Growth of Ca3Fe2Ge3O12 and Ca3Fe(Al,Cr) Ge3O12 garnets in molten bismuth germanate glasses

Additions of Fe₂O₃ to CaO·Bi₂O₃·2 GeO₂ cause Ca₃Fe₂Ge₃O₁₂ garnets to precipitate from the resultant melt at 1250°C. Garnets with the composition Ca₃Fe(Al, Cr) Ge₃O₁₂ are also precipitated by adding either $\text{Al}{}_2\text{O}{}_3\text{Fe}{}_2\text{O}{}_3$ or $\text{Cr}{}_2\text{O}{}_3\text{Fe}{}_2\text{O}{}_3$ mixtures. The well-formed crystals range from several to 100 μm in size and are obtained in 50 to 70% yields at $\text{Fe}\text{Bi}\text{= 0.4}$. Additions of Fe₂O₃ (up to $\text{Fe}\text{Bi}\text{= 1.0}$) to compositions containing ZnO, CdO, SrO, and BaO yield only dark glasses. The physical properties of these glasses suggest that Fe(III), in contrast to AL(III) & Ga(III), prefers octahedral coordination.     

Synthese et etude des proprietes spectrales de monocristaux de vanadate a structure grenat Ca2NaMg2V3O12 active par l'europium trivalent

Single crystals of garnet-type vanadates Ca_2?2xNa_1+xEu_xMg₂V₃O₁₂ have been obtained by the flux and Czochralski methods. Spectroscopic studies of Eu³⁺ ion in these single crystals confirm the precedent results obtained on powdered samples especially the lowering of the D₂ point symmetry of the dodecahedral site.     

Sol-gel synthesis of single-phase Ca5MgSi3O12: Eu, Mn phosphors for white-light emitting diodes

A novel Ca₅MgSi₃O₁₂: Eu²⁺, Mn²⁺ phosphor has been prepared by a sol-gel method. X-ray diffractometer, spectrofluorometer were used to characterize structural and optical properties of the samples. The results indicate that Ca₅MgSi₃O₁₂: Eu²⁺, Mn²⁺ phosphors show two emission bands excited by ultraviolet light. Blue (around 450 nm) and green (around 502 nm) emissions originate from Mn²⁺ and Eu²⁺, respectively. With appropriate tuning the concentration ratios of Eu²⁺ to Mn²⁺, Ca₅MgSi₃O₁₂: Eu²⁺, Mn²⁺ phosphors exhibit different hues and relative color temperatures, which have potential to act as a single-phase phosphor for white-light emitting diode.     

Mössbauer resonance studies on the Ca2Fe2O5-LaFeO3 system

Phases between Ca₂Fe₂O₅ and LaFeO₃ with La_1?2yCa_2yFe³⁺O_3?y formulation have been prepared in order to study the oxygen defects in perovskite-related ferrites. X-ray-diffraction analysis and magnetic measurements have confirmed the previous results i.e. the existence of a critical concentration of vacancies above which long range ordering appears. A detailed Mössbauer resonance study shows a continuous evolution of the iron environment, tetrahedra being formed even for low values of y. This result has been discussed and compared with previous ones for homologous CaTi_1?2yFe_2yO_?y phases.     

Caracterisation physico-chimique du ferrite de calcium et de lanthane Ca2LaFe3O8

In the Ca₂Fe₂O₅LaFeO₃ system a new ferrite of formula Ca₂LaFe₃O₈ has been isolated. As its precise cristallographic structure is not known, the structural model previously proposed has been tested by Mössbauer spectroscopy and magnetic measurements. The Fe³⁺ ions occupy one tetrahedral and two octahedral sites. The magnetic behaviour of Ca₂LaFe₃O₈ is close to those of Ca₂Fe₂O₅ and LaFeO₃. The Néel temperature is 735 K. The various exchange integrals have been calculated using a method previously applied to Ca₂Fe₂O₅.     

