[en] Deuteron magnetic resonance (DMR) experiments were carried out using highly deuterated NH₄HSeO₄ (AHSe) monocrystals in order to study the peculiarities and parameters of the hydrogen bonds of the orthorhombic structure of AHSe. The orientational dependences of the splittings between the DMR lines of deuterons in the hydrogen bonds were measured at room temperature for three orthogonal rotations about the crystallographic a, b, and c axis. Results show that in highly deuterated AHSe there is one structurally non-equivalent asymmetrical hydrogen bond in which the deuteron is ordered

[en] A new crystal modification of ammonium hydrogen selenate generated after long keeping times in the temperature range 252 to 271 K is studied. It is established that the structure of this modification is identical with those of the orthorhombic modification of highly deuterated AHS single crystals measured earlier. Thermodynamic stability regions of the AHS orthorhombic and monoclinic structures with different deuteration degree are found. It is discovered by direct optical observations that the polymorphic phase transition from the monoclinic phase to the orthorhombic one occurs by means of the embryo formation and new-phase crystal growth in the solid matrix of the initial phase. The conditions necessary for the fast crystal growth are found and discussed. (author)

[en] To study physical properties of CsLiSO₄ temperature dependences of integral intensities of 006, 021 X-ray reflections and monoclinity angle deviation from 90 deg are investigated. Studies of NMR in the temperature range from 150 to 300 K are carried out as well as of Cp heat capacity, thermal expansion, permittivity and hydrostatic pressure effect on phase transition in CsLiSO₄. Presence of structural phase transformation in CsLiSO₄ at 202.07 K is established unambiguously. Absence of concealed heat, character of heat capacity anomaly and structural data lead to the conclusion that the phase transformation of the second type without changing a number of atoms in an elementary cell takes place. Mechanical stresses, applied to the [110] direction, cause reversible changes of the domain structure