Geometric Frustration Suppresses Long-Range Structural Distortions in NbxV1-xO2

Though NbO2 and VO2 both exhibit similar electrostructural phase transitions, alloying the two completely suppresses ordering of either kind. It is mostly accepted that impurity ions act as electron-localizing defects, which weaken ordering in a percolative manner. This work reports total X-ray scattering measurements across the NbxV1-xO2 phase diagram that challenge this prevailing notion. The observations are instead more consistent with the geometric frustration of displacement models recently used to explain the suppression of long-range structural order in the V1-xMoxO2 phase diagram. Two separate short-range ordered phases are observed in the NbxV1-xO2 phase diagram, one of which has not been observed before. The structure of this new phase was not determined in the present study, and it is provisionally named 2D-u to indicate that the phase is likely two-dimensional and that it is unknown whether it is a new or existing structure. The other phase, stabilized around 10% Nb substitution, is shown to be structurally equivalent to the 2D-M2 phase that appears at or above 19% Mo substitution, except that it coincides with the insulator side of a metal-to-insulator transition (MIT), while the Mo analog does not. This shows that the MIT intrinsic to VO2 can persist into the quasi-two-dimensionally ordered state.