Stabilization of lattice period in quinary solid solutions of the <Emphasis Type="Italic">A</Emphasis><Superscript>3</Superscript><Emphasis Type="Italic">B</Emphasis><Superscript>5</Superscript> compounds

The stabilizing effect of elastic strains on the lattice period of a quinary solid solution is considered. The expression for the stabilization factor for quinary solid solutions of the A_xB_1?xC_yD_zE_1?y?z type is derived. It is shown that the stabilizing influence of the substrate sharply increases in the vicinity of the region of the chemical spinodal. The stabilization factors are calculated for the Ga_xIn_1?xP_yAs_zSb_1?y?z and A1_xGa_yIn_1?x?yP_zAs_1?z quinary solid solutions isoperiodic to InAs, GaSb, and GaAs. It is shown that in the region of thermodynamic instability the stabilization factor has negative values. The changes in the composition of the above elastically strained quinary solid solutions are analyzed with respect to the equilibrium composition. It is also shown that stabilization of the lattice period does not signify the stabilization of its composition.     

Study of structural properties of In<Subscript>x</Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>As/In<Subscript>y</Subscript>Al<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>As heterosystems on InP substrates

The structural properties of In_xGa_1?xAs/In_yAl_1?yAs samples on InP substrates are studied as functions of growth conditions by the method of high-resolution diffractometry. The results obtained and the photoluminescence spectroscopy data are used to optimize the technology of preparation of high-quality heterostructures with sharp interfaces. The parameters of the two-dimensional electron gas of such heterostructures measured at 77 and 300 K are comparable with the best world standards in this field, so these heterostructures may be used to manufacture transistors and integral amplifiers operating at the frequency 40 GHz and even higher.     

X-ray diagnostics of P-HEMT AlGaAs/InGaAs/GaAs structures

The structural characteristics of the P-HEMT AlGaAs/InGaAs/GaAs heterostructure have been studied by high-resolution X-ray diffractometry. The parameters of the heterostructure layers were determined by simultaneous analysis of the X-ray reflection curves for the (004) and (113) crystallographic planes. Interface diffusion has been established for the In_yGa_1?yAs quantum well and the Al_xGa_1?x As spacer layer, which are characterized by reconstructed profiles of the lattice parameter distribution and anisotropic distribution of random displacements in the layer plane and in the perpendicular direction.     

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.     

On the effect of structural and symmetrical features of potassium titanyl phosphate crystals with different contents of niobium,antimony, and zirconium on the second-harmonic intensity

A model is proposed, which shows that, at small deviations from the centrosymmetric state of the atomic structure, the quadratic nonlinear susceptibility of a crystal monotonically decreases with approach of the degree of central symmetry \(\eta _{\overline 1 } \)[φ(r)] of the electric potential function of the crystal structure to unity. The quadratic nonlinear susceptibility of K_{1 ? x} Ti_{1 ? x} Nb _x OPO₄ (x = 0, 0.02, 0.04, 0.11), K_{1 ? x} Ti_{1 ? x} Sb _x OPO₄ (x = 0.01, 0.07, 0.17), and KTi_{1 ? x} Zr _x OPO₄ (x = 0.03, 0.04) crystals has been measured. The degree of central symmetry \(\eta _{\overline 1 } \)[φ(r)] has been calculated for the structures of K_{1 ? x} Ti_{1 ? x} Nb _x OPO₄ (x = 0, 0.04, 0.11), K_{1 ? x} Ti_{1 ? x} Sb _x OPO₄ (x = 0.01, 0.07, 0.17), and KTi_{1 ? x} Zr _x OPO₄ (x = 0.03, 0.04) crystals. It is shown that, at \(\eta _{\overline 1 } \)[φ(r)] > 0.7, the relationship between the quadratic nonlinear susceptibility of the investigated crystals and the degree of their central symmetry \(\eta _{\overline 1 } \)[φ(r)] is in qualitative agreement with the proposed model.     

Acoustic Modes at the Twist Boundary in Piezoelectric Crystals

The transformations of the shear (SH and SV) and longitudinal (L) acoustic waves at the twist boundary in piezoelectrics of ?62m and ?42m the and classes have been investigated. This boundary is formed at a hard contact between two identical crystal half-spaces rotated relative to each other around the Z||?6||?4 axis by an angle 2?. Propagation occurs along the bisector of this angle (х axis) in the (x, y) twist plane. Situations have been established in which the shear modes are localized at the twist boundary exclusively due to the combination of the piezoelectric effect and half-space rotation, as well as the joint effect of the elastic anisotropy and rotation. The cases of independence of a bulk wave of the half-space rotation and occurrence of leaky waves are described.     

