Czochralski growth and characterization of Dy: La3Ga5.5Nb0.5O14 single crystals

Dysprosium (Dy) doped La₃Ga_5.5Nb_0.5O₁₄ single crystals were grown by the traditional Czochralski method along z‐axis. The structure of the crystal has been studied by X‐ray powder diffraction method, and the unit‐cell parameters are calculated to be a=8.22070 Å, c=5.12533 Å and V=299.965 ų. The segregation coefficient of Dy³⁺ in La₃Ga_5.5Nb_0.5O₁₄ crystal was measured by X‐ray fluorescence analysis. For 1 mol% doping level in the melt, the distribution coefficient of Dy³⁺ was determined to be 0.341 wt%. Specific heat, thermal expansion and transmission spectrum of Dy: La₃Ga_5.5Nb_0.5O₁₄ single crystals have been measured. The fluorescence spectra of Dy³⁺: La₃Ga_5.5Nb_0.5O₁₄ crystals were measured at room temperature, and there were four emission transitions occurring at 479, 576, 662 and 754 nm, respectively. The fluorescent lifetimes measurement results show 1.0% Dy: La₃Ga_5.5Nb_0.5O₁₄ possesses shorter fluorescence decay time (303.4 μs) than does 1.0%Dy:LGS (436.12 μs). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Neutron Diffraction Investigation of Langanite Crystals

The upper and lower parts of a single crystal with a nominal composition La₃Ga_5.5Nb_0.5O₁₄grown by the Czochralski method (Ir crucible, 99%Ar + 1%O₂, growth direction) have been studied by neutron diffraction for the first time. The compositions of the upper and lower parts are found to differ ((La_2.935(2)□_0.065)(Ga_0.450Nb_0.550(3))Ga₃(Ga_1.965(4) □_0.035)(O_13.90(1) □_0.10) and (La_2.940(1) □_0.060)(Ga_0.590Nb_0.410(2))Ga₅(O_13.82(1) □_0.18), respectively), and microtwin formation was observed in the upper part, with two unit cells related by the translation 1/2Z. Based on a comparison of the refined crystal composition and optical transmission spectra, the absorption bands at 30 000, 24 500, and 20 500 cm_-1 were assigned to divacancies (2 V _La^', 3 V _O^..), oxygen vacancies, and color centers, respectively. A relationship between the crystal color and oxygen content is established.     

<Emphasis Type="Italic">Ab Initio</Emphasis> calculation of elastic and electromechanical constants of langasite family crystals

Elastic and electromechanical (piezoelectric) constants for six langasite family crystals Ca₃TaGa₃Si₂O₁₄, Sr₃TaGa₃Si₂O₁₄, Ba₃TaGa₃Si₂O₁₄, La₃Ta_0.5Ga_5.5O₁₄, La₃Nb_0.5Ga_5.5O₁₄, La₃Ga₅SiO₁₄ are calculated within the density-functional perturbation theory.     

Comparison of the Spectral Characteristics of Chromium-Containing Crystals of the Calcium Gallogermanate Type with Their Structural Features

The molecular refractions and characteristics of the circular dichroism spectra of unactivated and chromium_activated La₃Ga_5.5Ta_0.5O₁₄, La₃Ga_5.5Nb_0.5O₁₄, La₃Ga₅GeO₁₄, La₃Ga₅SiO₁₄, Sr₃Ga₂Ge₄O₁₄, and Ca₃Ga₂Ge₄O₁₄crystals are compared with their structural parameters. Correlations between the molecular refractions and the structural distortions responsible for the optical activity of crystals and correlations between the characteristics of circular dichroism bands of defects and chromium ions and the interatomic distances in polyhedra and their distortions are found. The calcium gallogermanate structures exhibit a unique correspondence between the electronic configuration of host ions and the local symmetry.     

Crystal structure of La3Nb0.5Ga5.5O14 at 20 K

The atomic structure of La₃Nb_0.5Ga_5.5O₁₄ single crystals (space group P321, R = 2.21%, R _w = 2.31%) at 20 K was determined using X-ray diffraction. A comparative analysis of the structural characteristics determined at temperatures of 293 and 20 K did not reveal a noticeable redistribution of bond valences that could be considered precursors of a possible phase transition accompanied by the reduction of the symmetry of the crystal. The assumption was made that the anomalous behavior of the permittivity e ₃₃ of the La₃Nb_0.5Ga_5.5O₁₄ single crystals is associated with the decrease in the possible relative displacements of the Ga(1) and Nb atoms occupying the cation position with point symmetry 32 in a 1: 1 ratio.     

