Li2VO(Si,Ge)O4, a Prototype of a Two-Dimensional Frustrated Quantum Heisenberg Antiferromagnet

NMR and magnetization measurements in ${\mathrm{Li}}_2{\mathrm{VOSiO}}_4$ and ${\mathrm{Li}}_2{\mathrm{VOGeO}}_4$ are reported. The analysis of the susceptibility shows that both compounds are two-dimensional $S=1\mathrm{}/2$ Heisenberg antiferromagnets on a square lattice with a sizable frustration induced by the competition between the superexchange couplings $J_1$ along the sides of the square and $J_2$ along the diagonal. ${\mathrm{Li}}_2{\mathrm{VOSiO}}_4$ undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for $J_2/J_1>0.5\mathrm{}$. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.