Li₂VO(Si,Ge)O₄, a prototype of a two-dimensional frustrated quantum Heisenberg antiferromagnet

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NMR and magnetization measurements in Li₂VOSiO₄ and Li₂VOGeO₄ are reported. The analysis of the susceptibility shows that both compounds are two-dimensional S=1/2 Heisenberg antiferromagnets on a square lattice with a sizeable frustration induced by the competition between the superexchange couplings J₁ along the sides of the square and J₂ along the diagonal. Li₂VOSiO₄ undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for J₂/J₁ > 0.5. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.