Electronic origin of negative thermal expansion in vanadium oxyphosphate

We have studied the negative thermal expansion (NTE) behaviour of oxyphosphate V2OPO4 over a wide temperature range using X-ray and neutron diffraction, and have discovered that the NTE arises due to an electronic charge ordering transition from a larger-volume charge-ordered phase to a smaller-volume charge-disordered phase as temperature increases. The highly anisotropic thermal expansion is dominated by the difference in size of V2+ and V3+, with NTE most apparent along the directions of face-sharing octahedral chains. This material is unusual in remaining semi-conducting throughout the charge order transition, in contrast to many other electronically-driven NTE materials.

Crystal structure and lattice parameter variation graph

E. Pachoud, J. Cumby, J.Wright, B. Raguž, R. Glaum and J. P. Attfield, Electronic origin of negative thermal expansion in V2OPO4, Chemical Communications, 56, 2020, 6523.