Alteration of the stacked 2(pi)/3 magnetic structure in the triangular lattice anti-ferromagnet LiCrS2

LiCrS2 has magnetic Cr ions with spin S=3/2 on a triangular lattice. Other researchers (J. Solid State Chem. 3, 590, 1971) concluded that this compound has an anti-ferromagnetic order with a stacked 2(pi)/3 structure below a transition temperature of the order of TN=50 K. We have re-examined this system with neutron scattering methods on powder samples, paying attention to the temperature dependence of the magnetic scattering. The overall characteristics of the magnetic ordering are in agreement with the earlier study but we have observed significant modifications of the stacked 2(pi)/3 structure well below the critical temperature. The main new features observed on cooling from T=25 K down to T=1.5 K are (i) the gradual decrease from 2.5 (mu)B to 2.3 (mu)B of the ordered moment attributed to the 2(pi)/3 structure, (ii) the appearance of new correlations corresponding to a multiplication of the magnetic unit cell, and (iii) the growth of a diffuse elastic scattering component that can be ascribed to a frozen disorder. These observation are not explained in the framework of the standard theoretical treatments concerning the stacked triangular anti-ferromagnet with Heisenberg or slightly anisotropic interactions.