results

0 DTA and DSC, heating

0 DTA and DSC, cooling

bib2Cr04 MO1 % CaCrOL

- 621 -

'The system is characterized by a large field of solid solution

of Na2CrO4(1) and by two low temperature compounds 2:l =

Na4Ca(CrO4)3 and 3:l = Na6Ca(Cr04)4. Their small stability ranges

below 3OO'C were not determined in detail. During investigations

in the corresponding system Na2SeO4-Case04 the isostructural

Na4Ca(Se04)3 and a new modification of Case04 with scheelite struc-

ture were found (Table 1). The latter was obtained by sintering a

pellet at 4OO'C for 5 days with subsequent quenching. The 3:l

selenate could not be prepared. In contrast to the corresponding

Na2S04(I)ss (ref.4) the Na2Cr04(I)ss could not be quenched.

Table 1

Lattice parameters of new compounds

Complete powder data were sent to JCPDS for publication in the Powder Data file

Compound Symmetry a(A) c(R)

Na4Ca(Cr04j3 hexagonal 16.315 22.747

Na4Ca(Se04j3 hexagonal 16.348 22.853

CaSe04 tetragonal 5.047 11.664

IONIC CONDUCTIVITY

The large field of the solid solutions (Na2_2xCaxOx)Cr04

implies a massive concentration of cation vacancies 0. They pro-

vide "hopping paths" for the remaining Na+ ions and are the basis

of a remarkable ionic conductivity (Fig.3).

10-l

I

-$ 10-z

$ 10-3

10-4

% 0 Fig.3 Ionic conductiv-

0 5 10 15 20 25 ity of

No2 GO4 Mol % CaCr04 Na2Cr04(I)ss

- 622 -

Starting from pure Na2CrO4, the conductivity increases strong-

ly and reaches its maximum at about 7.5% vacancies. The following

decrease is probably due to defect clustering. The maximum value

measured was u = 10 -1 (a cm)-' at 5OO'C.

A comparison with the isostructural solid solutions of

Na2SO,(I) shows a.surprising agreement, since also for these

materials the maxima of the conductidity curves appear at 7.5%

vacancies (3). Due to decomposition reactions corresponding in-

vestigations in .the system Na2SeO4 -Case04 were not possible.

ACKNOWLEDGEMENTS

We are indepted to W. Nagel, G. Roth and H. Maltry for experi-

mental help and to the Deutsche Forschungsgemeinschaft for finan-

cial support.

REFERENCES

1 W. Eysel & K.-H. Breuer in: Analytical Calorimetry, Vo1.V (Editors: J.F. Johnson & P.S. Gill), Plenum Publ. Corp., 1984, pp. 67-80

2 G. Roth, W. Nagel, H. BiShm, Z. Krist. 159, 105-106, 1982 3 H.H. HGfer, W. Eysel and U. v.Alpen, J. Sol. State Chem. 36,

365-370 (1981) 4 W. Eysel; H.H; Hbfer, K.L. Keester and Th. Hahn, Crystal Chemis-

try and Structure of Na2S04(I) and its Solid Solutions. Acta Cryst. B, in press