Ferroelectric Phase Transition in the Whitlockite‐Type Ca₉Fe(PO₄)₇; Crystal Structure of the Paraelectric Phase at 923 K.

Abstract

Structure of the Paraelectric Phase at 923 K. -The ferroelectric phase transition of the title compound occurring at 890 K is characterized by high-temperature powder XRD, electron diffraction, HREM, second-harmonic generation, DSC, and dielectric and electrical conductivity measurements. The phase transition from a polar ferroelectric form (space group R3c) to a centrosymmetric paraelectric form (space group R3c) is reversible and of first order. The structural parameters of the paraelectric phase are refined by the Rietveld method from XRD data at 923 K in the space group R3c with Z = 6. -(LAZORYAK, B. I.; MOROZOV, V. A.; BELIK, A. A.; STEFANOVICH, S. Y.; GREBENEV, V. V.; LEONIDOV, I. A.; MITBERG, E. B.; DAVYDOV, S. A.; LEBEDEV*, O. I.; VAN TENDELOO, G.; Solid State Sci. 6 (2004) 2, 185-195; EMAT, Univ. Antwerp, B-2020 Antwerpen, Belg.; Eng.) -W. Pewestorf 43-005

High-Temperature Phase Transition in the Whitlockite-Type Phosphate Ca9In(PO4)7

“…Disappearance of the SHG signal on heating and its reappearance with definite thermal hysteresis on cooling is in accordance with the first-order polar-to-centrosymmetric phase transition which has previously been observed in Ca 9 Fe(PO 4 ) 7 (24). A noticeable difference in the value of the SHG signal before (I + = 1.32) and after (I À = 0.87) the heating-cooling cycle may be explained by the thermal history of the polycrystalline sample with a variable domain microstructure.…”

“…4. A detailed examination of different areas and grains only revealed a single phase (space group R3c) in contrast to Ca 9 Fe(PO 4 ) 7 where R3m (or R3) structure microdomains were found to be present inside the R3c matrix (24). Apparently, the formation of R3m (or R3) microdomains inside the R3c whitlockite-type Ca 9 R(PO 4 ) 7 phase is the result of a changing composition (R 3+ cations partially occupy the M5 site).…”

“…Kustova et al were the first to pay attention to changes in temperature of the IR-spectrum of Cr 3+ -stabilized bCa 3 (PO 4 ) 2 and related them to a new phase transition in this compound (23). We have recently discovered a hightemperature ferroelectric phase transition at 890710 K in Ca 9 Fe(PO 4 ) 7 (24). This reversible first-order b2b 0 phase transition changes the crystal structure from polar (R3c) to centosymmetric (R 3 3c) and is accompanied by a change in orientation of half of the P1O 4 tetrahedra.…”

Whitlockite-Related Phosphates Sr9A(PO4)7 (A=Sc, Cr, Fe, Ga, and In): Structure Refinement of Sr9In(PO4)7 with Synchrotron X-Ray Powder Diffraction Data

“…Ferroelectric phase transitions have recently been found in Ca 9 A(PO 4 ) 7 (A=Fe and In) (24,25). Low-temperature forms of Ca 9 A(PO 4 ) 7 crystallize in space group R3c with lattice parameters of aE10:3 ( A and cE37:3 ( A (Z=6) in a similar way to b-Ca 3 (PO 4 ) 2 (8,9,24,25).…”

“…Low-temperature forms of Ca 9 A(PO 4 ) 7 crystallize in space group R3c with lattice parameters of aE10:3 ( A and cE37:3 ( A (Z=6) in a similar way to b-Ca 3 (PO 4 ) 2 (8,9,24,25). In Ca 9 A(PO 4 ) 7 , the Ca1-Ca3 sites are occupied by Ca 2+ ions, the M4 and M6 sites are vacant, and the M5 site is occupied by A 3+ ions.…”