An analysis of the phenomenon of thermostimulated long-term memory of LiNbO<Subscript>3</Subscript> crystals

The way that the difference in the refractive indices of lithium niobate crystals changes upon the rapid heating of a sample with electroconductive masks deposited on its surface has been studied. It is shown that the phenomena observed are fully described by the pyroelectric effect in the presence of temperature gradient on polar surfaces of the sample.     

Characteristic of spontaneous polarization in Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> crystals

The temperature behavior of the spontaneous polarization of lead tetragermanate, a uniaxial ferroelectric, is studied in the range from 4.2 to 300 K. The results obtained along with the data from the literature make it possible to reconstruct a complete pattern of the behavior of P _s (T) both in the vicinity of the phase transition and at lower temperatures. In the range from 290 K to T_C, the crystal behavior is found to change from the dipole type (β = 1/2) to the pseudoquadrupole type (β = 1/4). This specific crossover manifests itself in the change in the behavior of P _s ^1/β as a function of (T_C-T). In the low-temperature range, weak anomalies in the dependenceP _s >(T) are found, which point to the occurrence of contributions from the dipole moments of separate structural fragments of Ge₂O₇ and GeO₄, which have internal degrees of freedom and are weakly bound to the dynamics of the crystal lattice.     

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.     

Central peak in the raman spectra of LiNbO<Subscript>3</Subscript> crystals far below the phase transition

A central peak has been revealed in the Raman spectrum of ferroelectric crystals of lithium niobate LiNbO₃ in the temperature range 80–300 K. It is shown that the extrapolation of the central peak, which manifests itself in inelastic-light-scattering spectra near the ferroelectric transition (T_c ～ 1400 K), cannot account for the existence of the central peak so far from the phase transition.     

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.     

Crystal Growth and Structures of New Niobium and Tantalum Oxides: Sr<Subscript>3</Subscript>LiNbO<Subscript>6</Subscript> and Sr<Subscript>3</Subscript>LiTaO<Subscript>6</Subscript>

Abstract  

Single crystals of two new niobium and tantalum oxides, Sr₃LiNbO₆ and Sr₃LiTaO₆ were grown out of a Sr(OH)₂/LiOH·H₂O/KOH flux and characterized by single-crystal X-ray diffraction. The materials crystallize in the trigonal R-3c space group with a = 9.8029(14) ? (9.8111(11) ?), b = 9.8029(14) ? (9.8111(11) ?), c = 11.200(2) ? (11.2056(12) ?), α = β = 90°, γ = 120° for Sr₃LiNbO₆ (Sr₃LiTaO₆). The oxides exhibit the K₄CdCl₆ structure-type, and consist of alternating face-shared BO₆ octahedra (B = Nb⁵⁺ or Ta⁵⁺) and LiO₆ trigonal prisms.     

Determination of the parameters of vector OOE phase matching in LiNbO<Subscript>3</Subscript>

Regularities in changes in the dimension of the ring of the second-harmonic radiation upon the rotation of a LiNbO₃ sample have been studied experimentally and confirmed by calculations for the case of vector ooe phase matching. It is shown that the angle δ of the external cone of the second-harmonic radiation in the case of vector ooe phase matching can be determined from the change in the ring dimension upon the rotation of an arbitrarily oriented crystal, provided that second-harmonic radiation is observed.     

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.     

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.     

Curved-face growth of NaNO<Subscript>3</Subscript> crystals

Curved-face growth of NaNO₃ crystals under the action of the (NH₄)₂Mo₂O₇ impurity is studied experimentally and possible causes of face bending are analyzed. Selective impurity adsorption on the basal pinacoid results in growth of platelike crystals with spherically bent faces. Growth-rate measurements show that the impurity decelerates pinacoid-face growth by the Bliznakov kink-poisoning mechanism. Face bending can be explained by gradual deceleration of step growth by the impurities accumulated before step fronts. A model of step deceleration is suggested that is based on the assumption of slow impurity adsorption resulting in the instability of a step train. The model allows one to qualitatively interpret the experimental data.     

Crystal structure and microtwinning of monoclinic La<Subscript>3</Subscript>SbZn<Subscript>3</Subscript>Ge<Subscript>2</Subscript>O<Subscript>14</Subscript> crystals of the langasite family

The crystal structure of monoclinic La₃SbZn₃Ge₂O₁₄ crystals from the langasite family is determined by X-ray diffraction analysis [a = 5.202(1) Å, b = 8.312(1) Å, c = 14.394(2) Å, β = 90.02(1)°, sp. gr. A2, Z = 2, and R/R _w = (5.2/4.6)%]. The structure is a derivative of the Ca₃Ga₂Ge₄O₁₄-type structure (a = 8.069 Å, c = 4.967 Å, sp. gr. P321, Z = 1). The crystal studied is a polysynthetic twin with the twin index n = 2, whose monoclinic components are related by pseudomerohedry by a threefold rotation axis of the supergroup P321.     

