Charge Order and Negative Thermal Expansion in V2OPO4

The semivalent oxyphosphate V 2 OPO 4 is found to have long-range V 2+ /V 3+ 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 V 2+ spin up and V 3+ spin down states is observed below a magnetic ordering transition at 164 K, and susceptibility measurements evidence local spin pairing correlations to higher temperatures.