Les tellurates mixtes Li2ZrTeO6 et Li2HfTeO6: un nouveau phénomène d'ordre dans la famille corindon

Li2ZrTeO6 and Li2HfTeO6, synthesized at 700°C, are hexagonal-rhombohedral, space group R3, with a = 5.172 (5.164) Å and c = 13.847 (13.782) Å for Zr(Hf). Their structure, deduced from X-ray powder diffraction intensities, may be derived from that of LiNbO3 by an additional Zr(Hf)Te ordering. The order phenomena exhibit two components: (i) within each octahedral layer there is a Li Zr(Hf) or a Li Te order, and (ii) two adjacent layers are connected by pairs of face-sharing octahedra, alternatively a LiZr(Hf) and a LiTe. Measurements of the dielectric properties point to a diffuse ferroelectric transition near Tc = 660°C for Li2ZrTeO6. Their Raman and IR spectra are reported and discussed in terms of symmetry properties and mass effects. © 1988.