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 the Ca<Subscript>3</Subscript>TaGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> compound

The absolute crystal structure of the Ca₃TaGa₃Si₂O₁₄ piezoelectric compound is refined using X-ray diffraction analysis. The unit cell parameters and final R factors are as follows: a = 8.112(1) Å, c = 4.9862(6) Å, space group P321, Z = 1, R = 0.98%, and R _w = 1.42%. It is shown that the configuration of the absolute crystal structure inherited from the seed material determines the positive sense of the optical activity of the crystal under investigation. The structural and acoustical characteristics of the Ca₃TaGa₃Si₂O₁₄ crystals are compared with those of the La₃Ga₅SiO₁₄ crystals.     

Magnetoplastic effect: Basic properties and physical mechanisms

This paper presents a review of the main results of investigations into the magnetoplastic effect, which manifests itself in motion of dislocations in crystals exposed to magnetic fields. The dependences of the mean free path of dislocations on the induction and direction of the magnetic field, the magnetic treatment time, the temperature, and the type and concentration of impurities are studied for NaCl, LiF, CsI, Zn, Al, and InSb crystals. The threshold magnetic field B _c below which the effect is absent, the saturation field B₀ above which the mean free paths of dislocations remain constant with an increase in the magnetic induction B, and the critical frequency v _c of rotation of a sample in the magnetic field above which the effect disappears are examined. The quantities B _c , B₀, and v _c are investigated as functions of the basic physical parameters. It is found that the magnetoplastic effect is highly sensitive to X-ray radiation at low doses and to simultaneous action of an electric field or mechanical loading. The hardening of NaCl(Pb) crystals in the magnetic field is revealed. The theoretical interpretation is proposed for all the findings and dependences observed.     

Dimorphism of <Emphasis Type="Italic">R</Emphasis>F<Subscript>3</Subscript> (<Emphasis Type="Italic">R</Emphasis> = La–Nd) crystals based on the data of X-ray diffraction studies

The ratio of two forms of tysonite in nominally pure single crystals of RF₃ (R = La–Nd) obtained from melts under identical conditions (the as-grown state) is studied for the first time by X-ray diffraction. Crystals of RF₃ with R = La–Nd belong to the β-LaF₃ structural type (space group \(P\bar 3c1\), Z = 6) and form twins. Samples 0.2–0.4 in diameter contain inclusions of the high-temperature a form (space group P6₃/mmc, Z = 2). It is shown that twinning and dimorphism of the RF₃ crystals (R = La–Nd) have a common structural basis.     

Growth of single crystals of the high-pressure phase of α-PbO<Subscript>2</Subscript>

The technique and results of hydrothermal growth of single crystals of the high-pressure phase of α-PbO₂ are described. Dark brown crystals of predominantly prismatic habit with characteristic sizes of 550 × 100 × 150 μm³ were obtained at p = 4 GPa by cooling an aqueous solution in the temperature range t = 600–300°C.     

Strain of interatomic bonds and multiple-well potential in crystals with perovskite structure

A method for determining the strain characteristics of interatomic bonds in crystals of ABX₃ compounds with perovskite structure is developed. The bond strain energy ΔU _s is estimated. This energy is responsible for the formation of ordered lattice distortions related to the rotation of octahedra and/or the cooperative displacement of cations. It is shown that ΔU _s correlates with the characteristics of ordered distortions and the temperature of the phase transition of compounds to the cubic structure. It is ascertained that the interatomicinteraction potential in crystals with strained bonds is a local multiple-well potential, the form of which depends on the nature of interacting atoms. It is shown that the occurrence of the ferroelectric state in the noted crystals may be due to the presence of bond strains and the anisotropy of the covalent component of the cation-anion interaction.     

Piezo-optic properties of TGS crystals doped with <Emphasis Type="Italic">D</Emphasis>-serine

The effect of uniaxial mechanical pressure along the main crystallophysical axes on the principal values of birefringence Δn _i in TGS crystals doped with 5% D-serine is investigated. It is ascertained that the values of Δn _i are rather sensitive to the action of uniaxial stresses. The temperature shift coefficients ?T _c /?σ_m are determined for the phase-transition points. The temperature and spectral dependences of the combined piezo-optic constants π _im ⁰ are calculated. The contribution of the secondary electro-optic effect to the changes in the birefringence and the piezo-optic constants of doped crystals is determined. It is found that the piezooptic constants π ₂₃ ⁰ and π ₁₂ ⁰ have the same values, which indicates a decrease in the anisotropy of the optical indicatrix of doped crystals under the action of uniaxial stress. The optical and deformation contributions to the relaxation effect of piezo-optical birefringence in doped TGS crystals are found.     

