The metal-to-insulator transition (MIT) in rutile VO$_2$ has proven uniquely difficult to characterise because of the complex interplay between electron correlations and atomic structure. A number of approaches have been explored since its discovery, relying on some combination of Mott localization, Peierls distortions, superexchange, and orbital ordering to explain the structural instability at the MIT. Here we report the discovery of the sudden collapse of three-dimensional order in the low-temperature phase of V$_{1-x}$Mo$_x$O$_2$ at $x=0.17$ and the emergence of a novel frustrated two-dimensional order at $x=0.19$, with only a slight change in electronic properties. Single crystal diffuse x-ray scattering reveals that this transition from the 3D M1 phase to a 2D variant of the M2 phase results in long-range structural correlations along symmetry-equivalent (11L) planes of the tetragonal rutile structure, yet extremely short-range correlations transverse to these planes. These findings emphasise the fragility of structural order in VO$_2$, the role of geometric frustration, and the need for improved electronic structure models.
Comment: 8 pages, 5 figures