A Microscopic Theory on the Phase Transitions in NH₄Br —An Ising Spin Phonon Coupled System—

Abstract

CsCl type NH₄Br crystal has been treated as an Ising spin-phonon coupled system to explain the phase transitions associated with the orientational order of NH₄⁺ ions. The appearance of two ordered phases is explained as essentially due to the fact that the direct interaction between NH₄⁺ ions stabilizes parallel ordering while the indirect interaction through phonons stabilizes the antiparallel ordering. Based on a microscopic hamiltonian describing the spin-phonon coupled system, several experimental results have been analized such as (i) anomalous lattice expansions (ii) orientational order parameters (iii) spontaneous displacement of Br⁻ ions (iv) P–T phase diagram (v) X-ray critical diffuse scattering. Semiquantitative agreements are obtained. Especially the property of correlated fluctuations of displacement of Br⁻ ions seems to give a support on the validity of the microscopic model used.