Recombinant formate dehydrogenase from <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>: Preparation,crystal growth in microgravity,and preliminary X-ray diffraction study

Crystals of high-purity recombinant NAD⁺-dependent formate dehydrogenase from the higher plant Arabidopsis thaliana (AraFDH) were grown in microgravity in the Modul’-1 protein crystallization apparatus on the International Space Station. The space-grown crystals have larger sizes than those grown on Earth. X-ray diffraction data suitable for determining the three-dimensional structure were collected from the space-grown crystals to a resolution of 1.22 Å using an X-ray synchrotron source. The crystals belong to sp. gr. P4₃2₁2; the unit-cell parameters are a = b = 107.865 Å, c = 71.180 Å, α = β = γ = 90°.     

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.     

Water-soluble ferroelectric crystals with inhomogeneous impurity distribution

Stabilization of the unipolar state via the formation of inhomogeneous impurity distribution in crystal bulk is considered. Possible growth of crystals with stable characteristics is demonstrated on triglycine sulfate (TGS) crystals with a regular inhomogeneous impurity distribution. The properties of TGS crystals with an inhomogeneous distribution of chromium ions grown above and below the Curie temperature T_C are studied. Inhomogeneous TGS crystals of three types are obtained: type-I crystals with a smooth variation of the concentration gradient along the growth direction, type-II crystals with a periodic layer variation of the impurity concentration, and type-III crystals with a sawtooth-like variation of the impurity concentration along the sample length. The TGS crystals with the regular inhomogeneous impurity distribution in the ferroelectric phase are characterized by higher values of the internal bias field E _b , unipolarity coefficient k, and pyroelectric coefficient γ than the inhomogeneous crystals in the paraelectric phase and the crystals with the statistic impurity distribution grown by the conventional method.     

The Influence of Cation Substitution on the Kinetics of Phase Transitions in Crystals of (K,NH<Subscript>4</Subscript>)<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> Solid Solutions

The structure of (K_0.967(NH₄)_0.033)₃H(SO₄)₂ crystals, belonging to the K₃H(SO₄)₂–(NH₄)₃H(SO₄)₂–H₂O salt system, has been investigated by X-ray structural analysis. The room-temperature characteristics of the atomic structure of these crystals are found to be as follows: sp. gr. C2/c, Z = 4, a = 14.7025(4) Å, b = 5.6859(2) Å, c = 9.7885(3) Å, and R/wR = 0.021/0.030%. The thermal and optical properties of (K,NH₄)₃H(SO₄)₂ and K₃H(SO₄)₂ single crystals have been investigated and compared in a temperature range of 295–500 K.     

Relation between the bond lengths in an O-H…O bridge

The data in the literature on the bond lengths of O-H…O bridges formed in organic and inorganic crystals have been analyzed. The results of generalizations of these data on the basis of the generally accepted relation exp(?(r ₁ ? r ₀/b) + exp(?(r ₂ ? r ₀/b) = 1 are considered. The factors limiting its application are revealed and its limiting accuracy is estimated. A more universal expression relating the lengths of the same bonds is proposed: exp(?((r ₁ ? r ₀)/b) ^h ) + exp(?((r ₂ ? r ₀)/b) ^h ) = 1. On the basis of the generalization of all known neutron diffraction data on bridges with angles exceeding 165°, the coefficients in this relation are determined to be r ₀ = 0.950 ± 0.005, b = 0.33018 Å, and h = 5/3.     

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.     

Electronic nature of substituents and the structure of 4,4′-dipyridylium dications: Crystal structure of <Emphasis Type="Italic">N,N′</Emphasis>-di(2-carboxyethyl)-4,4′-dipyridylium diperchlorate

The crystal structure of N,N′-di(2-carboxyethyl)-4,4′-dipyridylium diperchlorate, C₁₆H₁₈Cl₂N₂O₁₂, is determined using X-ray diffraction analysis at 293 K. The crystals are orthorhombic, a = 20.084(4) Å, b = 8.687(2) Å, c = 11.725(2) Å, space group Pna2₁, and Z = 4; 1688 reflections measured; and R = 0.056 and R _w = 0.059 for 1244 reflections with I > 3σ(I). It is found that, in the absence of charge transfer or short intramolecular O?N contacts, the nucleus of the dication adopts a twist conformation (the angle of rotation between the planes of the pyridine rings is equal to 23°).     

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.     