The distribution of Ga and Sb impurities in Ge-Si crystals grown by the Bridgman method using a feeding rod

Gallium- and antimony-doped Ge_{1 ? x} Si _x crystals (0 ≤ x ≤ 0.25) have been grown by the improved Bridgman method using a silicon seed and a macrohomogeneous feeding Ge-Si rod of the corresponding composition. The impurity concentration profiles along the grown crystals were determined from Hall measurements. The mathematical problem of impurity distribution along two-component mixed crystals grown from a melt with uniform and graded composition is solved in the Pfann approximation and within the virtual-crystal model for the solid solution. It is shown that the experimental impurity distributions in Ge_{1 ? x} Si _x crystals are described well by the data calculated on the assumption of linear change in the impurity segregation coefficient with the crystal composition.     

Behavior of ammonium ion in β-LiRb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>(NH<Subscript>4</Subscript>)<Subscript>x</Subscript>SO<Subscript>4</Subscript> mixed crystals (0.5 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 1.0)

The influence of replacement of an ammonium ion by rubidium on the x-T phase diagram in the concentration range 0.50 ≤ x ≤ 1.0 has been studied by X-ray and neutron powder diffraction over a wide temperature range. It is shown that a decrease in the ammonium concentration is accompanied by an increase in the temperature of the II ? I phase transition and stabilization of phase II up to low temperatures (20 K). The changes occurring in the dynamics of mixed crystals are studied by inelastic incoherent neutron scattering. The spectra of the generalized vibrational density of states obtained allowed one to establish the difference in the phonon modes corresponding to phase III of β-LiNH₄SO₄ and phase II of β-LiRb_1?x(NH₄)_xSO₄ mixed crystal with x = 0.91 and x = 0.77 at 20 K. It is shown that a mixed crystal with x = 0.77 at 20 K is in the orientational glass state.     

Thermal Expansion of EuF<Subscript>2 + <Emphasis Type="Italic">x</Emphasis></Subscript> Single Crystals and Their Thermal Stability

Thermal expansion of an EuF_2.136 nonstoichiometric crystal with the fluorite structure type (Eu _0.864 ²⁺ Eu _0.136 ³⁺ F_2.136, lattice parameter 5.82171(5) Å) has been experimentally investigated in the temperature range of 9–500 K. The coefficient of thermal expansion is α = 15.8 × 10^–6 K^–1 at T = 300 K. The observed anomalies in the behavior of the coefficient of thermal expansion at T > 400 K are related to the oxidation processes with partition of Eu²⁺ ions. It is established by differential scanning calorimetry that the onset temperature of EuF_{2 + x} oxidation in air is 430 K and that this process occurs in three stages. X-ray diffraction analysis shows that the oxidation is accompanied by the formation of a phase mixture based on two modifications of the Eu _{1– y} ³⁺ Eu _y ²⁺ F_3–y solid solution with the structure types of tysonite (LaF₃), orthorhombic β-YF₃ phase, and europium oxyfluorides of variable composition EuO_1–xF_{1 + 2x}, with dominance of the latter.     

Strontium in the collinsite structure: Rietveld refinement

The crystal structure of a strontium variety of a rare phosphate—mineral collinsite (Ca_{2 ? x}Sr_x)₂Mg[PO₄] · 2H₂O was solved from powder X-ray diffraction data (λ CuK_α radiation, Ni filter, 12.36° ≤ 2θ ≤ 100.00°, scan step 0.02°, exposure time per step 15 s) by the Rietveld method (R_wp = 4.15%, R_F = 1.03%, R_B = 2.46%); a = 5.8219(1) Å, b = 6.8319(2) Å, c = 5.4713(1) Å, α = 96.965(2)°, β = 108.846(2)°, γ = 107.211(2)°, sp. gr., \(\bar P1\), Z = 1, ρ_calcd = 3.12 g/cm³ (at x = 0.72). The new mineral was discovered in carbonatites from the Kovdor alkaline-ultrabasic massif. The crystallochemical data for collinsite were analyzed and compared with those for isotypic minerals of the fairfieldite group. Characteristic features of the low-temperature geochemistry of strontium were established.     

Effect of the sign of misfit strain on the formation of a dislocation structure in SiGe epitaxial layers grown on Si and Ge substrates

Comparative analysis of the specific features of the formation of a dislocation structure in the single-layer epitaxial heterostructures Si_1?xGe_x/Si and Ge_1?ySi_y/Ge is performed. It is ascertained that, at a relatively low lattice mismatch between an epitaxial layer and a substrate, the sign of misfit strain at the interface significantly affects the processes of defect formation. The most probable reasons for the observed phenomena are analyzed with allowance for the specific features of the state of the ensemble of intrinsic point defects in epitaxial layers subjected to elastic strains of a different sign.     

