Melt nonstoichiometry and defect structure of ZnGeP<Subscript>2</Subscript> crystals

The defect structure of ZnGeP₂ crystals grown from a melt by the vertical Bridgman method has been investigated. A deviation of the melt composition from stoichiometric leads to the formation of striations and the inclusions of other phases which are observed as structures (chains) oriented parallel to the growth axis. According to the microanalysis data, the inclusion composition corresponds to a mixture of ZnGeP₂, Zn₃P₂, and Ge. Nanoinclusions of germanium phosphide are detected by transmission electron microscopy. X-ray topography reveals defects of four types. The main defects in the central part of an ingot are related to the composition fluctuations, and the newly formed dislocations are basically single ones. Most dislocations are formed at the crystal periphery.     

Oxygen-conducting crystals of La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript>: Growth and main properties

Single crystals of the anionic conductor La₂Mo₂O₉ are grown by crystallization from a nonstoichiometric melt. Their polymorphism and domain structure, as well as the temperature dependences of conductivity and dielectric permittivity, are studied. In the temperature range 750–600°C, the conductivity of these crystals is as high as 10^?1–10^?2 Ω^?1 cm^?1.     

Growth of filamentary KDP crystals from solution with the addition of Al(NO<Subscript>3</Subscript>)<Subscript>3</Subscript> · 9H<Subscript>2</Subscript>O

To elucidate the mechanism of nucleation and growth of filamentary crystals on the bipyramid faces of KH₂PO₄ (KDP) crystals from solution with the addition of Al(NO₃)₃ · 9H₂O, the growth rates and transverse dimensions of the crystals were measured at various supersaturations, temperatures, and impurity concentrations. The dependences obtained can be interpreted with due regard for the competition between the intrinsic and impurity particles in the presence of Cabrera-Vermilyea stoppers.     

Growth of α-NiSO<Subscript>4</Subscript> · 6H<Subscript>2</Subscript>O crystals at high rates

The high-rate growth of nickel sulfate hexahydrate NiSO₄ · 6H₂O (α-NSH) crystals up to 120 × 120 × 65 mm³ in size is described for the first time. The data on the distribution of related impurities in the {011} and {001} growth sectors of α-NSH crystals grown at different rates are reported. The transmission spectra of both growth sectors of these crystals are obtained. The structural quality and the optical properties of rapidly and slowly grown α-NSH crystals are compared. It is established that the {011} growth sector of crystals grown at rates exceeding 5 mm/day shows the best characteristics for application in UV filters.     

Dilatometric and ultrasound study of [NH<Subscript>2</Subscript>(CH<Subscript>3</Subscript>)<Subscript>2</Subscript>]MnCl<Subscript>3</Subscript> · 2H<Subscript>2</Subscript>O crystals

The unit-cell parameters of [NH₂(CH₃)₂]MnCl₃ · 2H₂O crystals are determined by X-ray diffraction analysis and the velocities of longitudinal ultrasonic waves in these crystals are measured by the echo-pulse method in the temperature range 100–315 K. The coefficients of thermal expansion along the principal crystallographic axes are derived from the temperature dependences of the unit-cell parameters. The temperature dependences of the characteristics studied reveal kink anomalies at temperatures of ～125, 179, 203, 260, and 303 K. These anomalies are indicative of structural transformations in the [NH₂(CH₃)₂]MnCl₃ · 2H₂O crystals, which may be related to the dynamics of dimethylammonium cations.     

Electrical conductivity of Cs<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> crystals

The electrical conductivity of Cs₂CuCl₄ single crystals, synthesized by crystallization from aqueous solutions in the CsCl–CuCl₂–H₂O system, has been investigated. The temperature dependence of the electrical conductivity of crystals in a temperature range of 338–584 K exhibits no anomalies. The electrical transfer activation enthalpy is ΔH_σ = 0.72 ± 0.05 eV and the conductivity is σ = 3 × 10^–4 S/cm at 584 K. The most likely carriers in Cs₂CuCl₄ are Cs⁺ cations, which transfer electric charge according to the vacancy mechanism.     

