Charge Order and Negative Thermal Expansion in V2OPO4.

The semivalent oxyphosphate V2OPO4 is found to have long-range V2+/V3+ charge ordering up to 605 K where a monoclinic to tetragonal structural transition and a switch from positive to negative thermal expansion are observed. V-V bonding within orbital polymer chains is proposed as the key factor in the novel switch of thermal expansion behavior, as loss of V-V bonding enables transverse oxygen motions to dominate the thermal expansion at high temperatures. Ferrimagnetic order of V2+ spin up and V3+ spin down states is observed below a magnetic ordering transition at 164 K, and susceptibility measurements evidence local spin pairing correlations to higher temperatures.