A rietveld study of the cation substitution between uvarovite and yttrium-aluminum synthetic garnets,obtained by sol-gel method

A series of synthetic garnets solid solutions is compositions between Y₃Al₂Al₃O₁₂ (Y-Al garnet) (YAG) and Ca₃Cr₂Si₃O₁₂ (uvarovite) was synthesized using the sol-gel method. The expected general formula is (Ca_xY_1-x)₃ (Cr_xAl_1-x)₂ (Si_xAl_1-x)₃O₁₂ where x = 0 to x = 1. The cation distribution in dodecahedral, octahedral and tetrahedral sites and bond distances in these synthetic garnets were determined using the Rietveld method. It shows the incomplete substitution in small sites. The smaller site the smaller substitution there is.     

Ab initio calculation and spectral properties of nano- and bulk materials

This paper presents the development of ab initio calculation of the electronic structure of either clusters, nano-crystals, doped and unperfected bulk crystals. In addition, analysis of selected experimental data for γ- or plasma irradiated pure and doped wide-band gap oxides such as sapphire, α-Al₂O₃, garnet, Y₃Al₅O₁₂, Gd₃Sc₂Al₃O₁₂ and perovskites YAlO₃, SrTiO₃ is presented. Change in the crystals surface morphology and spectroscopic properties of sapphire, perovskites, garnets as well as ion oxidation state in pure and doped γ- and plasma irradiated crystals are discussed in detail using the optical and X ray spectroscopy experimental results.     

Production of Y3Al5O12 and Y2O3 nanopowders for optical ceramics

The physical and technical conditions for reproducible production of nanodispersed yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) and yttrium oxide (Y₂O₃) powders by chemical coprecipitation have been investigated. It is established that the obtained YAG nanopowders have enhanced reactivity, which significantly decreases the temperature range of interaction in the Y₂O₃-Al₂O₃ system in comparison with ceramic synthesis. It is shown that vacuum heat treatment may lead to reversible transformation of the YAG crystal structure from cubic to tetragonal.     

Growth and properties of Lu3Al5O12-RE3+ single crystals

Optically transparent garnet single crystals were grown from Lu₃Al₅O₁₂ melts containing different RE³⁺ ions. The distribution coefficient of Nd³⁺ ions is found to be a function of the growth rate. Lu₂O₃–Al₂O₃ system studies have been partially carried out. Measurements of the lattice data, absorption spectra and comparison with some properties of Y₃Al₅O₁₂ single crystals have also been made.     

Optical spectroscopy and local structure of Er3+ luminescence centres in СaO–Ga2O3–GeO2 glasses

Optical absorption, luminescence excitation and emission spectra of Er³⁺ centres in Ca₃Ga₂Ge₃O₁₂:Er glass with Er content of 1.46 wt% are presented and analysed. Luminescence kinetics for the main Er³⁺ transitions was satisfactorily described by single exponential decays with characteristic lifetimes. Oscillator strengths, phenomenological Judd–Ofelt intensity parameters, radiative decay rates (emission probabilities of transitions), branching ratios and radiative lifetimes for Er³⁺ centres in Ca₃Ga₂Ge₃O₁₂:Er glass are calculated and compared with the corresponding parameters of the Ca₃Sc₂Ge₃O₁₂:Er³⁺ garnet and other crystals and glasses. Quantum efficiency, η, of the ⁴I_13/2 → ⁴I_15/2 Er³⁺ transition is determined. Incorporation peculiarities and local structure of Er³⁺ luminescence centres in Ca₃Ga₂Ge₃O₁₂:Er³⁺ glass are discussed in comparison with garnet crystals and oxide glasses. On the basis of the presented results and referenced EXAFS data for Er, Eu and Ho impurities (L₃-edge) it has been shown that Er³⁺ centres in Ca₃Ga₂Ge₃O₁₂ glass occupy network sites with the coordination number to oxygen of N = 6.     

