Ferroelectric phase transition in the whitlockite-type Ca 9Fe(PO 4) 7; crystal structure of the paraelectric phase at 923 K

Abstract

A newly discovered phase transition in Ca 9Fe(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 μC cm −2 is estimated from SHG. The phase transition from a polar ferroelectric form β (space group R3 c), to a centrosymmetric paraelectric form β′ (space group R 3c ), 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 9Fe(PO 4) 7 are refined by the Rietveld method from XRD data at 923 K in the space group R 3c ( 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 9Fe(PO 4) 7 are related to the whitlockite-type structure. The β→ β′ phase transition in Ca 9Fe(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.