Point defects in langatate crystals

Langatate crystals of the general composition La₃(Ga_0.5Ta_0.5)Ga₅O₁₄, grown by the Czochralski method, have been investigated by neutron diffraction (single crystals) and X-ray diffraction (ground single crystals). The crystals were grown in an atmosphere of 99% Ar + 1% O₂ in the Y54° direction (rotation by 54° with respect to the y axis), without subsequent annealing (orange crystal) or with vacuum annealing (colorless crystal). It is established that colorless crystals have a higher gallium content and, therefore, a larger number of oxygen vacancies in comparison with colored crystals; this is a possible reason for their lower microhardness.     

Circular dichroism spectra of langasite family crystals in the range of electronic transitions of structure defects

The absorption and circular dichroism spectra of langasite family crystals are studied. Wide bands in the range of 285–500 nm that are related to the structure lattice defects are found. For all the crystals, these bands are in approximately the same spectral region, have identical structures, and can be attributed to one type of defects formed by cation vacancies and excess oxygen atoms in the optically active positions.     

Circular dichroism of some Nd-doped crystals of the langasite family

The absorption and circular-dichroism spectra of neodymium-doped crystals of langasite La₃Ga₅SiO₁₄ and the compounds La₃Ga_5.5Nb_0.5O₁₄ and Pb₃Ga₂Ge_3.92Si_0.08O₁₄, isostructural with langasite, are investigated in the wavelength range 320–940 nm. Electronic transitions in Nd atoms, which substitute the main cation of the salts (La or Pb) in the positions with the local symmetry C₂, are observed in the spectra. All transitions observed in the absorption spectrum are also active in the circular-dichroism spectrum. Detailed analysis is performed for several (well separated) bands: Dipole strengths D _ok , rotation strengths R _ok , and the anisotropy factors G _ok are calculated. Some specific features of the spectra obtained are noted and their possible relationship with the structure of impurity centers of neodymium and effect on the intensity of the Stark components are discussed.     

The absorption and circular dichroism spectra of langasite family crystals doped with chromium ions

The absorption and circular dichroism spectra of langasite family crystals, La₃Ga₅SiO₁₄, La₃Ga₅GeO₁₄, Ca₃Ga₂Ge₄O₁₄, Sr₃Ga₂Ge₄O₁₄ (red), Sr₃Ga₂Ge₄O₁₄ (green), La₃Ta_0.5Ga_5.5O₁₄, and La₃Nb_0.5Ga_5.5O₁₄, which were doped with chromium ions, have been investigated in the range of 240–850 nm. It is shown that chromium ions are incorporated into the structure of the investigated crystals both in the octahedrally (Cr³⁺ ion in 1a octahedron) and tetrahedrally (Cr⁴⁺ ion in 2d tetrahedron) coordinated positions. The ion ratio Cr³⁺/Cr⁴⁺ changes in a wide range in the crystals studied.     

Point defects isomerism in lithium fluoride crystals and nanocrystals

It has been established that the intrinsic radiation defects F₃ in lithium fluoride crystals and nanocrystals exist in three different configurations (isomers). It is found that at influence on a sample by a radiation with a wavelength that falls in the absorption band of one of the isomers this isomer is transformed to another one. After treatment cessation the equilibrium ratio of concentrations for these isomers is restored. The value of the energy barrier separating equilibrium states of isomers and the contribution of entropy in this barrier are defined.     

Point defects in GaP single crystals investigated by mechanical damping

The dependence of mechanical damping of LEC (liquid encapsulated Czochralski) crystals of GaP: S was measured in the temperature range 300… 600 K and at frequencies near 70 and 140 kHz, respectively. In samples of different stoichiometry two peaks could be observed: Peak A at 350 K and peak B at 450 K. Peak A is attributed to the relaxation of point defects near the surface, peak B is probably due to Ga vacancies.     