Low-temperature X-ray diffraction studies of [(CH<Subscript>3</Subscript>)<Subscript>2</Subscript>NH<Subscript>2</Subscript>]<Subscript>5</Subscript>Cd<Subscript>3</Subscript>Cl<Subscript>11</Subscript> crystals

[(CH₃)₂NH₂]₅Cd₃Cl₁₁ crystals are grown by the method of isothermal evaporation from saturated aqueous solutions containing dimethylamine and cadmium chlorides, [(CH₃)2NH₂]Cl and CdCl_2.5H₂O. The crystal grown are studied by the X-ray diffraction method. It is established that the crystals are orthorhombic with the unit-cell parameters at room temperature a = 18.115 ± 0.004 Å, b = 11.432 ± 0.002 Å, and c = 15.821 ± 0.003 Å. The unit-cell parameters a, b, and c of the [(CH₃)₂NH₂]₅Cd₃Cl₁₁ crystals are measured as functions of temperature in the temperature range 100–320 K. The data obtained were used to determine the thermal expansion coefficients along the main crystallographic axes. The temperature curves of the unit-cell parameters and thermal expansion coefficients showed pronounced anomalies in the vicinity of the temperatures T ₁ = 120, T ₂ = 150, and T ₃ = 180 K corresponding to the phase transitions in the [(CH₃)₂NH₂]₅Cd₃Cl₁₁ crystals. The crystals are also characterized by a pronounced anisotropy of thermal expansion.     

Growth and structure of La<Subscript>3</Subscript>Zr<Subscript>0.5</Subscript>Ga<Subscript>5</Subscript>Si<Subscript>0.5</Subscript>O<Subscript>14</Subscript> crystals

The complete X-ray structure determination of Czochralski grown La₃Zr_0.5Ga₅Si_0.5O₁₄ single crystals with the Ca₃Ga₂Ge₄O₁₄ structure is performed (sp. gr. P321, a = 8.226(1) Å, c = 5.1374(6) Å, Z = 1, Mo K_α1 radiation, 1920 crystallographically independent reflections, R = 0.0166, R_w = 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.     

Growth and properties of ZnMoO<Subscript>4</Subscript> single crystals

The crystallization conditions for obtaining zinc molybdate single crystals by the Czochralski and Kyropoulos methods from a melt have been studied. The growth parameters of large single crystals of optical quality are determined. The physicochemical and spectral-luminescent characteristics of the single crystals are studied.     

The domain structure of Cs<Subscript>3</Subscript>H(SeO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals

The characteristics of the changes in the domain structure of Cs₃H(SeO₄)₂ crystals have been studied in a wide temperature range. An anomalous change in the domain structure in the temperature range near the superprotonic phase transition was found. The anomalies in this temperature range were also found by differential scanning calorimetry. The characteristics of the changes in the domain structure in different phases are compared.     

Crystal structure of La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals doped with bismuth

Precision X-ray diffraction studies of La_{2 − x} Bi _x Mo₂O₉ (x = 0.04, 0.06, and 0.18) single crystals are performed. It is found that in the compounds doped with bismuth, analogously with the structure of the metastable β_ms phase of pure La₂Mo₂O₉ (LM), the La, Mo1, and O1 atoms deviate from the threefold axis on which they are located in the high-temperature β phase. It is shown that bismuth atoms substitute for part of lanthanum atoms and occupy a position at the threefold axis in the neighborhood of the split lanthanum position. The implantation of bismuth atoms in the LM structure results in the return of a part of the molybdenum atoms to the position at the threefold axis. The occupancy of this position is equal to the occupancy of the bismuth atomic position.     

Structure and superconductivity of layered oxycarbonate Bi<Subscript>2</Subscript>Sr<Subscript>4</Subscript>Cu<Subscript>2</Subscript>CO<Subscript>3</Subscript>O<Subscript>8</Subscript> crystals

A complex study of the elemental composition, structure, and superconducting properties of single crystals of layered oxycarbonate Bi₂Sr₄Cu₂CO₃O₈ has been performed taking into consideration the growth and postgrowth annealing conditions.     

Structure of Cs<Subscript>4</Subscript>(HSO<Subscript>4</Subscript>)<Subscript>3</Subscript>(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>) single crystals

Single crystals of Cs₄(HSO₄)₃(H₂PO₄) are synthesized and studied for the first time. The new compound is found in the course of studies of the phase diagram of the CsHSO₄–CsH₂PO₄–H₂O triple system. Data on the atomic crystal structure of single-crystalline and powder specimens, as well as on structural phase transitions, are obtained.     

