Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR),
magnetization measurements and electronic structure calculations in VOMoO4 are
presented. It is found that VOMoO4 is a frustrated two-dimensional
antiferromagnet on a square lattice with competing exchange interactions along
the side J1 and the diagonal J2 of the square. From magnetization measurements
J1+J2 is estimated around 155 K, in satisfactory agreement with the values
derived from electronic structure calculations. Around 100 K a structural
distortion, possibly driven by the frustration, is evidenced. This distortion
induces significant modifications in the NMR and EPR spectra which can be
accounted for by valence fluctuations. The analysis of the spectra suggests
that the size of the domains where the lattice is distorted progressively grows
as the temperature approaches the transition to the magnetic ground state at
Tc=42 K