Refinement of the crystal structures of the La3Ta0.5Ga5.5O14 and La3Nb0.5Ga5.5O14 compounds

This paper reports on the results of accurate X-ray structural investigations of single crystals La₃Ta_0.5Ga_5.5O₁₄ (a = 8.2260(1) ?, c = 5.1207(1) ?, R/R _w = 1.09%/1.10%, 3868 unique reflections) and La₃Nb_0.5Ga_5.5O₁₄ (a = 8.2237(1) ?, c = 5.1247(1) ?, R/R _w = 1.02%/1.03%, 3735 unique reflections) (space group P321, Z = 1, sinϑ/λ|_max ≈ 1.34 ?⁻¹ for both compounds). Using sets of data with a large number of high-angle reflections makes it possible to reliably compare the crystal structures by applying statistical tests. The structural differences between the La₃Ta_0.5Ga_5.5O₁₄ and La₃Nb_0.5Ga_5.5O₁₄ crystals are weakly pronounced, which correlates with the closeness of their piezoelectric characteristics. Original Russian Text ? A.P. Dudka, B.V. Mill, Yu.V. Pisarevsky, 2009, published in Kristallografiya, 2009, Vol. 54, No. 4, pp. 599–607.     

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.     

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.     

Electro‐optic,Piezoelectric and Dielectric Properties of Langasite (La3Ga5SiO14), Langanite (La3Ga5.5Nb0.5O14) and Langataite (La3Ga5.5Ta0.5O14)

Refractive indices were determined of single crystals of La₃Ga₅SiO₁₄ (langasite, LGS), La₃Ga_5.5Nb_0.5O₁₄ (langanite, LGN) and La₃Ga_5.5Ta_0.5O₁₄ (langataite, LGT) in the wavelength region between 0.36 μm and 2.33 μm. While phase‐matched optical second harmonic generation is not possible in LGS it occurs in the isotypic compounds LGN and LGT. Temperature‐dependent examination of the dielectric properties of LGS up to 600 °C showed anomalous behaviour. For all three substances the electro‐optic [r^σ_ijk] (“unclamped”) and the piezoelectric [d_ijk ] tensors were determined at room temperature. In addition, the temperature‐dependence of these properties was studied for LGS between –200 °C and +200 °C by a Jamin interferometer in combination with a modified Sénarmont compensator.     

Phase transitions in compounds with the Ca<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> structure

Possible structural changes described by the group-subgroup relationships in the Ca₃Ga₂Ge₄O₁₄-type structure (sp. gr. P321) are considered. The most probable phase transitions seem to be those accompanied by lowering of the symmetry to the maximal non-isomorphic subgroups P3 and C2. It is shown that only destructive phase transitions accompanied by symmetry rise up to the minimal non-isomorphic supergroups for the given structure type can take place. The change of the trigonal symmetry to monoclinic is revealed in La₃SbZn₃Ge₂O₁₄, whose crystal structure is refined as a derivative structure of the Ca₃Ga₂Ge₄O₁₄ structure type within the sp. gr. A2 (C2). At ～250°C, La₃SbZn₃Ge₂O₁₄ undergoes a reversible phase transition accompanied by symmetry rise, A2 ? P321. Similar phase transitions, P321 ? A2, are also observed in La₃Nb_0.5Ga_5.5O₁₄ and La₃Ta_0.5Ga_5.5O₁₄ under the hydrostatic pressures 12.4(3) and 11.7(3) GPa, respectively. The mechanisms of compression and phase transition are based on the anisotropic compressibility of a layer structure. With the attainment of the critical stress level in the structure, the elevated compressibility in the (ab) plane gives rise to a phase transition accompanied by the loss of the threefold axis. Attempts to reveal low-temperature phase transitions in a number of representatives of the langasite family have failed.     

Growth and characterization of La3Ga5.5Ta0.5O14 crystal

La₃Ga_5.5Ta_0.5O₁₄ (LGT) crystal was grown by using the Czochralski method. The as‐grown crystal is transparent, free from inclusions and with no cracks. Specific heat, thermal expansion, dielectric constants, transmission spectrum and optical damage threshold of LGT have been measured, and the results show general properties of LGT are similar to that of La₃Ga₅SiO₁₄ (LGS) crystal. The experiment to research the Q‐switch properties of LGT has been performed and the results show LGT possesses smaller electrooptic coefficients than that of LGS and may not be an ideal material used as a Q‐switch. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

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.     

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.     

Effects of Er3+ doping concentration and calcination on fluorescence properties of La3Ga5.5Nb0.5O14 nanocrystals

The effects of Er³⁺ doping concentration and calcination were examined on the fluorescence properties of La₃Ga_5.5Nb_0.5O₁₄ (Er:LGN) nanoparticles for the first time. High quality Er:LGN nanoparticles were synthesized by sol‐gel method. The room temperature fluorescence spectra showed a green emission, which can be attributed to ²H_11/2‐⁴I_15/2 and ⁴S_3/2‐⁴I_15/2 transition. The relationship between the relative emission intensity and the doping concentration was investigated. The maximum of the Er³⁺ doping concentration in LGN nanopowders is 2.0%. The fluorescent lifetime of 2.0% Er:LGN nanoparticles is 1.45ns. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of Nd:Sr3Ga2Ge4O14 crystals by a modified Bridgman method