Neutron diffraction and X-ray diffraction investigations of langasite crystals

Crystals of langasite La₃Ga₄(GaSi)O₁₄ grown by the Czochralski method are studied using neutron diffraction for the first time. It is established that the compositions of the upper and lower parts of an orange crystal grown from the La₃Ga_5.14Si_0.86O₁₄ seed in an (Ar + O₂) atmosphere (the 〈 0001 〉 growth direction) can be written as(La_2.85(2)□_0.15)(Ga_0.95(2)□_0.05) Ga₃(Ga_1.15 Si_0.85(5))(O_13.72□_0.28(7)) and(La_2.89(1)□_0.11)·(Ga_0.98(1)□_0.02) Ga₃(Ga_1.06Si_0.94(4))(O_13.81□_0.19(9)), respectively. The La content in the upper and lower parts of this crystal is lower and higher than the Ga content, respectively, and the Ga content exceeds the Si content in the (GaSi) position. By contrast, in a colorless crystal of the composition(La _2.97(4)□_0.03) Ga(1)(Ga_2.94(9)□_0.06)(Ga_0.7(1) Si_1.3)(O_13.9□_0.1(1)), which is grown from the La₃ Ga₅SiO₁₄ seed in an argon atmosphere (the 〈 01\(\bar 1\)1 ?growth direction), the Ga content in the (GaSi) position is lower than the Si content. A relation between the Ga: Si ratio and the (Ga,Si)-3O interatomic distances is found.     

Temperature dependence of lattice parameters of langasite single crystals

To determine the coefficient of thermal expansion of trigonal langasite (La₃Ga₅SiO₁₄) the two independent lattice parameters a and c are measured over a temperature range of 800 °C using X‐ray diffraction on single crystal samples. From the given nonlinear temperature dependence the linear and quadratic thermal coefficients of expansion α₁₁, β₁₁ and α₃₃, β₃₃ for the two lattice parameters a and c could be deduced. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The color of langatate crystals and its relationship with composition and optical properties

Langatate crystals grown by the Czochralski method in 〈0001〉 and 〈01 1〉 directions both without subsequent annealing (from colorless to orange crystals) and with vacuum (colorless and pale‐green crystals) and air (orange crystal) annealing have been investigated by neutron diffraction, X‐ray diffraction and optical spectroscopy. The main types of point defects were compared with absorption bands: oxygen vacancies ‐ V_O^•• (0.35‐0.36 µm), the vacancies in the lanthanum position ‐ V_La‴ (0.29‐0.30 µm); the absorption band at 0.48‐0.49 µm is typical for the colored samples. The relationship between the X color coordinate (red component) and the oxygen contents has been obtained. The green color of the sample annealed in vacuum is due to the presence of Ta⁺³ ions in the La(Ta³⁺,Ga)O₃ phase that has the structural and geometric agreement with langatate structure. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical activity and crystalline structure of crystals of the langasite family

The dependences of the refraction on the structural parameters are considered for the La₃Ga_5.5Nb_0.5O₁₄, La₃Ga_5.5Ta_0.5O₁₄, La₃Ga₅SiO₁₄, Ca₃Ga₂Ge₄O₁₄, and Sr₃Ga₂Ge₄O₁₄ crystals of the langasite family. It is shown that the angle of deviation of the 1a octahedron faces that are normal to the optical axis from 60° is one of the main sources of optical activity of these crystals. The interaction of the cations belonging to the 3e dodecahedron and 1a octahedron, cations of the 3e dodecahedron and 2d tetrahedron, and the repulsion of O²⁻ ions are believed to be the basic interactions affecting the angle value. The dependences of the angle on the crystal-chemical characteristics of the considered crystals are analyzed. The role of the relative sizes of the structural polyhedra is demonstrated.     