High-pressure synthesis and magnetic properties of Bi<Subscript>0.5</Subscript>D<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> (<Emphasis Type="Italic">D</Emphasis> = Pb,Ba) solid solutions

Perovskites Bi_0.5D_0.5MnO₃(D = Pb, Ba) were prepared under high pressure (4 GPa) at 1200–1300°C. According to the X-ray diffraction data, crystalline Bi_0.5Pb_0.5MnO₃ has a tetragonal unit cell with the parameters a = 3.940 Å and c = 3.800 Å, whereas Bi_0.5Ba_0.5MnO₃ crystals are cubic with a = 3.940 Å. It is concluded from magnetic studies that lead-containing manganite is an antiferromagnet with T_N = 120 K, whereas Bi_0.5Ba_0.5MnO₃ is a spin glass with spin-freezing temperature T _f = 38 K. Both compounds are decomposed upon heating in air at temperatures above 500°C. With the use of synthesis in air, Bi_0.5Ca_{0.5 ? x}D _x MnO₃ solid solutions with x as high as 0.25 were obtained.     

Cationic transport mechanism in α-Ag<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>I(0 < <Emphasis Type="Italic">x</Emphasis> < 0.25): Molecular-dynamics simulation

The structural and transport characteristics of an Ag_{1 ? x} Cu _x I(0 < x < 0.25) solid solution have been simulated by the molecular-dynamics method. It is found that the cation diffusion coefficient decreases with increasing copper concentration; this correlation is in agreement with the experimentally observed decrease in ionic conductivity. It is shown that the cationic transport in disordered Ag_{1 ? x} Cu _x I phases is mainly due to the migration of silver cations, whereas the mobility of copper cations is much lower. Cu⁺ cations are found to reside in the 8c positions in a bcc cell; this finding suggests the existence of nanoscale α-CuI regions.     

Electrical and nonlinear optical properties of KTiOPO<Subscript>4</Subscript> single crystals doped with niobium,antimony, and tantalum

Single crystals of K_{1 ? x}Ti_{1 ? x}Nb_xOPO₄ (KTP: Nb), K_{1 ? x}Ti_{1 ? x}Sb_xOPO₄ (KTP: Sb), and K_{1 ? x}Ti_{1 ? x}Ta_xOPO₄ (KTP: Ta) solid solutions are grown and their dielectric, conducting, and nonlinear optical properties are investigated. The maximum contents x of niobium, antimony, and tantalum impurities in the crystals are equal to 0.11, 0.23, and 0.25, respectively. The doping of the KTiOPO₄ crystals with niobium, antimony, and tantalum brings about the formation of additional potassium vacancies and additional potassium positions and, as a consequence, an increase in the ionic conductivity σ₃₃. An increase in the doping level leads to a smearing of the ferroelectric phase transitions and a decrease in the phase transition temperatures. The permittivity of the doped crystals exhibits a broad relaxation peak in the temperature range 200–600°C.     

Birefringence of (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals under the action of uniaxial pressures

The influence of uniaxial mechanical pressure σ _m ≤ 150 bar on the spectral (300–800 nm) dependence of the birefringence Δn _i of (NH₄)₂SO₄ crystals is studied. The dispersion Δn _i (λ) is shown to be normal and greatly increases when approaching the absorption edge. Uniaxial pressure changes the value of dispersion dΔn _i /dλ but not its character. It is found that the simultaneous action of pressures σ _x ～ σ y ～ 560 bar results in the occurrence of a uniaxial isotropic state. Piezoelectric constants of the crystals are estimated.     

Crystal structure of sakhaite from the Solongo deposit in connection with the crystallochemical interpretation of the sakhaite-harkerite mineral series

The crystal structure of mineral sakhaite Ca₄₈[Mg_13.2(Fe, Mn)_2.8](CO₃)₁₆{ Al[SiO_3.75(OH)_0.25]₄}(BO₃)₂₈[(H₂O)_3.3(HCl)_3.3] from the Solongo deposit, Zabaykalye, is established (R 0.047) by X-ray diffraction analysis (Bruker Smart CCD diffractometer, 2θ: θ scan, λMoK_α radiation, graphite monochromator): a = 14.679(2) Å, sp. gr. Fd3m, Z = 1, ρ_calcd = 2.99 g/cm³. It is shown that some part of BO₃ triangles in the structure of sakhaite from Solongo is replaced with five-member complexes {Al[SiO₃(O,OH)]₄}, established previously in the harkerite structure. Three schemes of isomorphism, which are characteristic of the sakhaite-harkerite mineral series, are selected and the general formula is proposed: Ca₄₈(Mg,Fe,Mn)₁₆(CO₃)₁₆{Al[SiO₃(O, OH)]₄}_y(B₃)_32?y · n(H₂O,HCl) (Z = 1), y_max = 8, n_max = 16 ? y. The structural relationship of sakhaite-harkerite minerals with borate-sulfates of the tychite family Na₆(Mg,Fe,Mn)₂(BO₃)₄(SO₄) is revealed. The correlation between the number of harkerite fragments forming the mineral structure, the structural symmetry, and the degree of imperfection is ascertained.     