Characteristic of spontaneous polarization in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The temperature dependence of spontaneous polarization in Li₂Ge₇O₁₅, a ferroelectric with one polarization axis, is studied in the temperature range from 4.2 K to T_C. An unusual character of spontaneous polarization associated with the change of the crystal behavior from the Ising to multipole type is revealed in the vicinity of T_C. The low value of spontaneous polarization is explained not by the relation between certain physical quantities, but rather by the interaction of the components of higher order (n > 2) electric moments which, by definition, have rather low values. The Li₂Ge₇O₁₅ structure is built by macrofragments consisting of two types of mesotetrahedra whose distortion results in the appearance of pseudopolarization of the nonvector type.     

Piezo-optic properties of incommensurately modulated K<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystals

The effect that uniaxial pressures along the principal crystallophysical directions have on the dispersion and temperature dependences of the birefringence in incommensurate K₂ZnCl₄ crystals has been investigated in a wide temperature range. This parameter has been established to be fairly sensitive to uniaxial pressures. The spectral and temperature dependences of combined piezo-optic coefficients are analyzed. Significant anomalies of these coefficients are found at the incommensurate-commensurate phase transition. They are due to the change in the induced birefringence that results from spontaneous polarization and to the significant effect of uniaxial stress on the soliton structure of the crystal. The contributions that the electrooptic effect, the elasto-optic effect, and the order parameter make to the spontaneous changes in the piezooptic coefficients are separated.     

Growth,structure, and magnetic properties of CuFeTe<Subscript>2</Subscript> single crystals

CuFeTe₂ single crystals were grown and the temperature dependence of their magnetic susceptibility in the temperature range 1.8–400 K was investigated. It is found that the magnetic susceptibility shows anomalies at temperatures T _s = 65 and T _N = 125 K. At T > 125 K, the crystal is in the paramagnetic state controlled by Fe²⁺ and Cu²⁺ ions with an effective magnetic moment of 1.44 μB.     

Analysis of chemical bonding in Zn<Subscript>3</Subscript>P<Subscript>2</Subscript> crystals from X-ray diffraction data

The refined data on the crystal structure of Zn₃P₂ (I) provide evidence for a tetrahedral coordination of metal atoms, which is characteristic of covalent structures with sp³-hybridized orbitals. Deformation-electron-density maps for crystal I, which were constructed from high-precision X-ray diffraction data, confirm the predominantly covalent character of bonds in this compound. The deformation-density maps clearly show covalent bridges with their peaks shifted toward electronegative phosphorus atoms. The longer the bond, the larger the shift. The presence of delocalized electrons indicates that the bonding mechanism is partially metallic.     

Dispersion of the polarization processes in Ca<Subscript>0.32</Subscript>Ba<Subscript>0.68</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> crystals

The switching processes in calcium barium niobate crystals CaxBa_1–x Nb₂O₆ with x = 0.32 have been investigated. The dielectric hysteresis loops observed in the samples exposed to alternating fields of different frequencies and constant amplitude have been analyzed. It is shown that the loop formation is caused by the sample heating. The sample temperature increases due to the dielectric loss energy release only up to a certain frequency, above which complete switching becomes impossible.     

Exchange of sodium and silver ions in Na<Subscript>3</Subscript>Sc<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

Sodium-and silver-ion exchange in single crystals of two polymorphous modifications of the Na₃Sc₂(PO₄)₃ compound has been studied. It is established that in the process of ion exchange, the samples undergo phase transitions similar to the well-known temperature transformations observed in these systems. It is shown that the phases with ferroelectric, ionic, and superionic properties may simultaneously coexist in one sample.     

Growth of Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> and Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> crystals by the low-thermal gradient Czochralski technique

Bi₁₂SiO₂₀ crystals have been grown for the first time by the low-thermal gradient Czochralski technique in the 〈111〉 and 〈110〉 directions. The conditions for reproducible crystal growth with a high-quality polyhedral faceted front are found. The systematic features of shaping Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ crystals, grown by the low-thermal gradient Czochralski technique, are compared. The defect formation in these crystals is studied and their optical homogeneity is analyzed by interferometry.