Y2O3-Al2O3-Nd2O3 phase diagram and the growth of (Y,Nd)3 Al5O12 single crystals

The optimum compositions of the melts used for the growth of yttrium-aluminum garnet (YAG) single crystals with different neodymium contents are determined using the phase diagram of the ternary system Y₂O₃-Al₂O₃-Nd₂O₃ with the binary sections Y₃Al₅O₁₂-Nd₂O₃ and Y₃Al₅O₁₂-Nd₃Al₅O₁₂. A number of melt compositions characterized by one garnet phase, namely, (Y,Nd)₃Al₅O₁₂, are established. Single crystals of yttrium-aluminum garnets with a high content of the activator (up to 2.6 wt % Nd) are grown by the Czochralski method. __________ Translated from Kristallografiya, Vol. 48, No. 5, 2003, pp. 945–949. Original Russian Text Copyright ? 2003 by Soboleva, Chirkin. Dedicated to the 60th Anniversary of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences     

Luminescence properties of Y3Al5O12:Eu3+-coated submicron SiO2 particles

Silica submicron spherical particles coated with an yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) layer doped with Eu³⁺ were prepared by the sol–gel method. The structure and morphology of samples determined by the X-ray powder diffraction measurements and transmission electron microscope images, respectively, indicated that well-crystallized garnet nanocrystallites were formed with successive coating cycles. Similar trends were deduced from the evolution of the luminescence spectra. The ratio of integrated intensities of the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions was used to analyze the structural variations in the surroundings of the Eu³⁺ ion. The effect of coating was analyzed by comparing the luminescence properties of the Y₃Al₅O₁₂:Eu³⁺ nanocrystalline powders and composite Y₃Al₅O₁₂:Eu³⁺/SiO₂ materials.     

On the Formation of the Garnet Phase in Oxidic High-Temperature Solutions

The paper informs on the formation of the garnets Y₃Fe₅O₁₂, Y₃Al₅O₁₂ and Gd₃Ga₅O₁₂ as primary phase in the ternary solvent mixtures PbO PbF₂ B₂O₃ and PbO MoO₃ B₂O₃. There are correlations with formerly measured oxygen ion concentrations (OIC) in the solvents:

• i each garnet grows in a special region of the OIC, the phase boundaries are parallel to the lines of constant OIC and
• ii the garnet solubility increases distinctly with decreasing OIC. This emphasizes the important role of the oxygen ions for the garnet formation.

Solubility models are discussed taking into consideration both previous and present results. We conclude that the garnet formation is described best by a model reaction of the type RE rare earth elements, Y; M Al, Fe, Ga.     

Crystal structure of holtite I

The crystal structure of As-containing holtite I is refined (Ital Structures diffractometer, 939 crystallographically independent reflections, anisotropic approximation, R = 0.047). The parameters of the orthorhombic unit cell are a = 4.695(1) Å, b = 11.906(3) Å, c = 20.38(3) Å, sp. gr. Pnma, Z = 4. On the whole, the structural formula obtained, (Si_2.43Sb_0.36As_0.21)BO₃[(Al_0.62Ta_0.26□)Al₂(Al_0.98□)₂(Al_0.94□)₂O₁₂](O,OH,□)_2.65, corresponds to the electron-probe analysis data. The statistical replacement of (Si,As)O₄ tetrahedra by pyramidal [SbO₃] groups is confirmed. The X-ray diffraction spectra of holtite I are compared with those of holtite II.     

Crystal structures of low-symmetry cancrinite and cancrisilite varieties

The high-sodium variety of cancrinite [Si_6.3Al_5.7O₂₄][Na₂(H₂O)₂][Na_5.7(CO₃)_0.9(SO₄)_0.1(H₂O)_0.6] (Kovdor Massif, Kola Peninsula, Russia) and the calcium-containing variety of cancrisilite [Si_6.6Al_5.4O₂₄][(Na_1.2Ca_0.4)(H₂O)_1.6][Na₆(CO₃)_1.3(H₂O)_1.2] (Khibiny Massif, Kola Peninsula, Russia) are studied. The trigonal unit cell parameters of the crystal structures under investigation are as follows: a = 12.727(4) Å, c = 5.186(2) Å, and space group P3 for the former mineral and a = 12.607(4) Å, c = 5.111(1) Å, and space group P3 for the latter mineral. The reduced symmetry of the new varieties as compared to the symmetry of typical cancrinite and typical cancrisilite is associated with the specific features in the arrangement of the carbonate groups and water molecules in channels. This inference is confirmed by the IR spectroscopic data.     

