Study of Frustration Effects in Two-Dimensional Triangular Lattice Antiferromagnets–Neutron Powder Diffraction Study of VX2, X≡Cl, Br and I

In order to study the frustration effects, we have taken powder diffraction patterns of VX 2 , X≡Cl, Br and I) which are thought to be quasi-two-dimensional triangular lattice antiferromagnets. The spins in VCl 2 , VBr 2 and VI 2 were found to order at 36.0, 29.5 and 16.3 K respectively forming a three sublattice structure in the basal plane. Contrary to our expectation, a partially ordered model gives a better fit to the observed diffraction rather than the Néel state with 120° structure though the spins are of Heisenberg symmetry. In VCl 2 and VBr 2 , strong diffuse scattering can be observed not only at T > T N but also at T ≪ T N . No critical scattering characteristic for 2D could be seen but the temperature variation of sublattice magnetization is rather close to that for the Ising system.