Study of the specific features of single-crystal boron microstructure

A complex study of the structure of β-boron single crystal grown by the floating-zone method, with sizes significantly exceeding the analogs known in the literature, has been performed. The study includes X-ray diffraction analysis and X-ray diffractometry (measurement of pole figures and rocking curves), performed on both laboratory and synchrotron sources; atomic-resolution scanning transmission electron microscopy with spherical aberration correction; and energy-dispersive microanalysis. X-ray diffraction analysis using synchrotron radiation has been used to refine the β-boron structure and find impurity Si atoms. The relative variations in the unit-cell parameters a and c for the crystal bulk are found to be δa/a ≈ 0.4 and δc/c ≈ 0.1%. X-ray diffractometry has revealed that the single-crystal growth axis coincides with the [\(2\bar 2013\)] crystallographic axis and makes an angle of 21.12° with the [0001] threefold axis. Electron microscopy data have confirmed that the sample under study is a β-boron crystal, which may contain 0.3–0.4 at % Si as an impurity. Planar defects (stacking faults and dislocations) are found. The results of additional measurements of the temperature dependence of the thermal conductivity of the crystal in the range of 50–300 K are indicative of its high structural quality.     

Synthesis and crystal structure of <Emphasis Type="Italic">N</Emphasis>-(6methoxylbenzothiazol-2-yl)-1-(4-fluorophenyl)<Emphasis Type="Italic">O</Emphasis>,<Emphasis Type="Italic">O</Emphasis>-dipropyl-<Emphasis Type="Italic">α</Emphasis>-aminophosphonate

Molecular and crystal structure of N-(6-methoxybenzothiazol-2-yl)-1-(4-fluorophenyl)-O,O-dipropyl-α-aminophosphonate, C₂₁H₂₆FN₂O₄PS, have been determined by single-crystal X-ray diffraction study. The title compound is tetragonal, with a = 21.35(3) Å, c = 20.16(5) Å, Z = 16, D _x = 1.308 Mg/m³, μ(MoKα) = 0.247 mm^?1, and space group is I4₁/a. The structure was solved by direct method and refined to a final R = 0.0687 for 2370 reflections with I > 2σ(I). The compound shows a fully delocalized benzothiazole system with a sp² hybridization of the N(2). There is a strong intermolecular hydrogen bond between P=O and NH. The crystal structure is stabilized by a strong intermolecular N–H?sO hydrogen bond.     

Use of a fluorine-containing solvent in the synthesis of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> single crystals

A laboratory technological procedure has been developed for the synthesis of high-temperature superconducting YBa₂Cu₃O_{7 ? δ} single crystals (T _c ～ 90 K, ΔT _c ～ 1.0 K) up to 0.25 cm² in size from a nonstoichiometric fluorine-containing flux of (YO_1.5)(BaO)_{4 ? x}(BaF₂)_x(CuO)₁₀, where 2 ≥ x ≥ 0, using a combination of enhanced nucleation and directional crystallization by the Czochralski method. Studies using differential thermal analysis demonstrated that the addition of BaF₂ decreased the eutectic-crystallization temperature and increased the crystal-growth rate. The optimum concentration of BaF₂ in the starting melt composition was found (x = 0.2). The single-crystal surface was studied by atomic-force nanoscopy. The morphology of single crystals that have been synthesized from a melt of their own components differs substantially from that of crystals grown from a BaF₂-containing melt.     

Synthesis of Nonstoichiometric Samarium Fluoride SmF<Subscript>2 + <Emphasis Type="Italic">x</Emphasis></Subscript>

The products of interaction of SmF₃ with metallic Sm⁰, Si, and molecular hydrogen in quartz, graphite, and molybdenum containers have been investigated by X-ray diffraction. It is shown that reduction occurs with the formation of nonstoichiometric cubic SmF_{2 + x} (sp. gr. Fm3?m, unit-cell parameter a = 5.8517(2) Å) with a content x = 0.108 only in the case of synthesis of SmF₃ with metallic Sm⁰ in a quartz ampoule. The transformation of the SmF_{2 + x} phase composition in air at room temperature has been studied for the first time. It is found that SmF_2.108 passes into the superstructural phase Sm₁₃F_32–δ (sp. gr. R3?) with the unit-cell parameters a = 14.7354(4) Å and c = 10.1156(4) Å for 60 days. The use of Sm⁰, Si, and H₂ as reducing agents for SmF₃ is inefficient because of the weak tendency of the latter to reduction: the process occurs also with a low yield of the Sm₁₃F_32–δ phase and of the α-SmF₃-based phase as impurity in the case of direct reduction by hydrogen.