Isosymmetrical Phase Transition and Charge Ordering in the Mixed Valence Vanadate β-YbV₄O₈

The mixed valence vanadate β-YbV₄O₈ crystallizes in a structure related to CaFe₂O₄. The structure is built of a three-dimensional framework of VO₆ octahedra in which the Yb ions are incorporated. At 185 K the compound undergoes an isosymmetrical phase transition which is accompanied by discontinuous changes in the lattice parameter, as well as in the interatomic distances and angles. While above the phase transition the bond valence sums indicate no clear charge separation of the tri- and tetravalent V ions over the four symmetrically independent octahedral sites, below the transition temperature a complete charge ordering is observed. The structural transition is accompanied by an anomaly in the magnetic susceptibility data, which corresponds to a paramagnetic-paramagnetic transition. This anomaly can be understood assuming that the Curie-Weiss type d electrons of the vanadium cations separate at 185 K into d electrons, which maintain their Curie-Weiss character, and others, which lose their spin moment with decreasing temperature. Thus, the unusual complete charge ordering can be attributed not only to Coulomb repulsion between the V and Yb cations but also to the spin annihilation of the vanadium cations.