The anisotropic thermal expansion and anisotropy of lattice volume change in Y4Al2O9at 1370°C were confirmed by high-temperature neutron diffraction of the sintered polycrystalline sample. The cell volume of the high-temperature phase is approximately 0.4% smaller than that of the low-temperature phase. The crystal structure of the high-temperature phase was determined by Rietveld analysis. It crystallizes in a monoclinic cell,a=7.4804(4) Å,b=10.5461(5) Å,c=11.2057(7) Å, andβ=108.927(4)° at 1518°C,Z=4 with space groupP21/c, which is identical to the space group of the low-temperature phase. The phase transition was concluded to be diffusionless by comparison of the crystal structures 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 parallel to the (010) plane along theaaxis.
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