Ferroelectric phase transition in the whitlockite-type Ca9Fe(PO4)7;: crystal structure of the paraelectric phase at 923 K

A newly discovered phase transition in Ca9Fe(PO4)(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 muC cm(-2) is estimated from SHG. The phase transition from a polar ferroelectric form beta (space group R (3) over barc), to a centrosymmetric paraelectric form beta' (space group R (3) over barc), 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 beta'-Ca9Fe(PO4)(7) are refined by the Rietveld method from XRD data at 923 K in the space group R (3) over barc (Z = 6) with lattice parameters a = 10.3878(2) Angstrom and c = 37.8387(4). A (R-wp = 1.48% and R-I = 2.16%). Both beta- and beta'-Ca9Fe(PO4)(7) are related to the whitlockite-type structure. The beta --> beta' phase transition in Ca9Fe(PO4)(7) is accompanied by a disordering of the Ca2+ ions at the Ca3 sites and orientational disordering of the PlO(4) tetrahedra. (C) 2003 Elsevier SAS. All rights reserved.