Frustration-driven structural distortion in VOMoO 4

Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), magnetization measurements, and electronic structure calculations in ${\mathrm{VOMoO}}_{4}$ are presented. It is found that ${\mathrm{VOMoO}}_{4}$ is a frustrated two-dimensional antiferromagnet on a square lattice with competing exchange interactions along the side ${(J}_{1})$ and the diagonal ${(J}_{2})$ of the square. From magnetization measurements ${J}_{1}{+J}_{2}$ is estimated around $155\mathrm{K},$ in satisfactory agreement with the values derived from electronic structure calculations. Around $100\mathrm{K}$ a structural distortion, possibly driven by the frustration, is evidenced. This distortion induces significant modifications in the NMR and EPR spectra which are possibly due to valence fluctuations. The analysis of the spectra suggests that the size of the domains where the lattice is distorted progressively grows on cooling as the temperature approaches the transition to the magnetic ground state at ${T}_{c}\ensuremath{\simeq}42\mathrm{K}.$