Fragile 3D Order in V_{1-x}Mo_{x}O_{2}.

The metal-to-insulator transition in rutile ${\mathrm{VO}}_{2}$ has proven uniquely difficult to characterize because of the complex interplay between electron correlations and atomic structure. Here, we report the discovery of the sudden collapse of three-dimensional order in the low-temperature phase of ${\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{O}}_{2}$ 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 suggest that this two dimensionality results from a novel form of geometric frustration that is essentially structural in origin.