Investigation of the crystallization features,atomic structure,and microstructure of chromium-doped monticellite

A series of Cr⁴⁺:CaMgSiO₄ single crystals is grown using floating zone melting, and their microstructure, composition, and crystal structure are investigated. It is shown that regions with inclusions of second phases, such as forsterite, akermanite, MgO, and Ca₄Mg₂Si₃O₁₂, can form over the length of the sample. The composition of the single-phase regions of the single crystals varies from the stoichiometric monticellite CaMgSiO₄ to the solid solution Ca_{(1 ? x)}Mg_{(1 + x)}SiO₄(x = 0.22). The Cr:(Ca_0.88Mg_0.12)MgSiO₄ crystal is studied using X-ray diffraction. It is revealed that, in this case, the olivine-like orthorhombic crystal lattice is distorted to the monoclinic lattice with the parameters a = 6.3574(5) Å, b = 4.8164(4) Å, c = 11.0387(8) Å, β = 90.30(1)^o, Z = 4, V = 337.98 ų, and space group P2₁/c. In the monoclinic lattice, the M(1) position of the initial olivine structure is split into two nonequivalent positions with the center of symmetry, which are occupied only by Mg²⁺ cations with the average length of the Mg-O bond R _av = 2.128 Å. The overstoichiometric Mg²⁺ cations partially replace Ca²⁺ cations (in the M(2) position of the orthorhombic prastructure) with the average bond length of 2.347 Å in the [(Ca,Mg)-O₆] octahedron. The average distance in SiO₄ distorted tetrahedra is 1.541 Å.     

Liquid-phase epitaxy and optical absorption of gadolinium-gallium garnet films grown from a PbO-B2O3 solution melt on Gd3Ga5O12 substrates

The optical absorption due to impurity ions was studied in gadolinium-gallium garnet single crystal films with a stoichiometric composition Gd₃Ga₅O₁₂ grown by liquid-phase epitaxy from a supercooled PbO-B₂O₃ solution melt on (111)-oriented Gd₃Ga₅O₁₂ substrates.     

Growth of single crystals of monticellite CaMgSiO4: Cr from melt by floating zone technique and the study of their composition

Crystallization of monticellite CaMgSiO₄ from melt by the floating-zone technique was studied at different compositions of feed rods. The compositions of rapidly cooled melts and different regions of grown crystals were investigated by the X-ray phase and microprobe analyses. The inclusions of secondary phases were identified. The range of most promising compositions of the charge for technology of growing large high-quality crystals is established. The mathematical model of the changes in the melt composition during monticellite crystallization is constructed.     

Long-term stability of Bi(2223) and Dy(123) superconducting tapes in the direct current circuit

The stability of critical parameters T _c and I _c of commercial high-temperature superconducting wires upon long-term passage of transport current (about 0.7I _c) in liquid nitrogen (77 K) is studied. Voltage-current characteristics U(I), as well as the critical current and critical temperature, are investigated for the case of Bi(2223) hermetic multifilament wires and Dy(123) superconducting tapes covered by a thin Ag layer. In the former case, a considerable decrease in the critical current (by ～30%) and swelling of the wires after passage of the current for 323 h are observed. The same is true for a reference sample, which does not experience the action of current and stays in liquid nitrogen for 700 h. The decrease in the critical current in the Bi(2223) sample is likely to be associated with penetration of a liquid coolant into the composite conductor: evaporating and expanding as a result of heating, it severely deforms the material. The Dy(123) sample grown epitaxially demonstrates high stability of the critical current after it has experienced the action of current for 400 h and been kept in liquid nitrogen for 1000 h.     

GSAG and YSAG: a study on isomorphism and crystal growth

The composition of gadolinium-scandium-aluminium (GSAG) and yttrium-scandium-aluminium (YSAG) garnet crystals grown by the Czochralski method and by floating zone melting was analysed. It is shown that Sc³⁺ and Al³⁺ ions in GSAG and Y³⁺, Sc³⁺ and Al³⁺ ions in YSAG can simultaneously occupy two types of sites in the lattice. The congruent melting composition of GSAG is {Gd_2.88Sc_0.12} [Sc_1.77Al_0.23] (Al₃)O₁₂. The homogeneity range of GSAG is rather narrow. In the Y₂O₃-Sc₂O₃-Al₂O₃ system a wide range of garnet-structured solid solutions exist. However, there is no ternary garnet that melts congruently. Distribution coefficients of components in YSAG are slightly varied from unity in the whole concentration range, and as the melt composition approaches YAG they tend to unity. Cr³⁺ and Nd³⁺ distribution coefficients in GSAG and YSAG are presented.

     

Preparation of nanopowdered M1? x R x F2+ x (M = Ca, Sr, Ba; R = Ce, Nd, Er, Yb) Solid Solutions

The synthesis procedure has been worked out, and nanopowders of fluoride solid solutions (ss) Ca_1−x R _x F_2+x (R = Er, Yb), Sr_1−x Nd _x F_2+x , and Ba_1−x Ce _x F_2+x have been manufactured by coprecipitation from aqueous solutions. The powders consist of rounded particles with sizes from 50 to 150 nm. The particles with sizes of about 150 nm are aggregates of adhered nanoparticles. The nanoparticles are highly reactive and sinter at low temperatures (T < 0.35T _m). Original Russian Text ? S.V. Kuznetsov, I.V. Yarotskaya, P.P. Fedorov, V.V. Voronov, S.V. Lavrishchev, T.T. Basiev, V.V. Osiko, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 3, pp. 364–369.     

Optical absorption by Nd<Superscript>3+</Superscript> and Gd<Superscript>3+</Superscript> ions in epitaxial films grown on Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> substrates from a lead-containing solution melt

Epitaxial films of composition (Gd,Nd)₃Ga₅O₁₂ or (Gd,Y,Nd)₃Ga₅O₁₂ with a neodymium content varying from 0.3 to 15 at. % are grown by liquid-phase epitaxy from a supercooled PbO-B₂O₃-based solution melt on Gd₃Ga₅O₁₂(111) substrates. The optical absorption spectra of the epitaxial films grown are measured in the wavelength range 0.2–1.0 µm. The results of interpreting the absorption bands observed in the spectra are used to construct the energy level diagrams of Nd³⁺ and Gd³⁺ ions in the matrices of the epitaxial films.     

Investigations in the field of perinone compounds

A simple method for preparing perinones from peri-dinitro compounds has been obtained. It has been shown that the reaction takes place via the formation of naphthalene-1, 8-diamine and 2-(2-carboxyphenyl)-perimidine.