Temperature-Induced structural phase transformations in Cu<Subscript>1.50</Subscript>Zn<Subscript>0.30</Subscript>Te and Cu<Subscript>1.75</Subscript>Cd<Subscript>0.05</Subscript>Te single crystals

The results of studying the temperature-induced polymorphic phase transformations in Cu_1.50Zn_0.30Te and Cu_1.75Cd_0.05Te single crystals are presented. The single crystals have been investigated in the range of 290–1100 K using X-ray diffraction analysis. The temperature dependences of the unit-cell parameters, X-ray density, and coefficients of thermal expansion for each polymorphic modification of Cu_1.50Zn_0.30Te and Cu_1.75Cd_0.05Te single crystals are presented. The influence of cationic substitution (replacement of Cu²⁺ with Zn²⁺ and Cd²⁺ cations) on the phase-formation processes, number of polymorphic modifications, and temperature of structural phase transition in single crystals of these solid solutions is demonstrated.     

Influence of partial replacement of Cu by Cd and Zn atoms on polymorphous transformation in Cu<Subscript>1.75</Subscript>Te crystal

A layer Cu_{1.75 ? x}Te single crystal in which some Cu atoms were replaced by Cd and Zn atoms (x = 0.05) was studied by high-temperature X-ray diffractometry. It was established that this replacement substantially affects the number and temperature of polymorphous transformations.     

Structure and properties of antimony-doped lanthanum molybdate La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript>

Polycrystalline samples of the composition La₂Mo_{2 − x} Sb _x O_{9 − y} , where 0 ≤ x ≤ 0.05, were prepared by solid-phase synthesis. Single crystals of La₂Mo_1.96Sb_0.04O_8.17 were obtained by spontaneous crystallization from flux. The structure of the metastable β _ms phase of this compound was determined at room temperature by X-ray diffraction. It was found that the La, Mo, and O1 atoms are displaced from the threefold axis on which they are located in the high-temperature β phase. It was shown that molybdenum atoms in the crystal structure are partially replaced by antimony atoms, which are located on the threefold axis. In antimony-doped crystals, lanthanum atoms partially return to the site on the threefold axis and the coordination environment of molybdenum cations becomes more ordered, thus facilitating the stabilization of the cubic phase at room temperature. Calorimetric measurements (DSC) showed that the introduction of Sb as the dopant into the La₂Mo₂O₉ structure leads to a decrease in the temperature of the α → β phase transition from 570 to 520°C and to the partial suppression of this transition. The temperature behavior of the conductivity confirms the DSC data. Thus, doping with Sb contributes to the stabilization of the cubic phase at room temperature.     

Structural transformations in Cu<Subscript>1.95</Subscript>Ni<Subscript>0.05</Subscript>S crystals

A solid solution of the Cu_1.95Ni_0.05S composition has been synthesized for the first time due to the partial replacement of Cu with Ni atoms in Cu₂S. The polymorphic transformations in the polycrystalline samples in the temperature range of 300–1400 K have been investigated by X-ray diffraction and differential thermal analysis. It is established that, at room temperature, the synthesized Cu_1.95Ni_0.05S samples have an orthorhombic lattice with unit-cell parameters a = 26.50 Å, b = 15.39 Å, and c = 13.85 Å (sp. gr. Abm2). Heating to T = 379 ± 2 K leads to its transformation into a hexagonal lattice with parameters a = 3.960 Å and c = 6.78 Å (sp. gr. P6₃/mmc). At 750 ± 2 K, the hexagonal modification is transformed into a cubic one with period a = 5.788 Å (sp. gr. Fm\(\bar 3\)m). The phase transition in this crystal is enantiotropic.     

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.     

Investigation of the structural conditionality for changes in physical properties of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals

With the aim of elucidating the nature of anomalies in the physical properties of K₃H(SO₄)₂ crystals that arise as the temperature grows, the dielectric and optical properties of the crystals are studied, an X-ray diffraction analysis of single-crystal and polycrystalline specimens are performed, and the morphology and chemical composition are studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. As a result of the studies performed, a phase transition from the phase with the monoclinic symmetry (space group C2/c) to the phase with the trigonal symmetry (space group R $ \bar 3 $ \bar 3 m) is found in a number of K₃H(SO₄)₂ specimens at a temperature of ≈457 K, the responsibility of the dynamically disordered hydrogen-bond system for the rise of high proton conductivity in the high-temperature phases of the crystals of this family is confirmed, and data on the solid-phase reactions proceeding at high temperatures are obtained.     

