Anisotropy of microhardness of β-BaB2O4 single crystals

The anisotropy of microhardness of β-barium borate single crystals β-BaB₂O₄ (BBO) is studied by the sclerometry method on the (0001) basal plane, the $(10\bar 10)$ plane of the hexagonal prism, and the $(11\bar 20)$ plane of the trigonal prism. It is shown that the anisotropy observed in the crystal is determined by the directions of covalent B-O bonds. It is established that the anisotropy of microhardness correlates with the system of cleavage planes.     

Program for calculating the scattering parameters used in the X-ray standing wave method

A computer program for calculating the complex kinematic scattering parameters χ₀, χ_h, and $\chi _{\bar h} $ of X rays, which are Fourier components of the crystal complex susceptibility, as well as some of their combinations, is presented. The values calculated by the program can be used for computer simulation of experimental results obtained by the X-ray standing-wave method. Methods for calculating these parameters on the basis of well-known tables are described in detail. For crystals of complex structure, it is necessary to know their structure and the Debye temperature or specific heat capacity in order to calculate χ_h and $\chi _{\bar h} $ . To calculate χ₀, it is sufficient to know the chemical formula and the density of a material.     

Regular domain structure in a lithium niobate crystal—Period stabilization

High stability of the period and homogeneity of a regular domain structure was attained in Nd: Mg: LiNbO₃ crystals grown from melt with an excess of lithium oxide by the Czochralski method along the normal to the close-packed $\{ 01\bar 12\} $ face.     

Formation of zinc oxide quasibicrystal structures

Quasibicrystal structures with interblock boundaries of epitaxial zinc oxide layers on a sapphire substrate along a given direction have been obtained for producing submicron electronic devices. It is shown that the use of the buffer technique allows one to grow on one (10 $\bar 1$ 2) α-Al₂O₃ substrate ZnO layers oriented in the (11 $\bar 2$ 0)and (0001) planes with clearly pronounced interlayer boundaries. The morphology and structural characteristics of these layers are studied.     

Flexoelectric effect and statistical properties in polar liquid crystals

The structural properties and flexoelectric coefficients e ₁ and e ₃ of polar liquid crystals (LCs) such as 4-n-pentyl-4′-cyanobiphenyl (5CB) are studied in the nematic phase by the molecular-dynamics and statistical-mechanics methods. A number of order parameters, $\bar P_{2L} (L = 1,2,3)$ and $\bar P_2 (r)$ , and orientational correlations ξ_λ for the nearest and next-nearest neighbors were investigated. The calculations show the absence of spontaneous polarization $(\bar P_1 \simeq 0)$ in the nondeformed polar 5CB liquid crystal over the entire range of temperatures corresponding to the nematic phase. The origination of spontaneous polarization (characterized by two independent flexoelectric coefficients e ₁ and e ₃) in response to an external deformation of a 5CB sample is studied. The calculated e ₁ and e ₃ coefficients agree well with the experimental data for 5CB obtained by the pyroelectric method.     

Modification of birefringence properties of nanostructured silicon with a change in the level of substrate doping with boron

Birefringence of porous-silicon films prepared by electrochemical etching of boron-doped Si(110) wafers with a resistivity of 25–45 mΘ cm has been studied. The samples are found to exhibit the properties of a negative uniaxial crystal with the optical axis oriented along the [1 $\bar 1$ 0] crystallographic direction. The possibility of using porous-silicon films as phase plates for light-polarization control in the near and mid-IR ranges is demonstrated.     

The first observation of dislocation blocking in pure metal without external stress

Self-blocking of c + a edge dislocations at second-order pyramidal slip in magnesium single crystals whose axis is parallel to the c axis has been found. Self-blocking is confirmed by the dislocation extension along a selected direction in the absence of external stress. Transmission electron microscopy (TEM) images of (c + a) dislocations extended in the $\left\langle {1\bar 100} \right\rangle$ directions are obtained. A model of a two-valley potential relief in Mg is proposed: the Peierls relief determines the second-order slip of (c + a) dislocation in the pyramid plane, while the immersion in deep valleys is related to splitting of the c + a edge segment (or the spread of its core) in the basal plane.     

Study of synthetic hydroxyapatite by the method of high-resolution transmission electron microscopy: Morphology and growth direction

Hydroxyapatite crystals grown from aqueous solutions have been studied by the methods of high-resolution electron microscopy and transmission electron diffraction. Processing of the experimental electron micrographs with the use of the Digital Micrograph program and the study of the corresponding Fourier trans-forms showed that the submicron microcrystals grow mainly along the [0001] direction. The (0001) and the $(01\bar 10)$ planes are perpendicular and parallel to the long edge of the crystals, respectively. The good accord between the experimental electron-microscopy images and the electron microscopy images calculated by the EMS program was attained only for crystals with the thicknesses ranging from one to five lattice periods. This allows us to state that hydroxyapatite grows from aqueous solutions in the form of very thin (with the thickness of the order of several lattice parameters) platelike crystals.     