Reflection spectra of NaClO<Subscript>3</Subscript>, NaBrO<Subscript>3</Subscript>, and LiIO<Subscript>3</Subscript> gyrotropic crystals in the vacuum UV region

The reflection spectra in the fundamental-absorption region, 5–25 eV (250–40 nm), of optically active crystals with cubic symmetry (NaClO₃, NaBrO₃) and uniaxial optically active crystal (LiIO₃) have been investigated. It is shown that the reflection spectra of cubic crystals have a similar structure, which is determined by the electronic transitions in the XO₃ group. The comparison of these spectra with the corresponding spectrum of lithium iodide made it possible to determine the type of transition in the spectra of cubic crystals. Using the projection operator method, it was shown that the sign of optical rotation of cubic crystals with symmetry T is independent of the screw axis sign. Possible reasons for the unprecedentedy large optical rotation of lithium iodide crystal in the optical axis direction are considered.     

Absolute structure of La<Subscript>3</Subscript>Ga<Subscript>5</Subscript>SiO<Subscript>14</Subscript> langasite crystals

The absolute structure of La₃Ga₅SiO₁₄ piezoelectric crystals (a = 8.1746(6) Å, c = 5.1022(4) Å, space group P321, Z = 1) with the positive sense of rotation of the plane of polarization is refined using X-ray diffraction analysis (R = 1.37%, R _w = 1.71%, 2413 unique reflections, max sinθ/λ = 1.15 Å^?1). The contributions from the anharmonicity of thermal vibrations of lanthanum atoms are calculated with the use of the components of the third-and fourth-rank tensors. It is demonstrated that these contributions can have a significant effect.     

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.     

Ionic conductivity of ScF<Subscript>3</Subscript> single crystals (ReO<Subscript>3</Subscript> type)

Electrical conductivity σ of ScF₃ single crystals (sp. gr. \(Pm\overline 3 m\), ReO₃ structure type) has been studied by impedance spectroscopy and compared with the electrical conductivity of rare earth HoF₃ (β-YF₃ type) and LaF₃ (tysonite type) trifluorides. ScF₃ crystals obtained by Bridgman directional solidification have ionic conductivity σ = 4 × 10^–8 S/cm at 673 K. It is smaller than the σ values for LaF₃ (sp. gr. \(P\overline 3 c1\)) and HoF₃ (sp. gr. Pnma) single crystals by a factor of 10⁴–10⁵. The low conductivity of ScF₃ crystals is due to the weak coordinating ability (coordination number CN = 6) and low electronic polarizability (α_cat = 1.1 ų) of Sc³⁺ ions. Mobile V_F⁺ vacancies and less mobile interstitial V_i^- ions (defects are formed according to the Frenkel mechanism) are involved in the ion transport. HoF₃ and LaF₃ single crystals have a high coordinating ability (CN = 9 for Ho³⁺ and CN = 11 for La³⁺) and a high electronic polarizability of cations (α_cat = 1.6–1.9 ų for Ho³⁺ and α_cat = 2.2 ų for La³⁺). Only mobile V_F⁺ vacancies (defects are formed according to the Schottky mechanism) are involved in ion transport.     

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.     

Structure of Gd<Subscript>0.95</Subscript>Bi<Subscript>0.05</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystals at 293 and 90 K

The structure of GdFe₃(BO₃)₄ single crystals has been studied by X-ray diffraction at 293 and 90 K. The crystals are grown from a flux in the Bi₂Mo₃O₁₂–B₂O₃–Li₂MoO₄–Gd₂O₃–Fe₂O₃ system. The results of chemical analysis and structural study show that these crystals contain bismuth as an impurity. It is found that bismuth atoms are located at gadolinium sites in the structure. A decrease in the temperature is accompanied by a lowering of the symmetry from sp. gr. R32 (at 293 K) to sp. gr. P3₁21 (at 90 K). The presence of two types of iron chains with different geometries at 90 K promotes a change in the magnetic properties of these crystals with a decrease in the temperature.     

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.     

Comparative analysis of the heat transfer processes during growth of Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> and Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> crystals by the low-thermal-gradient Czochralski technique

The heat transfer processes occurring in the solid and liquid phases during growth of Bi₁₂GeO₂₀ and Bi₄Ge₃O₁₂ crystals by the low-thermal gradient Czochralski method are analyzed and compared. It is experimentally found that, under similar growth conditions, the deflection of the crystallization front for the Bi₁₂GeO₂₀ crystal is considerably smaller than the deflection of the crystallization front for the Bi₄Ge₃O₁₂ crystal and the faceting of the former front is observed at the earlier stage of pulling. The results of the numerical simulation demonstrate that the different behavior of the crystallization fronts is associated with the difference between the coefficients of thermal absorption in the crystals.     

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.