[en] The paper is divided into six sections. The introduction consists of two short subsections. Section 1.1 is intended for readers not versed in radiospectroscopy. The basic principles of the NQR in modulated systems are treated in section 1.2. Section 2 is the essential one, it includes a short historical review, experimental data and calculations of NQR spectra in incommensurate phases of Rb₂ZnCl₄ and Rb₂ZnBr₄. Some suggestions about further applications of NQR in the study of modulated systems are made in section 3. (Auth.)

[en] A research of commensurate phase-incommensurate-commensurate (P-I-F) phase transitions in the Rb₂ZnCl₄ ferroelectric is carried out by the nuclear quadrupole resonance method. The number of ³⁵Cl NQR lines and correlation of their intensities in the paraelectric P and ferroelectric F commensurate phases corresponds to the Pmcn(P) and P2sub(1) cn(F) space groups with the trebling of the elementary unit volume. An unusual behaviour of widths and intensities of NQR lines is observed in the incommensurate I phase. This phenomenon is caused by the appearance of non-uniform shifts above Tc. From the widths of NQR lines and temperature behaviour of frequencies clear information about the character of space modulation of the incommensurate structure in the regions, adjoining points of phase transitions can be obtained

[en] From neutron-diffraction data the authors have investigated the atomic structure of the para phase of the crystals of NH₄HSeO₄ at 293 and 400 K. They locate the H atoms and analyze the character of the hydrogen bonds. Comparison of the geometry of the hydrogen bonds and the thermal vibration parameters of the atoms led to the conclusion that the model of dynamic disordering of an H atom at one of the H-bonds is preferable. The nonequivalence of the H bonds is reflected in the nonequivalence of the [SeO₄] tetrahedra

[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] The first results of studying crystal phase transitions in Cs₂CdI₄ by the ¹²⁷I NQR method are presented. NQR frequencies as a function of temperature in the range 77 to 450 K show the existence of three phase transitions. An analysis of the number of spectral lines and their intensities gave the sequence of space groups of different phases. A moisture stimulated polymorphic transition was observed

[en] ¹²⁷I NMR, X-ray diffraction and dielectric measurements were conducted in Cs₂ZnI₄ crystal. Phase transition sequence is presented

No abstract available

[en] By means of ¹³³Cs nuclear magnetic resonance one studied two samples of A₃B₂X₉ family, that was, Cs₃Bi₂I₉ and Cs₃Sb₂I₉. The latter crystal was furthermore studied by means of NQR. One tried to synthesize Cs₃Sb₂I₉ crystal hexagonal modification. It was determined that in the course of phase transitions only one of cesium atom types experienced essential changes

[en] Direct spin-lattice relaxation. causted by local thermal fluctuations inthe atomic displacement wave amplitude in the incommensurate crystal phase can effect the resonance spectrum line shape by reducing the intensity of edge temperature-dependent peaks of frequency distribution and almost not affecting temperature-independent peculiarities of this distribution. That is evidently thereason of anomalous behjaviour of the spectrum of ¹²⁷I NQR in Cs₂ZnI₄ in a certain temperature range below the 'normal'-to-incommensurate phase transition

[en] In Rb₂ZnCl₄ crystals the curly polar phase, which changes into the incommensurable phase below 192 K, has been investigated by pulsed NMR. The angular correlations of the second-order quadrupole shifts have been measured and the corresponding tensors of the electric field gradient have been calculated. The six Rb layers change differently in magnitude and orientation compared to the paraelectric phase. The temperature dependences within the C-phase are also different. The spin-lattice relaxation times have been measured and discussed for each layer in the C-phase. All relaxation times show an anomaly at a further phase transition occurring at 72 K connected with an increase of the number of spectral lines