Synthesis and Crystal Structure of [Li(THF)<Subscript>4</Subscript>]Cu<Subscript>2</Subscript>Br<Subscript>4</Subscript>

The mixed-valence compound [Li(THF)₄]Cu₂Br₄ was synthesized in a redox reaction from 1,4-dihydroxynaphthalene, CuBr₂ and LiNtBuSiMe₃ in THF. X-ray quality crystals of [Li(THF)₄]Cu₂Br₄ (monoclinic, P2₁/c) are obtained from the mother liquor at ambient temperature. In the solid state, infinite chains of anionic [Cu₂Br₄]⁻ units are established. These chains are separated by [Li(THF)₄]⁺ cations.     

Domain Structure and Orientational Ordering of NO<Subscript>2</Subscript> Groups in K<Subscript>2</Subscript>Ba(NO<Subscript>2</Subscript>)<Subscript>4</Subscript> Crystals

Polarization-optical studies of the domain structure of K₂Ba(NO₂)₄ crystals and differentialscanning calorimetric measurements have been performed in the vicinity of the high-temperature phase transition. The orientational ordering of NO₂ atomic groups is analyzed and the temperature dependence of the birefringence coefficient is theoretically described.     

Structural reasons for the nonlinear optical properties of KTi<Subscript>0.96</Subscript>Zr<Subscript>0.04</Subscript>OPO<Subscript>4</Subscript> single crystals

This paper reports on the results of the precision X-ray structural investigations of KTi_0.96Zr_0.04OPO₄ single crystals at room temperature. It is established that the incorporation of zirconium atoms into the structure of KTiOPO₄ (KTP) crystals does not lead to substantial changes in the framework structure and results only in an insignificant decrease in the scatter of the distances in the PO₄ tetrahedra and the formation of more symmetric (TiZr)O₆ octahedra as compared to the TiO₆ octahedra. However, the incorporation of zirconium atoms into the KTP structure is accompanied by the redistribution of the electron density in the crystal as a whole, so that the electron density increases in the region of the positions occupied by the potassium atoms. This changes the nonlinear optical properties of the given series of crystals, which are estimated from the intensity of the second harmonic generation signals.     

Flux crystallization of trigonal GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> competing with the crystallization of α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

The formation of trigonal GdFe₃(BO₃)₄ crystals in the Bi₂Mo₃O₁₂-B₂O₃-Li₂MoO₄-Gd₂O₃-Fe₂O₃ system was studied. The flux compositions for which GdFe₃(BO₃)₄ is the high-temperature phase with a wide range of crystallization were determined. The features of nucleation of these crystals and their growth near the phase boundary with α-Fe₂O₃ were analyzed. The growth of GdFe₃(BO₃)₄ single crystals involving preliminary nonequilibrium crystallization of α-Fe₂O₃ is described.     

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.     

On the nature of fracture of SrB<Subscript>4</Subscript>O<Subscript>7</Subscript> and PbB<Subscript>4</Subscript>O<Subscript>7</Subscript> single crystals

The crack resistance and peculiarities of the internal structure of isostructural SrB₄O₇ and PbB₄O₇ single crystals of the framework type have been studied. It is shown that the cleavage of these crystals, which is atypical of this type and inherent in strontium and lead tetraborates, is due to the presence of boron-oxygen layers (bound by a relatively small number of covalent bonds) in their 3D boron-oxygen frameworks; crystals are cleaved along these layers. It is established that cracks propagate in SrB₄O₇ and PbB₄O₇ single crystals as a result of breakage of both the bonds between bridge atoms and the bonds in B₃O₃ boron-oxygen cycles—the main elements of the boron-oxygen framework. The break of bonds in the boron-oxygen cycles is explained by the presence of an unusual oxygen position in these cycles, which is shared by three boron-oxygen tetrahedra and whose B-O bonds are much weaker in comparison with the bonds typical of BO₄ groups.     