An X-ray diffraction refinement of the structure of natural thomsonite

Thomsonite Na₂Ca₄Al₁₀Si₁₀O₄₀, orthorhombic Pncn, a = 1.3124 nm, b = 1.3078 nm, c = 0.662 nm, V = 1.1369 nm³, Z = 2, D_m = 2.30, D_c = 2.26, μ = 12.4 cm⁻¹. The average length of the bond T–O = 0.1685 nm. The final R-value for 456 independent observed reflections is 0.085.     

Magnetic properties and X‐ray photoelectron spectroscopy of nanocrystalline Ho3Al5O12

Nanocrystalline holmium aluminium garnet (Ho₃Al₅O₁₂) has been prepared for the first time by modified Pechini's reaction after sintering the precursor gel at 1223 K. The nanomaterial has been characterized by X‐ray diffraction (XRD), selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). The XRD pattern confirms the formation of single‐phase Ho₃Al₅O₁₂; the average size of the nanoparticles has also been determined. X‐ray photoelectron spectroscopy (XPS) has been used to study the chemical composition and bonding in the as‐prepared samples. The binding energies of core‐level electrons in Ho, Al and O in the title material have been found slightly shifted compared to the values of the respective elements. DC magnetic susceptibility has been measured in the temperature range 2 – 260 K. Low effective magnetic moment of Ho³⁺, μ_eff = 1.35 µ_B and Weiss constant have been derived from the inverse magnetic susceptibility–temperature linear plot. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Computer simulation of the self-assembly of crystal structures of zeolites Ca64(Sr,K,Ba)48(Cu12(O,Cl))4[Si192Al192O786](H2O) n (tschoertnerite, TSC, V = 31 614 Å3) and Ca2K2[Al6Si6O24](H2O)10 (willhendersonite, cha, V = 804 Å3) from template nanocluster precursors K48 and K12

The self-assembly of zeolites Ca₆₄(Sr,K,Ba)₄₈(Cu₁₂(O,Cl))₄[Si₁₉₂Al₁₉₂ O₇₈₆](H₂O) _n (tschoertnerite, TSC-type framework, V = 31614 ų) and Ca₂K₂[Al₆Si₆O₂₄] (H₂O)₁₀ (willhendersonite, CHA-type framework, V = 804 ų), which form paragenetic associations, has been simulated using computational methods (TOPOS program package). A new method for analyzing zeolites of any complexity has been used, which is based on the complete expansion of the three-dimensional structural graph (3D factor graph) in tiles and the selection of nonintersecting tiles forming a packing. The code of self-assembly of 3D structures from complementary linked nanocluster precursors is reconstructed: primary chain → microlayer → microframework. A supracluster precursor K48 with the symmetry $\bar 4$ 3m, formed of four K12 clusters corresponding to the t-hpr tile, is established for TSC. The K48 cluster contains Ca template cations, which stabilize its local region in the stages of K12 → K24 → K48 self-assembly. Bifurcations of evolution paths (structural branching points) during the self-assembly of TSC and CHA microframeworks are established in the stage of formation of the K24 supracluster from invariant templated K12 clusters. The models under consideration explain the 100% localization of B = Ca cations, which play the role of templates, and the 50% occupation of the positions of K, Sr, and Ba spacer cations (in TSC) and K spacer cations (in CHA).     

Refinement of the crystal structures of synthetic nickel- and cobalt-bearing tourmalines

The crystal structures of synthetic tourmalines with a unique composition containing 3d elements (Ni, Fe, and Co) have been refined: (Ca_0.12?_0.88)(Al_1.69Ni _0.81 ²⁺ Fe _0.50 ²⁺ )(Al_5.40Fe _0.60 ³⁺ )(Si_5.82Al_0.18O₁₈)(BO₃)₃(OH)_3.25O_0.75 I, a = 15.897(5), c = 7.145(2) Å, V = 1564(1) Å; Na_0.91(Ni _1.20 ²⁺ Cr _0.96 ³⁺ Al_0.63Fe _0.18 ²⁺ Mg_0.03)(Al_4.26Ni _1.20 ²⁺ Cr _0.48 ³⁺ Ti_0.06)(Si_5.82Al_0.18)O₁₈(BO₃)₃(OH)_3.73O_0.27 II, a = 15.945(5), c = 7.208(2) Å, V = 1587(1) ų and Na_0.35(Al_1.80Co _1.20 ²⁺ )(Al_5.28Co _0.66 ²⁺ Ti_0.06)(Si_5.64B_0.36)O₁₈(BO₃)₃(OH)_3.81O_0.19 III, a = 15.753(8), c = 7.053(3) Å, V = 1516(2) ų. The reliability factors are R ₁ = 0.038?0.057 and wR ₂ = 0.041–0.060. It is found that 3d elements occupy both Y- and Z positions in all structures. The excess positive charge is compensated for due to the incorporation of divalent oxygen anions into the O3(V)+O1(W) positions.     

