A newly discovered phase transition in Ca 9 Fe(PO 4 ) 7 is studied by high-temperature X-ray powder diffraction (XRD) and electron diffraction (ED), high-resolution electron microscopy, second-harmonic generation (SHG), differential scanning calorimetry, dielectric and electrical conductivity measurements. The phase transition temperature, T c , is 890±10 K. Dielectric measurements reveal the ferroelectric nature of the phase transition. A spontaneous polarization of about 2-3 μCcm -2 is estimated from SHG. The phase transition from a polar ferroelectric form β (space group R3c), to a centrosymmetric paraelectric form β′ (space group R3̄c), is reversible and of first order. The evolution of the crystal structure and lattice parameters is studied by high-temperature XRD and ED. The structural parameters of β′-Ca 9 Fe(PO 4 ) 7 are refined by the Rietveld method from XRD data at 923 K in the space group R3̄c (Z=6) with lattice parameters a=10.3878(2) Å and c=37.8387(4) Å (R wp =1.48% and R I =2.16%). Both β- and β′-Ca 9 Fe(PO 4 ) 7 are related to the whitlockite-type structure. The β→β′ phase transition in Ca 9 Fe(PO 4 ) 7 is accompanied by a disordering of the Ca 2+ ions at the Ca3 sites and orientational disordering of the P1O 4 tetrahedra. © 2003 Elsevier SAS. All rights reserved.
Lazoryak, B. I., Morozov, V. A., Belik, A. A., Stefanovich, S. Y., Grebenev, V. V., Leonidov, I. A., … Van Tendeloo, G. (2004). Ferroelectric phase transition in the whitlockite-type Ca 9 Fe(PO 4 ) 7 ; Crystal structure of the paraelectric phase at 923 K. Solid State Sciences, 6(2), 185–195. https://doi.org/10.1016/j.solidstatesciences.2003.12.007