Antiferroelastic structural transitions in PrAlO3 by means of neutron diffraction

Structural phase transitions of the perovskite-type PrAlO3 have been studied by using neutron diffraction. The results show that, on cooling, the structures are successively transformed from R anti 3c (Phase II) to Imma (Phase III), Imma (Phase IV), I4/mcm (Phase V) by the 215-K, 153-K, and 122-K transitions. The tilting schemes in Phase III, IV and V are mainly characterized by (ψa = ψc, ψb = 0), (ψa ≠ ψc, ψb = 0), and (ψc, ψa = ψb = 0), respectively, where ψa, ψb and ψc are rotational angles around a-, b-, and c-primitive axes of the AlO6 octahedron. The refreezes of the condensed R25yand R 25x optical soft modes occurring at about 215 K and 122 K are interpreted by correlation with the cooperative Jahn-Teller distortion (JTD) of the Pr 3+-4f orbitals in the PrO 12 polyhedra. The phase IV is characterized as an intermediate state, in which the ψa tilts are continuously reduced by the JTD and disappear just above the 122-K transition. The easy mobility of the domain walls suggests the PrAlO 3 crystal to be of antiferroelastic nature, an unusual property that results from the cooperative displacements of O ions induced by the correlation between the antiphase tilts along primitive axes of the AlO 6 octahedra and the JTD around Pr +3 ions in PrO 12 polyhedra. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)