Isosymmetrical phase transition in α-YbV4O8

Abstract

The structure of YbV4O8 is related to the CaFe2O4 structure type. VO6 octahedra form a three-dimensional framework with tunnels in which the Yb3+ ions are incorporated. Two different polymorphs α and β are known and differ mainly in the arrangement of the Yb ions within the framework. We studied the structure and magnetic properties of α-YbV4O8 as a function of temperature. At approximately 70 K α-YbV4O8 undergoes a first-order isosymmetrical phase transition (P21/n → P21/n). While in the high-temperature α phase the three V3+ and one V4+ are disordered over the four symmetrically independent octahedral sites, in the low-temperature α′ phase complete charge ordering is observed. The transition is accompanied by a paramagnetic–paramagnetic anomaly in the magnetic susceptibility data which can be interpreted on the basis of spin-gap formation. The transition mechanism in the α polymorph is very similar to that observed earlier in the β polymorph at 185 K.