High-temperature neutron diffraction study of Y4Al2O9

The anisotropic thermal expansion and anisotropy of lattice volume change i n Y4Al2O9 at 1370 degrees C were confirmed by high-temperature neutron diff raction of the sintered polycrystalline sample. The cell volume of the high -temperature phase is approximately 0.4% smaller than that of the low-tempe rature phase. The crystal structure of the high-temperature phase,vas deter mined by Rietveld analysis, It crystallizes in a monoclinic cell, a = 7.480 4(4) Angstrom, b=10.5461(5) Angstrom, c = 11.2057(7) Angstrom, and beta = 1 08.927(4)degrees at 1518 degrees C, Z = 4 with space group P2(1)/c, which i s identical to the space group of the low-temperature phase, The phase tran sition was concluded to be diffusionless by comparison of the crystal struc tures of the high- and low-temperature phases. At the transition, the atoms in the unit cell move cooperatively on the slip planes of oxygen atoms par allel to the (010) plane along the a axis. (C) 1998 Academic Press.