Crystal structure of the binary complex of cobalt and zinc chlorides with carbamide [Co(OCN<Subscript>2</Subscript>H<Subscript>4</Subscript>)<Subscript>5</Subscript>(H<Subscript>2</Subscript>O)][ZnCl<Subscript>4</Subscript>]

Mixed single crystals of [Co(OCN₂H₄)₅(H₂O)][ZnCl₄] were grown by the isothermal evaporation of an aqueous solution. The crystal structure of this complex was established by X-ray diffraction (R = 0.052 based on 7003 reflections). The crystals consist of [Co(OCN₂H₄)₅(H₂O)]²⁺ cations containing Co atoms in an octahedral coordination and [ZnCl₄]²⁻] anions containing Zn atoms in a tetrahedral coordination. The carbamide molecules are involved in both intramolecular and interionic hydrogen bonds. The H₂O molecule forms hydrogen bonds with the anions.     

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.     

Growth kinetics and adsorption trapping of impurity during crystallization of NH<Subscript>4</Subscript>Cl in the NH<Subscript>4</Subscript>Cl-CuCl<Subscript>2</Subscript>-H<Subscript>2</Subscript>O-CONH<Subscript>3</Subscript> system

The growth of NH₄Cl crystals and their trapping of copper impurity in the NH₄Cl-CuCl₂-H₂O-CONH₃ quaternary system have been experimentally studied. The epitaxial adsorption of copper complexes on (100) faces leads to a sharp decrease in the growth rate in good correspondence with the Bliznakov-Chernov equation. The copper impurity enters the crystal composition in amounts up to 6.5 mol %. The impurity distribution coefficient nonlinearly changes with the copper concentration in the solution: it is much larger than unity at low concentrations and sharply decreases with an increase in supersaturation. Such behavior is indicative of the adsorption mechanism of copper trapping by NH₄Cl crystals. Single-crystal X-ray study shows that the impurity is incorporated in NH₄Cl crystals in the form of oriented intergrowths of different complex coppercontaining compounds. The concentration and variety of impurity phases increase with an increase in the copper content in the solution and decrease with an increase in supersaturation. Heterogeneous 2D isomorphous trapping of copper impurity by NH₄Cl crystals induces high (up to 60 MPa) internal stresses in them, as a result of which anomalous birefringence and splitting of crystals occur.     

Specific features of phase transitions and the conduction of La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> oxide-ion conducting compound doped with vanadium

The X-ray powder analysis, calorimetric studies, and conductivity measurements of a series of ceramic La₂Mo_2−x V _x O _y specimens with different vanadium content are performed with the aim of following the dynamics of phase formation of the low-temperature α, high-temperature β, and metastable β _ms phases. At x ≥ 0.06, the cubic phase becomes stable and the monoclinic phase vanishes; therefore, the main α → β transition is suppressed. According to the data of differential thermal analyses, a weak thermal anomaly is observed in the range 450–470°C at x ≥ 0.06. This anomaly is indicative of the β _ms → β transition due to the conversion of the cubic phase with statically disordered oxygen atoms into the cubic phase with dynamic disorder. The conductivity of the high-temperature β phase obeys the Vogel-Tammann-Fulcher law.     

Structure refinement of cadmium cerium(IV) phosphate Cd<Subscript>0.5</Subscript>Ce<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Cadmium cerium orthophosphate Cd_0.5Ce₂(PO₄)₃ is synthesized by precipitation from aqueous solutions. The structure refinement from powder X-ray diffraction data is preceded by the sample preparation and structure solution. The refinement is carried out by the Rietveld method (ADP-2 diffractometer, Cu _Kα radiation, Ni filter, 15° < 2θ < 120°, 2θ-scan step 0.02°, counting time 10 s per step). All calculations are carried out using the WYRIET program (version 3.3) within the sp. gr. P2₁/n. The structure is refined with anisotropic displacement parameters for cations and isotropic displacement parameters for oxygen atoms.     

Neutron diffraction study of phase transitions in the superionic conductor Li<Subscript>0.25</Subscript>Cu<Subscript>1.75</Subscript>Se

Neutron diffraction studies of the superionic conductor Li_0.25Cu_1.75Se are reported. It is shown that gradual disordering in the cation sublattice with an increase in temperature leads to changes in the symmetry of the crystal lattice at 400 and 500 K. At temperatures close to these values, anomalies in the temperature dependence of both ionic and electron conductivity are observed.     

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.