Structural conditionality of the piezoelectric properties of langasite family crystals

The atomic displacements upon isomorphic substitutions in crystals of the langasite family have been analyzed. The thermal parameters are determined and the probability density function of atoms is analyzed. Local potential energy minima are found which can be occupied by atoms under external effects. The contributions of cations in all four independent crystallographic positions and anions in all three such positions to the piezoelectric properties are established. One specific structural feature is the constant (at isomorphic substitutions) or possible (under external effects) but always opposite displacements of two cations along symmetry axis 2. Large cations in eight-vertex polyhedra make the main contribution to the piezoelectric properties. The cations in the tetrahedra on symmetry axis 2 weaken these properties. The cations in the octahedra in the origin of coordinates and in the tetrahedra on symmetry axes 3 only slightly affect the piezoelectricity.     

Light scattering in langasite (La3Ga5SiO14) crystals

A way to plot the light-scattering indicatrix for crystals in the form of rectangular parallelepipeds of different crystallophysical orientations has been proposed. The experimental setup, which makes it possible to record the intensity of light scattered in these samples in the plane oriented perpendicular to the input face, has been designed. The light-scattering indicatrices for langasite crystals have been plotted in the XY and YZ planes. The reflectance and transmittance of the samples under the normal incidence of light have been estimated. The maximum angle at which the light scattered within the crystal bulk emerges from it without total internal reflection has been found. The angle by which langasite crystal should be rotated to record the scattered-light intensity by a photodetector has been experimentally determined.     

Point defects in barium-sodium niobate (Ba2NaNb5O15) crystals

The paper deals with studies of the nature and the kinetics of point defect formation and dissociation in barium-sodium niobate single crystals as a result of vacuum heat treatement and ionizing Co⁶⁰ γ-radiation.     

Optical activity along the optical axis of crystals with ordered langasite structure

Optical activity along the optical axis of four piezoelectric crystals Sr₃TaGa₃Si₂O₁₄ (STGS), Sr₃NbGa₃Si₂O₁₄ (SNGS), Ca₃TaGa₃Si₂O₁₄ (CTGS) and Ca₃NbGa₃Si₂O₁₄ (CNGS) with ordered langasite structure were studied via a convenient method. All the crystals investigated show unusually large values of the specific rotation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Point and extended defects in superconductors

The effect of point defects created by γ‐irradiation and extended defects created by Pb‐ion irradiation on the behavior of the critical current density, J_c in several superconductors have been investigated. We have found a limitation of the role of both types of defects on enhancing J_c. We report an explanation of the effect of γ‐irradiation and Pb‐ion irradiation doses on J_c in these materials. The explanation is based on combining several competing mechanisms of irradiation which we believe to take place mainly in the regions of the grain boundaries. The influences of these mechanisms were found to vary with the irradiation dose level. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Circular dichroism spectra of synthetic amethyst crystals

The axial circular-dichroism spectrum of a synthetic amethyst crystal has been studied for the first time. Bands both of the positive and negative signs are revealed. New information on the color centers of amethyst and their electronic structure is obtained.     

Growth defects in barium-strontium niobate crystals

The paper is concerned with the results of investigation of barium-strontium niobate chemical composition change, of the lattice parameter, microhardness of the crystal region. Analysis reveals the increase of the barium content up to 4% and the change of the crystal lattice parameter by 10₋₃ Å, leading to considerable tensions and crystal cracking. The etching of the slides perpendicular to the growth axis has been realized as well as the decoration of the fresh cleavages by anthraquinone vapours which indicate the electric inhomogenity in the crystal volume.     

Topological classification of defects in crystals

Here we review the topological approach to the classification of defects in crystals, which are deviations from crystallinity. First, we illustrate this approach using the simpler case of dislocations. We show how the fact that dislocations are completely characterized by their Burgers vectors only, is related via topological considerations to the fact that a perfect crystal is exactly reproduced by translations through a lattice vector. Later, we present the developments of the French school and that of Rogula, comparing and contrasting them. These are attempts to generalize the above idea to include consideration of other transformations that leave a perfect crystal invariant. Finally, we discuss briefly some of the problems that still remain with this approach.