Crystal structure of zinc(II) hydrated trinuclear complex with monoprotonated ethylenediaminetetraacetate anion(3−), [Zn<Subscript>3</Subscript>(H<Emphasis Type="Italic">Edta</Emphasis>)<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)<Subscript>6</Subscript>]

The crystal structure of the [Zn₃(HEdta)₂(H₂O)₆] complex (I) is determined by X-ray diffraction analysis. The crystals are orthorhombic, a = 14.780 Å, b = 29.699 Å, c = 7.032 Å, Z = 4, and space group Pna2₁. The structural units of crystals I are trinuclear linear molecules, in which the peripheral atoms Zn(1) and Zn(2) each coordinate two N atoms and three O atoms of the HEdta ^3? ligand and the O(w) atom of the H₂O molecule, whereas the central Zn(3) atom coordinates four O(w) atoms of the H₂O molecules and two terminal O atoms of the two HEdta ^3? ligands. The HEdta ^3? ligand fulfills a hexadentate chelating—bridging function. The bond lengths are as follows: Zn-O(HEdta), 2.006(4)–2.123(4) Å; Zn-N, 2.214(6) and 2.128(5) Å; and Zn-O(w), 2.006(6)–2.225(5) Å. In structure I, there is a specific contact formed by hydrogen bonds, owing to which the distance between the central atoms of individual molecules appears to be shorter than that in covalently bonded complexes.     

X-ray diffraction study of thiocarbamide hydrochloride

Thiocarbamide hydrochloride single crystals forming a salt with the composition [(NH₂)₂CSH]⁺Cl^? are studied by the X-ray diffraction method. The tetragonal crystals have the unit-cell parameters a = 7.556(1) Å and c = 18.329(3) Å, V = 1046.5 ų, Z = 8, ρ_calcd = 1.436 g/cm³, sp. gr. P4₁2₁2, and R = 0.050. Each chloride anion in the crystal forms five hydrogen bonds with thee different cations.     

Unclassical hydrogen bonds of C—H⋅<Emphasis Type="Italic">s</Emphasis>O and C—H⋅<Emphasis Type="Italic">s</Emphasis>N in the crystals of 2-amino-3-cyano-4-(3,4-dichlorophenyl)-5-oxo-1,4,5,6-tetrahydro-4H-pyrano[2,3-<Emphasis Type="Italic">d</Emphasis>]pyrimidine

The 2-amino-3-cyano-4-(3,4-dichlorophenyl)-5-oxo-1,4,5,6-tetrahydro-4H-pyrano[2,3-d] pyrimidine (1, C₁₇H₁₅Cl₂N₅O₃) was synthesized and characterized by IR,¹H NMR and elemental analysis. The molecular structure of1 was further studied by using X-ray crystallography. The crystals of compound1 are triclinic, space groupP-1,a = 6.0090(4) Å,b = 10.4056(7) Å,c = 15.6021(11) Å, α = 70.983(4), β = 84.056(6), γ = 84.611(6),Z = 2,V = 915.47(11) ų. Two types of unclassical hydrogen bonds C–H?sO and C–H?sN were presented in the crystals. In addition, there were classical hydrogen bonds in the crystal structure.     

Specific features of the crystal structure and magnetic properties of KTaO<Subscript>3</Subscript> produced by electrolysis of melts

The magnetic susceptibility χ(T) at 4.2 K < T < 293 K; the dependence of the magnetic moment on the magnetic field strength, M(H), at 4.2, 77, and 293 K; and the electrical resistivity ρ(T) at 4.2 K < T < 293 K are studied for samples of perovskite-phase KTaO₃ obtained by both solid-phase synthesis (KTaO ₃ ^s ) and deposition on a cathode during electrolysis of melts (KTaO ₃ ^e ). Yellowish white KTaO ₃ ^s powders are diamagnetic and reveal dielectric properties. Dark polycrystalline KTaO ₃ ^e samples with metallic luster are characterized by the dependence ρ(T) typical of metals and additional paramagnetic contribution to the paramagnetic susceptibility as compared with KTaO ₃ ^e . Changes in the properties of KTaO₃ during electrocrystallization are attributed to partial reduction of tantalum. They are revealed in the structural features of KTaO ₃ ^e (excess of tantalum as compared to the stoichiometric composition of KTaO ₃ ^e , deficiency of the oxygen sublattice, and clearly pronounced anharmonicity of atomic vibrations). A change of the cation-anion-cation interactions, occurring owing to the overlapping of oxygen p orbitals with tantalum t_2g orbitals and the formation of impurity levels near the conduction band, leads to the generation of free carriers, which make a paramagnetic contribution to the magnetic susceptibility.