Yu. V. Pisarevskii

Crystal structure and optical activity of La3Nb0.5Ga5.5O14 and Sr3Ga2Ge4O14 single crystals of the langasite family

Precision X-ray structural studies of the La3Nb0.5Ga5.5O14 and Sr3Ga2Ge4O14 single crystals were carried out. The space group P321 was confirmed. The anomalous X-ray scattering was taken into account to establish the absolute structures (chirality) of the crystals; they proved to be of different sign. The structural features responsible for the optical activity of crystals were revealed, and the dependence of the magnitude and sign of specific rotation on the structural parameters of these compounds were determined.

Phase transitions in compounds with the Ca3Ga2Ge4O14 structure

Possible structural changes described by the group-subgroup relationships in the Ca3Ga2Ge4O14-type structure (sp. gr. P321) are considered. The most probable phase transitions seem to be those accompanied by lowering of the symmetry to the maximal non-isomorphic subgroups P3 and C2. It is shown that only destructive phase transitions accompanied by symmetry rise up to the minimal non-isomorphic supergroups for the given structure type can take place. The change of the trigonal symmetry to monoclinic is revealed in La3SbZn3Ge2O14, whose crystal structure is refined as a derivative structure of the Ca3Ga2Ge4O14 structure type within the sp. gr. A2 (C2). At ∼250°C, La3SbZn3Ge2O14 undergoes a reversible phase transition accompanied by symmetry rise, A2 ⇄ P321. Similar phase transitions, P321 ⇄ A2, are also observed in La3Nb0.5Ga5.5O14 and La3Ta0.5Ga5.5O14 under the hydrostatic pressures 12.4(3) and 11.7(3) GPa, respectively. The mechanisms of compression and phase transition are based on the anisotropic compressibility of a layer structure. With the attainment of the critical stress level in the structure, the elevated compressibility in the (ab) plane gives rise to a phase transition accompanied by the loss of the threefold axis. Attempts to reveal low-temperature phase transitions in a number of representatives of the langasite family have failed.

X-ray acoustic resonators for controlling the spatial characteristics of X-radiation

The paper presents the results of studying the acoustic characteristics of special X-ray acousto-optical elements designed for precision control of X-ray beam parameters with time resolution. X-ray measurements revealed a number of specific features of oscillations of X-ray acoustic elements not exhibited in standard electromechanical measurements. The existence of these features is confirmed by numerical calculations computations of the acoustic fields in this type of elements.

Possibilities of Controlling an X-ray Beam with a Crystal Subjected to Long-Wave Ultrasonic Vibrations

X-ray diffraction is experimentally studied in the Laue geometry in a germanium crystal carrying a long-wave ultrasonic wave that creates an alternating lattice deformation along the sample surface. Stroboscopic equipment is used to separate different phases and, correspondingly, different profiles of a spatial deformation distribution from the periodic deformation. A uniform deformation is shown to change the angular position of the X-ray beam, and a nonuniform deformation broadens the angular region of reflection and decreases the peak intensity. Ultrasound can be used to compensate for the static deformation at the place where the single-crystal sample and the resonator are glued together. Apart from the fundamental long-wave harmonic, the crystal contains a parasitic deformation with a shorter wavelength. A simple theoretical model is developed, and it rather accurately describes the experimental results.

Elastic, piezoelectric, and dielectric properties of α-GeO2 single crystals

The elastic, piezoelectric, and dielectric characteristics of α-GeO2 crystals are studied at room temperature. The measurements were performed by the resonance method on crystals grown on α-quartz seeds from solutions in the recirculating mode. In germanium dioxide crystals, the piezoelectric moduli have high values. The characteristic features of variations of the electromechanical and elastic properties in crystals with the quartzlike structure observed earlier are confirmed.

New method for measuring rocking curves in X-ray diffractometry by ultrasonic modulation of the lattice parameter

A method for measuring double-crystal rocking curves with the aid of an X-ray-acoustic analyzer is proposed and implemented. This method makes it possible to perform precise measurements with a high time resolution and without the mechanical rotation of the crystal. The results of testing the method are presented.

In situ study of the growth and degradation processes in tetragonal lysozyme crystals on a silicon substrate by high-resolution X-ray diffractometry

The results of an in situ study of the growth of tetragonal lysozyme crystals by high-resolution X-ray diffractometry are considered. The crystals are grown by the sitting-drop method on crystalline silicon substrates of different types: both on smooth substrates and substrates with artificial surface-relief structures using graphoepitaxy. The crystals are grown in a special hermetically closed crystallization cell, which enables one to obtain images with an optical microscope and perform in situ X-ray diffraction studies in the course of crystal growth. Measurements for lysozyme crystals were carried out in different stages of the crystallization process, including crystal nucleation and growth, developed crystals, the degradation of the crystal structure, and complete destruction.

An electromechanical x-ray optical element based on a hysteresis-free monolithic bimorph crystal

A hysteresis-free electrically controlled X-ray optical element based on a monolithic bi-domain bimorphic piezoelectric actuator that is made of lithium niobate crystal was proposed and successfully tested in practice. This element is used for electronically controlled adjustment of the angular position of an X-ray optical monochromator in a range of up to 200″ and is characterized by a high relative linearity (up to 98%), repeatability (the repeatability error is no more than 2%), and low control voltages (up to 100 V). The hysteresis- free behavior of the dependence of the angular position of the element on the control element, which demonstrates the high efficiency of the hysteresis-free monolithic bimorphic piezoactuators as controlled elements of X-ray optics, is shown.

Certain Features of the Effect of Diffraction on the Acoustooptic Interaction in an Elastically Anisotropic Medium

With the use of the distribution of the ultrasonic-beam intensity calculated with allowance for elastic anisotropy, it is shown that the transverse divergence of acoustic waves affects the diffraction efficiency of an acoustooptic cell. The calculations are in agreement with measurements of the acoustic-field distribution in the acoustic lines of crystalline- and fused-quartz acoustooptic devices. The distribution is measured in the plane perpendicular to the diffraction plane.

Growth and structure of La3Zr0.5Ga5Si0.5O14 crystals

The complete X-ray structure determination of Czochralski grown La3Zr0.5Ga5Si0.5O14 single crystals with the Ca3Ga2Ge4O14 structure is performed (sp. gr. P321, a = 8.226(1) Å, c = 5.1374(6) Å, Z = 1, Mo Kα1 radiation, 1920 crystallographically independent reflections, R = 0.0166, Rw = 0.0192). The absolute structure is determined. It is shown that possible transition of some of La atoms (∼1.2%) from the 3e to 6g position may give rise to the formation of structural defects.