Isosymmetrical phase transition in α-YbV4O8

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 alpha and beta are known and differ mainly in the arrangement of the Yb ions within the framework. We studied the structure and magnetic properties of alpha-YbV4O8 as a function of temperature. At approximately 70 K alpha-YbV4O8 undergoes a first-order isosymmetrical phase transition (P2(1)/n -> P2(1)/n). While in the high-temperature alpha phase the three V3+ and one V4+ are disordered over the four symmetrically independent octahedral sites, in the low-temperature alpha' 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 alpha polymorph is very similar to that observed earlier in the beta polymorph at 185 K.