EPR and interpretation of thermodynamic behaviour of SASD-type of crystals

Based on EPR and analysis of modified Mitsui model in a wide region of temperature, the description of experimentally observed anomalies for some thermodynamic functions behaviour and an interpretation of differences in phase transition order for the NaNH4SO4·2H2O (SASD) and NaNH4SeO4·2H2O (SASeD) crystals is presented. These anomalies and the shift of the phase transition order are discussed from the point of view of competition of short- and long-range interactions between active SO4 and SeO4 groups.