Pressure Effect on the Anomal Behaviour of Physical Properties of NaNH4SO4 • 2H2O Type Crystals

All experimentally observed anomalies in dielectric susceptibility, heat capacity, lattice dynamics behaviour for the isomorphic crystals SASD and SASeD were consistently interpreted upon modified-Mitsui model taken into account the dielectric and EPR results of subjected systems. We have found that the inclusion of short and long range correlations which takes place for two inequivalent active groups SO4 (I) (SO4 (II) and SeO4 (I) (SeO4 (II) modifies temperature behaviour of the spin-Hamiltonian D parameter and gives much better explanation of experimental results. Theoretical considerations were based on two-time Green functions method and the modified Mitsui model. The spectrum of bonded quasispin-phonon vibration has been analysed.