EPR studies were performed on MgSiFs . 6Hz0 single crystals using MnZt paramagnetic probes substituted for Mg2+ ions. The spin-Hamiltonian parameters (SHPS) are determined accurately for the different phases: the monoclinic phase below T, = 301 K, the rhombohedral phase above To = 370 K and the intermediate phase between T, and To. The sets of SHFS are interpreted in the light of sUuchlral changes between these phases. The EPR line features in the intermediate phase are analysed in the framework of a sWic and an orderdisorder model. We interpret the EPR specwm behaviours around To -20 K as the occurrence of a crossover between static and dynamic disorder in the material.

X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr 3þ -doped dimethylammonium aluminium sulphate hexahydrate (DMAAlS) and dimethylammonium gallium sulphate hexahydrate (DMAGaS) are presented from room temperature down to liquid helium temperature. Both crystals undergo an order-disorder phase transition to ferroelectric phase at T C1 ¼ 152 K and T C1 ¼ 134 K, respectively. Additionally, DMAGaS undergoes a first-order transition into a low temperature non-ferroelectric intermediate phase at T C2 ¼ 116 K and into a low temperature antiferroelectric phase at about T C3 ¼ 60 K. The spin Hamiltonian parameters have been determined for all observed in both crystal phases. Remarkable EPR line width changes with observed triplets confirm the order-disorder character of the ferroelectric phase transition and demonstrate freezing-out of dimethyl ammonium reorientations, which is a primary reason for this transition. The doubled number of EPR lines in comparison to the paraelectric and ferroelectric phases in DMAGaS indicates a unit cell doubling in antiferroelectrics phase below 60 K.

X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr3+-doped dimethylammonium aluminium sulphate hexahydrate are presented from 100 K to room temperature. The crystal undergoes a phase transition at 152 K fi'om the ferroelastic to the ferroelectrie phase. The spinHamiltonian parameters have been determined for both phases. The spin-Hamiltonian pararneters in the ferroelectrie phase are: g = 1.980+0.003, b g = (1140+15). 10 -4 cm -t, b 2 = (214+10). 10 -4 cm -t. Remarkable EPR line width ehanges con¡ the order-disorder character of the ferroelectric phase transition on a microscopio level and demonstrate that the dimethylammonium reorientation freezing-out is the p¡ reason for this transition.

In this paper some results concerning an EPR study of Mn doped [Mg(H2 0 ) ISiF single crystal are reported. The temperature dependence of the kPR spectra shows t h a t the crystal undergoes two successive phase transitions. We focussed our attention on t h e intermediate phase where the occurence of the incommensurate phase was reported.

The birefringent and piezooptic properties of (MgSiF,) . 6 H,O crystals are investigated. The secondorder phase transition at = 353 K is observed for the first time. It is shown that the investigated crystals possess properties characteristic of an incommensurate phase in the temperature range 298 to 353 K. The transition temperatures are found to be strongly dependent on the history of the sample. MCCJlenOBaHLI nBynpeJIOMnR€OI4Me II ~L~~O O~T M Y~C K H~ CBOiiCTBa KpHCTannOB (MgSiF,) ' 6 H,O. Bnepmre Ha6nWoAaJIcR @a3OBbla UepeXOn npII = 353 K. nOKa3aH0, YTO MCCJIcAyeMLle KpIICTaJIJIbI 0 6 n a n a m~ CBOfiCTBaMII, XapaKTepHLIMIl AJlR HeCOpa3MepHOfi ($a361 B TeMIIepaTypHOfi 06nac-r~ 298 DO 353 K. 06HapyXeHa CHJILHaR 3aBPiCHMOCTh TeMnepaTyp ($a30BLIX IIepeXOnOB OT IIpeaLlCTOpHH o6pasua.