Spectroscopy of Rare-Earth Elements in Single-Crystal Films of Yttrium-Aluminium Garnets (YAG)

The utilization of liquid-phase epitaxy for growing thin yttrium-aluminium garnet films is discussed. By transfer method from lead solvents films were obtained with a composition Y₃Al₅O₁₂: Nd³⁺ on substrates with composition Y₃Al₅O₁₂, and films Y₃Al₅O₁₂: Er³⁺, Y₃Al₅O₁₂: Er³⁺, Ga³⁺ (about 40 wt% of erbium) on substrates Y₃Al₅O₁₂: Nd³⁺ Characteristics are given for simultaneous induced emission generation of the system film-substrate at 77 K Er³⁺ ion (λ_gen = 1.6602 μ) being a component of the film, and Nd³⁺ ion, being a component of the substrate (λ_gen = 1.061 μm). The results obtained are discussed.     

<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.     

X-ray quantitative determination of the phases in a batch for the synthesis of poly- and monocrystal yttriumaluminium garnet

For the synthesis of ceramics and single crystals of yttriumaluminium garnet, a batch consisting of Y₂O₃ and Al₂O₃ is first treated thermally in order to obtain the garnet phase Y₃Al₅O₁₂. It is possible to make a choice of the optimum technological parameters of the batch if the amounts of all oxide phases, present during the synthesis, are well known. A combined X-ray quantitative method is suggested for this purpose. The initial oxides and a garnet phase are determined by the method with external standard. The rest two impurity oxide phases are determined as binary system. The specific diffraction characteristics of the present compounds were taken into account. The amounts of all phases in the batch, treated thermally from 1373 to 2075 K at every 100 K are shown.     

Accurate structural study of langasite-family Ca<Subscript>3</Subscript>TaGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> crystal

An accurate X-ray diffraction study of Ca₃TaGa₃Si₂O₁₄ single crystal has been performed using two datasets obtained on a diffractometer equipped with a CCD area detector (a = 8.1056(2) Å, c = 4.9800(1) Å, sp. gr. P321, Z = 1, R/wR = 0.486/0.488%). A model structure is determined which is characterized by a high degree of reproducibility of structural parameters. Each site in Ca₃TaGa₃Si₂O₁₄ is occupied by atoms of only one type. Nevertheless, its structural feature is asymmetric disordering of sites of Ca, Ta, Ga, and two out of three oxygen atoms occupying special and general sites. A transition of some part of Ca atoms (~3%) from 3e sites on the twofold symmetry axis to general 6g sites is revealed.     

The effect of B2O3 on the solubility of (YSmLuCa)3(FeGe)5O12 garnet in PbO-B2O3 flux

The paper deals with the solubility of (YSmLuCa)₃ (FeGe)₅O₁₂ garnet in PbO—B₂O₃ flux. An anionic model explaining the effect of B₂O₃ on the solubility of rare earth garnet is suggested. This model is based on the assumption that PbO in the melted mixture of PbO, B₂O₃, Fe₂O₃, R₂O₃, CaO, GeO₂ dissociates into Pb²⁺ and O²⁻. The oxygen anions react with the other oxides under the formation of BO₃³⁻, FeO₃³⁻, RO₆⁹⁻, CaO₂²⁻ and GeO₃²⁻ anions, until the equilibrium is established. The garnet is dissociated into the anions mentioned above. The validity of the model is verified by an agreement of the experimentally determined saturation temperatures of the melts for LPE of garnet layers with the Arrhenius-type expression of the solubility product of garnet where x_i denotes the equilibrium concentrations of the anions in the melt and x° is the Ge content of the garnet film.     

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.