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

NMR and magnetization measurements in Li2VOSiO4 and LiVOGcO(4) 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 sizable frustration induced by the competition between the superxchange couplings J(1) along the sides of the square and J(2) along the diagonal. Li2VOSiO4 undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for J(2)/J(1) > 0.5. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.