X-ray diffraction study on the phase transition of Rb2CdI4

The structural phase transition of Rb2CdI4 single crystal is investigated b y means of X-ray diffraction. The crystal structure of the room temperature phase (phase I alpha) is decided to be the Sr2GeS4-type structure (space g roup P2(1)/m, Z=2) by structure analysis. In this phase, tetrahedral anion CdI42- and two crystallographically independent Rb+ ions occupy two positio ns related by mirror reflection normal to the b-axis with equal probability . It is confirmed that the transition from paraelectric phase (phase I alph a) to ferroelectric one (phase II alpha) takes place at 209.7 K. The crysta l structure of phase II alpha (space group P2(1), Z=2) is also decided. In phase II alpha, the positions of CdI42- tetrahedron and two independent Rb ions almost coincide with one of two disordered sites in phase I alpha. In Rb2CdI4, the alpha-beta transition which indicates the transformation from the Sr2GeS4-type structure (monoclinic system) to the beta-K2SO4-type one (orthorhombic system) is not observed up to the melting temperature. We dis cuss the alpha-beta transition in A(2)BX(4)-type compounds with the Sr2GeS4 -type structure on the basis of the ratio the ionic radius of A(+) to the a verage bond length B-X in the tetrahedral BX42-.