A Microscopic Theory on the Phase Transitions in NH 4 Br —An Ising Spin Phonon Coupled System—

摘要: CsCl type NH 4 Br crystal has been treated as an Ising spin-phonon coupled system to explain the phase transitions associated with orientational order of + ions. The appearance two ordered phases is explained essentially due fact that direct interaction between ions stabilizes parallel ordering while indirect through phonons antiparallel ordering. Based on a microscopic hamiltonian describing system, several experimental results have analized such (i) anomalous lattice expansions (ii) parameters (iii) spontaneous displacement - (iv) P T diagram (v) X-ray critical diffuse scattering. Semiquantitative agreements are obtained. Especially property correlated fluctuations seems give support validity model used.