High-temperature phase transition in the whitlockite-type phosphate <script type="math/tex">Ca_{9}In(PO_{4})_{7}</script>

Morozov, V.A.; Belik, A.A.; Stefanovich, S.Yu.; Grebenev, V.V.; Lebedev, O.I.; Van Tendeloo, G.; Lazoryak, B.I.

doi:10.1006/JSSC.2001.9521

Title

High-temperature phase transition in the whitlockite-type phosphate $Ca_{9}In(PO_{4})_{7}$

Author

Morozov, V.A.

Belik, A.A.

Stefanovich, S.Yu.

Grebenev, V.V.

Lebedev, O.I.

Van Tendeloo, G.

Lazoryak, B.I.

Abstract

The double phosphate Ca9In(PO4)(7) was obtained by solid-state reaction and was studied by room and high-temperature Xray powder diffraction, electron diffraction, high-resolution electron microscopy, second-harmonic generation (SHG) technique, differential scanning calorimetry, dielectric and conductivity measurements. The beta-Ca9In(PO4)(7) room-temperature phase is related to the mineral whitlockite and is similar to beta-Ca-3(PO4)(2). It has space group R (3) over barc, with unit cell parameters: a = 10.4008(1) Angstrom, c = 37.272(1) Angstrom, Z = 6. The structure was refined by the Rietveld method (R-wp = 4.69, R-I = 1.81). A reversible first-order beta <----> beta' phase transition of the ferroelectric type with T-c = 902 K is revealed by SHG, differential scanning calorimetry and dielectric measurements. The centrosymmetric beta'-Ca9In(PO4)(7) (973 K) has space group R (3) over barc, a = 10.4611(2) Angstrom, c = 37.874 (1) Angstrom (R-wp = 4.27, R-I = 4.11). Compared to the low-temperature beta-Ca9In(PO4)(7) structure, beta'-Ca9In(PO4)(7) can be described as an orientational disordering of the P1O(4) tetrahedra together with a calcium disordering on the M3 site. (C) 2002 Elsevier Science (USA).

Language

English

Source (journal)

Journal of solid state chemistry. - London

Publication

London : 2002

ISSN

0022-4596

DOI

10.1006/JSSC.2001.9521

Volume/pages

165 :2 (2002) , p. 278-288