Cooperative tilting scheme of VF6 octahedra related to the structural phase transitions of diluted layer compound Cs0.5Rb0.5VF4

The successive structural phase transitions of the diluted layer compound C s0.5Rb0.5VF4 are Studied by means of polarized light scattering and X-ray d iffraction. It is found that the structural transitions occurring at about 123 and 368 K are mainly characterized by the cooperative tilting of VF6:oc tahedra around [100], [010] and [001] axes in the two-dimensional networks of the octahedral The space group in each phase se is determined as follows : D-2(3)-P2(1)2(1)2 in Phase IV (123K<T less than or equal to 368K), and D- 4h(5)-P4/mbm: in Phase II (368K < T), on heating. The structure is determin ed at room temperature by using a four-circle X-rap diffractometer. The fol lowing tilting scheme of the VF6 octahedra characterizes the structural in Phase III phase transition of Cs0.5Rb0.5VF4; a(p)(+)b(p)(+)c(p)(+) (Phase I V) --> a(p)(+)a(p)(+)c(p)(+) (Phase III) --> a(0)a(0)c(p)(+) (Phase II) --> a(0)b(0)c(0) (Phase I).