| Popis: |
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 R3c (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 y and R 25 x 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 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. |
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