Fragile 3D Order in V_(1-x)Mo_xO₂

VO₂ displays a first-order metal-insulator phase transition near 340 K. Accompanying this electronic transition is a structural transition. The connection between these two transitions is unclear, with electronically-driven and structurally-driven models providing conflicting results. Electron doping with molybdenum enhances metallicity and reduces the structural transition temperature; between 17% and 19% Mo, we find that the long-range structural transition is suppressed entirely while the electronic transitions remain similar.
Diffuse x-ray scattering measurements were performed on a single crystal of the V_0.81Mo_0.19O₂. In the low-temperature insulating phase, sharp rods are observed, indicating two-dimensional ordering of atomic displacements. The slight oscillation of these rods about the [110] direction in reciprocal space can be explained by weak, inherently frustrated coupling between the ordered planes. Such fragile embedded order is predicted by an Ising-like ferrodistortive model proposed by Lovorn and Sarker. 3D-ΔPDF analysis of the diffuse scattering and structural simulations provide a clear picture of the local order in this system.