On the problem of transient hydrodynamic instability of nematics in magnetic field: I. Instability at splay deformation

A dependence of the square of dimensionless magnetic-field (MF) strength on the square of dimensionless wave vector of the domain structure, $h^2 \left( {\tilde q^2 } \right)$ , is analytically derived for the transient hydrodynamic instability arising at splay deformation under a MF. The domain formation is related to the fold catastrophe; the square of the critical field of domain formation, h _D ² , is determined from the condition ${{\partial \left[ {h^2 \left( {\tilde q^2 } \right)} \right]} \mathord{\left/ {\vphantom {{\partial \left[ {h^2 \left( {\tilde q^2 } \right)} \right]} \partial }} \right. \kern-0em} \partial }\left( {\tilde q^2 } \right) = 0$ . In the general case, the function $h^2 \left( {\tilde q^2 } \right)$ is a ratio of two polynomials whose coefficients for calamitics are determined by combinations of four dimensionless viscosities ν₂/ν₁, ν₆/ν₁, ν₇/ν₁, and ν₈/ν₁. Under some assumptions, the quotient of the polynomials at large $\tilde q^2$ is a quadratic function which allows one to experimentally determine the dimensionless viscosities ν₂/ν₁, ν₆/ν₁, ν₇/ν₁, and α₂/ν₁.     

New icosahedral nanoclusters in crystal structures of intermetallic compounds: Topological types of 50-atom deltahedra D50 in samson phases β-Mg2Al3 and ?-Mg23Al30

A database of intermetallic compounds has been compiled using the TOPOS program package. This database includes 514 topological types, containing 12- and 13-atom icosahedral i clusters. An isolated group of 1649 i clusters is described by 14 point groups and their maximum symmetry D _3d ( $\bar 3$ m) and T _h (m $\bar 3$ ) is established, respectively, in 47 and 25 types of crystal structures. A structural analysis of the outer quasispherical shells showed that local 63-atom i configurations 1@12@50, which contain 50 atoms in the second layer, are implemented in 8 out of 19 cases. Examples of new topologically different types of 50-atom D50 deltahedra in the Samson phases ?-Mg₂₃Al₃₀ and ??-Mg₂Al₃ are presented. Four topologically different sites with coordination numbers of 5, 6, 6, or 7 are established in the ? shell and seven sites with coordination numbers of 5, 5, 6, 6, 6, 6, or 7 are found in the ?? shell. The inner i clusters for the ??-Mg₂Al₃ structure (with the symmetry $\bar 3$ m) and the ?-Mg₂₃Al₃₀ structure (with the symmetry $\bar 3$ ) have a similar chemical composition, i.e., Mg₇Al₆ and Mg₆Al₇, and their 50-atom shells are chemically identical to 18Mg + 32Al. The configurations found supplement the series of known two-layer icosahedral Bergman and Mackay clusters in the form of deltahedra with 32- and 42-atom shells.     

Ba1−x R x F2 + x Phases (R = Gd-Lu) with distorted fluorite-type structures—products of crystallization of incongruent melts in the BaF2-RF3 Systems (R = Gd-Lu). III. Defect Ba0.75Lu0.25F2.25 structure. A new {Lu8[Ba6F71]} supercluster of defects

The structure of Ba_0.75Lu_0.25F_2.25 crystals grown from melt has been studied by X-ray diffraction analysis (4729 measured reflections, 269 independent reflections with I > σ (I), R = 1.1%, R _w = 0.7%). The crystals are crystallized in the cubic system, sp. gr. Pm $\bar 3$ m, with the lattice parameter a = 5.9870(9) Å. A new complex of defects is singled out—a supercluster of the composition {R ₈[Ba₆F₇₁]}. This supercluster differs from the well-known rare earth octahedral supercluster of the composition {Ba₈[R ₆F_68-69]} because its nucleus is formed not by RE cations but by an alkali earth cation, Ba. The {R ₈[Ba₆F₇₁]} supercluster has a configuration close to that of the [B₁₄F₆₄] fragment of the fluorite structure and can replace the latter isomorphously. The model of the Ba_0.75Lu_0.25F_2.25 crystals consisting of coherently intergrown isostructural microphases having different chemical compositions is characterized by the good agreement of the calculated and experimentally determined occupancies of the F^1? positions. The comparison of the Ba_0.8Yb_0.2F_2.2 (phase studied earlier) and Ba_0.75Lu_0.25F_2.25 structures demonstrates the evolution of the defect structure along the series of rare earths with the corresponding change of the sp. gr. Fm $\bar 3$ m by the sp. gr. Pm $\bar 3$ m.     

Crystal structure of tisinalite Na2(Mn,Ca)1 − x (Ti,Zr,Nb,Fe3+)[Si6O8(O,OH)10]

Crystal structure of tisinalite from the Lovozero alkaline massif (the Kola Peninsula) was established by single-crystal X-ray diffraction analysis (SYNTEX $\bar P1$ diffractometer, λMoK_α radiation, 2θ/θ scanning mode). The structure solution (SHELX97 program package, R _hkl = 0.0565, 951 independent reflections, anisotropic refinement of thermal atomic displacements) confirmed that tisinalite belongs to the lovozerite structure type (sp. gr., $\bar P1$ , a = 10.036(5) Å, c = 12.876(9) Å, Z = 3). The difference between the structure of tisinalite and the structures of the minerals of the lovozerite group established earlier consists in the nature of the occupancy of both cation and anion positions.     