Substitutions in the Ca2Na6Al6Si6O24(SO4)2 haüyne structure

Four new haüynes having the chemical formula Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, X = Se, Te, Mo and W, were synthesized by solid-state reaction. Various substitutions were then attempted for Ca, Na, Al and Si. The replacement of Ca²⁺ by Sr²⁺ or Cd²⁺ was successful in Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, when X = S, Cr, Mo or W, except for Cd, when X = Cr. The synthesis of Mn₂Na₆Al₆Si₆O₂₄(XO₄)₂ could be made when X was Mo or W, and, among the Pb substitutions tried, Pb₂Na₆Al₆Si₆O₂₄(SO₄)₂ was successful. The solubility of Li, K and Ag was partial and was different in different haüynes. Maximum solubility of Li for Na was three atoms in Ca₂Na₆Al₆Si₆O₂₄(SO₄)₂ and the minimum was half an atom in Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, X = Mo or W. Maximum replacement of K or Ag for Na was two atoms in Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, X = Mo or W and the minimum was 0.5 in Ca₂Na₆Al₆Si₆O₂₄(SO₄)₂. The solubility of Li, K and Ag was 1.5 atoms in Ca₂Na₆Al₆Si₆O₂₄(CrO₄)₂. The solubility of Ga³⁺ or Fe³⁺ for Al³⁺ was partial. A maximum of 0.5, 3 and 4 atoms of Ga³⁺ can be substituted for Al³⁺ in sulfate, chromate and molybdate or tungstate haüynes, respectively. The solubility of Fe³⁺ was one atom in all haüynes. The complete replacement of Si⁴⁺ by Ge⁴⁺ was possible in M²⁺₂Na₆Al₆Si₆O₂₄(XO₄)₂, when M = Ca, Cd or Sr, and X = Mo or W.     

A yellow-emitting Ca3Si2O7: Eu phosphor for white LEDs

A series of yellow-emitting phosphors based on a silicate host matrix, Ca_3 − xSi₂O₇: xEu²⁺, was prepared by solid-state reaction method. The structure and photoluminescent properties of the phosphors were investigated. The XRD results show that the Eu²⁺ substitution of Ca²⁺ does not change the structure of Ca₃Si₂O₇ host and there is no impurity phase for x < 0.12. The SEM images display that phosphors aggregate obviously and the shape of the phosphor particle is irregular. The EDX results reveal that the phosphors consist of Ca, Si, O, Eu and the concentration of these elements is close to the stoichiometric composition. The Ca_3 − xSi₂O₇: xEu²⁺ phosphors can be excited at a wavelength of 300-490 nm, which is suitable for the emission band of near ultraviolet or blue light-emitting-diode (LED) chips. The phosphors exhibit a broad emission region from 520 to 650 nm and the emission peak centered at 568 nm. In addition, the shape and the position of the emission peak are not influenced by the Eu²⁺ concentration and excitation wavelength. The phosphor for x = 0.045 has the strongest excitation and emission intensity, and the Ca_3 − xSi₂O₇: xEu²⁺ phosphors can be used as candidates for the white LEDs.     

Calcium-platinum oxide Ca4PtO6 - growth problems and polymorphism

Two forms of Ca₄PtO₆ single crystals were grown at different conditions, one from CaCl₂ flux and one from KF or PbF₂ flux. For the first form (I) the orthorhombic cell parameters are: a = 9, 164A, b = 9, 235A, c = 6, 502A. The unit cell dimensions of the hexagonal form (II) are a = 9, 26, c = 11, 26A. Both are semiconducting, with resistivities of 1, 04 × 10² and (4, 80 ? 22, 5) × 10³ ohm-cm for the orthorhombic and hexagonal forms, respectively. IR spectra were recorded and the thermal stability of Ca₄PtO₆ has also been studied. A contamination effect on crystal growth is discussed.     

Novel Dy-doped Ca2Gd8(SiO4)6O2 white light phosphors for Hg-free lamps application

The luminescent properties of Ca₂Gd_8(1−x)(SiO₄)₆O₂:xDy³⁺ (1% ≤ x ≤ 5%) powder crystals with oxyapatite structure were investigated under vacuum ultraviolet excitation. In the excitation spectrum, the peaks at 166 nm and 191 nm of the vacuum ultraviolet region can be assigned to the O²⁻ → Gd³⁺, and O²⁻ → Dy³⁺ charge transfer band respectively, which is consistent with the theoretical calculated value using Jφrgensen's empirical formula. While the peaks at 183 nm and 289 nm are attributed to the f-d spin-allowed transitions and the f-d spin-forbidden transitions of Dy³⁺ in the host lattice with Dorenbos's expression. According to the emission spectra, all the samples exhibited excellent white emission under 172 nm excitation and the best calculated chromaticity coordinate was 0.335, 0.338, which indicates that the Ca₂Gd₈(SiO₄)₆O₂:Dy³⁺ phosphor could be considered as a potential candidate for Hg-free lamps application.     

