A Structural Study of the Successive Transitions in LiRbSO₄

Abstract

The crystal structures of LiRbSO₄ are analyzed in the disordered phase (250, 220 and 210°C), the antiferroelectric phase (190°C), the ferroelectric phase (170°C) and the paraelectric phase (160 and −100°C) from X-ray diffraction data. In the disordered phase, it is concluded that the SO₄ tetrahedron is in disorder by using of the split atom method as well as the difference Fourier maps. Following the incommensurate-commensurate transition, the tetrahedra occupy one of two configurations alternately. The modulation period 2c₀ at 190°C is elongated to 5c₀ in the ferroelectric phase, where c₀ is the basic cell dimension in the disordered phase. The structural transformation is characterized by the change of the occupation pattern of the SO₄ tetrahedron correlated to the displacement of its center of mass and positions of Li and Rb atoms.