Bridgman growth of Nd:SGG (Sr₃Ga₂Ge₄O₁₄) crystals has been investigated for the first time. Pt crucible of ∅︁25mm×250mm with a seed well of ∅︁10mm×80 mm is used, and seed is SGG crystal of ∅︁10mm×50mm grown by Bridgman method in advance. The growth parameters are optimized as the furnace temperature is set to 1450∼1500°C, temperature gradient in the crystal‐melt interface is less than 25 K/cm and growth rate is less than 0.5mm/h. The Nd:SGG crystals with 25mm in diameter and 60mm in length are grown successfully from 1.5 to 8at% Nd³⁺ doped stoichiometric Sr₃Ga₂Ge₄O₁₄ melt. The distribution coefficient and concentration of Nd³⁺ in Nd:SGG crystals are obviously higher than those of Nd:YAG crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and properties of lanthanum galliogermanate glasses

Lanthanum galliogermanate glasses were prepared. Raman spectra, molar volumes, glass transition temperatures and activation energies for glass transition and crystallization were obtained. For glasses having the same La₂O₃/GeO₂ ratio, the molar volumes increase with the Ga₂O₃ content, and the glass transition temperatures, activation energies for glass transition and crystallization, increase initially then decrease as the ratio of Ga₂O₃/GeO₂ increases. For glasses having the same Ga₂O₃/GeO₂ ratio, the molar volumes increase with La₂O₃ content, and the glass transition temperatures increase as the ratio of La₂O₃/GeO₂ increases. The change of glass structure and its properties with composition is correlated with the concentration of lanthanum ion.     

Formation of new oxide glasses by laser spin melting and free fall cooling

Glass spherules have been made by laser spin melting and free fall cooling techniques from ceramic rods. Thus, the materials were not in contact with a container at any time. Glasses of 100 to 800 μm diameter were formed from at least 80 wt% of the oxides Al₂O₃, Ga₂O₃, In₂O₃, La₂O₃, ZrO₂, HfO₂, Nb₂O₅ and Ta₂O₅ with 20 or less weight percent of CaO + SiO₂. The best glass formers were in the Nb₂O₅- and Ta₂O₅-based systems. The indices of refraction of most of the glasses and Abbe numbers of many were measured.     

Single‐crystal growth and characterization of mullite‐type orthorhombic Bi2M4O9 (M = Al3+, Ga3+, Fe3+)

Pure and homogeneous single crystals of orthorhombic mullite‐type Bi₂M₄O₉ (M = Al³⁺, Ga³⁺, Fe³⁺), and a mixed Bi₂Fe_1.7Ga_2.3O₉ crystal from an equimolar Ga/Fe composition were grown by the top seeded solution growth (TSSG) method. All these compounds melt incongruently in the range of about 800 and 1100 °C. In case of bismuth gallate and ferrate inclusion‐free crystals with dimensions up to several cubic centimeters can be grown. Limited solubility in Bi₂O₃ and the high steepness of the liquidus curve are the reasons for getting only small imperfect bismuth aluminate crystals. In contrast to ceramic materials preparation reported in literature, divalent calcium and strontium could not be incorporated into the mullite‐type structure during the melt growth process. Several fundamental physical properties like heat capacity, thermal expansion, heat conductivity, elastic constants, high‐pressure behavior and oxygen diffusivity were determined by different research groups using single‐crystalline samples from the as‐grown materials. Furthermore, the refractive indices of Bi₂Ga₄O₉ were measured in the range of 0.430 and 0.700 μm. Such as many other bismuth containing compounds the refractive indices of Bi₂Ga₄O₉ are larger than 2, and Bi₂Ga₄O₉ is an optic biaxial positive crystal. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lattice Parameter Local Determination for Trigonal,Hexagonal, and Tetragonal Crystal Systems Using Several Coplanar X-Ray Reflections

The method for determining the local lattice parameters using quasi-multiple X-ray diffraction (which was proposed and used only for crystals of the cubic system) has been expanded to measure the local crystal lattice parameters for the trigonal, hexagonal, and tetragonal systems. Local variations in the lattice parameters in the tetragonal paratellurite (TeO₂) and trigonal lanthanum-gallium tantalate (La₃Ga_5.5Ta_0.5O₁₄) crystals have been investigated. Reflections necessary for implementing the quasi-multiple X-ray diffraction scheme are found for these crystals. The pairs of reflections that were found were used to complete quasi-multiple X-ray diffraction schemes on a laboratory X-ray source and determine the variation in the lattice parameter a along the surface of these crystals. The relative measurement error was 4 x 10^-6 at a spatial resolution of 140 μm. The accuracy of the method and the parameters determining this characteristic were analyzed based on the results. The locality of the method and the limits of the spatial resolution of an X-ray diffraction measurement of lattice parameters are estimated.