Peculiar reflections in diffraction patterns as indicators of structural type and quality

Reflections serving as indicators of the types of packets forming crystal structures of many layered semiconductors have been revealed in diffraction patterns. It is found that the values l for the strongest reflection in series 000l and 00l, as well as the next to the strongest reflection in series \(hh\bar 2\bar hl\) (h = const) and 0kl (k = const) for hexagonal and monoclinic structures, respectively, determine the number of polyhedral (Tand O) cation-filled layers per cell and indicate the types of packets TOT \(TO\bar TE\) , \(TO\bar T\bar TE\) , \(TOO\bar TE\) , \(TTO\bar T\bar TE\) , \(OOE_1 TO\bar TE_1 \) and OOE \(OOE_1 TO\bar TE_2 TO\bar TE_1 \) , where T and \(\bar T\) are inversely oriented tetrahedra, O is an octahedron, E is an empty interpacket layer, and E₁ and E₂ are partially filled (to less than 1/3) interpacket layers.     

Orientation of optical axes in absorptive crystals with arbitrary permittivity tensors

The direct relation has been established between the orientations of the optical axes and the components of the inverse complex permittivity tensor of an absorptive crystal possessing an arbitrary symmetry. The particular forms of the general relationships for triclinic crystals, characterized by an arbitrary degree of absorption, are obtained for crystals of all the other symmetry systems. The relationships obtained acquire a substantially simplified form for weakly absorptive crystals. The consideration is based on the relationship established for the components of the inverse permittivity tensor in the Cartesian coordinates and the diadic form of this tensor, which is related to the directions of the optical axes.     

Kinetics of hydrothermal crystallization of quartz in Na2CO3 solutions

The kinetics of crystallization of the {0001}c, {01 $\bar 1$ 1}r, and {10 $\bar 1$ 1}R faces of quartz in 0.5 M Na₂CO₃ (M is molarity) aqueous solutions has been studied in the temperature range 200–450°C. It is established that the dependence of the crystal growth rate on temperature in the logV-1/T, K coordinates is of a parabolic nature. It is most probable that the nonlinearity of this dependence is associated with a deficiency in the solution of silica monomers, taking part in the elementary event of quartz crystallization. The causes of a jumpwise decrease in the activation energy of the growth of the c, r, and R faces at t > 280–325°C are considered.     

Investigation into the growth and structure of thin-film solid solutions of iron-based superconductors in the FeSe0.92-FeSe0.5Te0.5 system

Thin films of FeSe_0.92 and FeSe_0.5Te_0.5 iron chalcogenide superconductors and solid solutions containing these components in different ratios have been grown on the surface of LaAlO₃ (10 $\bar 1$ 2) crystals by pulsed laser deposition. Films of solid solutions have been deposited by simultaneous laser ablation from two targets of the FeSe_0.92 and FeSe_0.5Te_0.5 stoichiometric compositions onto one substrate. An X-ray diffraction study of the film structure shows that the films grown are epitaxial and their lattice parameters regularly vary with the ratio of the deposited components, which was controllably varied by changing the ablation intensities from the targets.     

Refinement of the crystal structure of sanidine-like feldspar

The crystal structure of sanidine-like feldspar of the composition KAlSi₃O₈ from the Khibiny alkaline massif (the Kola Peninsula) has been refined (X-ray diffraction analysis; automated Syntex $P\bar 1$ diffractometer; 2θ: θ scanning technique; 2320 reflections; R _(hkl) = 0.0409; anisotropic refinement; AREN program package). The data obtained for KAlSi₃O₈ are: a = 8.615(9), b = 13.030(7), c = 7.200(5) Å, α = 89.99(5)°, β = 116.01(6)°, γ = 89.98(7)°, Z = 4, sp. gr. $C\bar 1$ . Microtwinning revealed in the crystal structure of the mineral explains the simultaneous existence of two structural-optical types in one sample—“high” and “low” sanidines.     

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.     

Refined crystal structure of iron-rich triclinic astrophyllite

Crystal structure of Fe-rich triclinic astrophyllite K₂NaFe₇ [Ti₂Si₈O₂₆F](OH)₄ is refined (a Syntex P $\bar 1$ automatic diffractometer, 3809 reflections, 2θ/θ scan, R = 0.041): a = 5.365(2), b = 11.88(1), c = 21.03(2) Å, α = 84.87(6)°, β = 92.25(5)°, γ = 103.01(4)°, sp. gr. A $\bar 1$ , Z = 2, d _calcd = 3.29 g/cm³. The refined structure is identical to that reported earlier. The structure is built by three-layer TOT sheets in which an O layer of Fe-octahedra is sandwiched between the T layers consisting of Si-tetrahedra and Ti-octahedra. It is established that differently chosen unit cells of the mineral are interrelated.