Antiferroelastic structural transitions in PrAlO 3 by means of neutron diffraction

Structural phase transitions of the perovskite‐type PrAlO 3 have been studied by using neutron diffraction. The results show that, on cooling, the structures are successively transformed from R $ \bar 3 $ c (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 ( ψ a = ψ c , ψ b = 0), ( ψ a ≠ ψ c , ψ b = 0), and ( ψ c , ψ a = ψ b = 0), respectively, where ψ a , ψ b and ψ c are rotational angles around a ‐, b ‐, and c ‐primitive axes of the AlO 6 octahedron. The refreezes of the condensed R   25   yand R   25   xoptical 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 ψ a 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. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)