Neutron diffraction study of the order-disorder transition in AgCrS 2

Abstract

In AgCrS 2 the sites available for silver form a honeycomb lattice consisting of two interpenetrating sublattices. At low temperature silver occupies only one of the two sublattices. At high temperature both sublattices are occupied, the transition being of second order nature (T c = 673 K). The transition was studied by neutron powder diffraction in the temperature range 300 – 693 K. It was found that the experimental curve of the long range order parameter L versus T/T c lies below the theoretical curve (exact solution in the Ising formalism). This might indicate that the interaction energy of silver ions on neighbouring sites increases with L; another possibility is that the forces are long range. It was also found that silver is displaced from the ideal positions; refinements were carried out with (partial) atoms displaced from the ideal sites. The distances Δr increase with temperature; at 300 K Δr= 0.28A˚, above T c Δr= 0.6A˚.