Phase developments in the Pb(Zn1/2W1/2)O3-Pb(Zn1/3Ta2/3)O3-Pb(Zn1/3Nb2/3)O3 pseudo-ternary system

The phase stability ranges in the B-site precursor (Zn_1/2W_1/2)O₂-(Zn_1/3Ta_2/3)O₂-(Zn_1/3Nb_2/3)O₂ were determined by X-ray diffraction (XRD), where wolframite, tri-αPbO₂, and columbite phases were identified. Next attempts were carried out (with the addition of PbO) for the system Pb(Zn_1/2W_1/2)O₃-Pb(Zn_1/3Ta_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃, where the perovskite phase did not develop in the entire compositions investigated. Instead, only the Pb₂WO₅ and pyrochlore phases (along with ZnO) resulted.     

Mössbauer resonance studies in the CaTiO3Ca2Fe2O5 system

A Mössbauer resonance study of the CaTiO₃-Ca₂Fe₂O₅ system - i.e. the Ca₂Fe_2xTi_2?2xO_6?x solid solution (0?x?1) - shows a continuous evolution of the local iron environment over the entire composition range. For 0.50?x?1 the investigation confirms a previous structural model of alterning layers of octahedra and tetrahedra. For lower concentrations of oxygen vacancies, evidence is given of Fe³⁺ ions randomly distributed in octahedral and even in tetrahedral sites. Titanium atoms apparently have only octahedral surroundings. A long range order is detected when $\text{x}\text{? 0.50}$.     

Layer-like cation ordering in a perovskite-type compound Ca3(CaTa2)O9 with monoclinic symmetry

A structural scheme to fit in a monoclinic cell of the ordered perovskites Ca₃(XZ₂)O₉ is proposed based on a layer-like ordering of X and Z ions. According to this scheme, the structure of compound Ca₃(CaTa₂)O₉ was analyzed with X-ray powder method. The structure is found to involve an alternate stacking of one Ca-layer and two Ta-layers in the octahedral cation sites, and has a base-centered monoclinic Bravais lattice with the constants a=9.808 A,b=5.533 A,c=7.061 A, and ß=89°1′. The space group is $\text{C}\text{2}\text{m-C}{}_{\mathrm{2h}}{}^3$. The unit cell contains six triclinic perovskite units with the lattice constants a′=b′=4.000 A, c′=3.996 A, α′=ß′=89°32′, and γ′=87°32′.     

Synthesis and sintering of Ca0.5Zr2P3O12 -a low thermal expansion material

The single phase compound Ca_0.5Zr₂P₃O₁₂ (CZP) was prepared by solid state reaction technique. This material shows a negative thermal expansion in the temperature region of 30°–500°C. The effect of MgO and ZnO addition on the sintering behavior and thermal expansion characteristics of Ca_0.5Zr₂P₃O₁₂ was investigated. Mg₃(PO₄)₂ and Zn₃(PO₄)₂ were observed as minor phases responsible for improving the overall thermal expansion of CZP + MgO, ZnO systems. SEM studies and density data are also discussed. Observed sintering kinetics suggest that a liquid phase is promoting the sintering reaction. 98+% of theoretical density and near zero expansion behavior in certain compositions were observed.     

Etude par diffraction X et microscopie electronique des composes inedits de formule AnBnO3n+2 dans les systems La2Ti2O7CaTiO3, Nd2Ti2O7CaTiO3 et Ca2Nb2O7CaTiO3

New compounds corresponding to the formula A_nB_nO_3n+2 with n = 4,5; 5 and 6 have been found in the two following systems Nd₂Ti₂O₇CaTiO₃ and Ca₂Nb₂O₇CaTiO₃. These compounds have been investigated by electron microscopy. The results obtained are in good agreement with our structural hypothesis.     

Development of mechanoluminescent micro-particles Ca2MgSi2O7:Eu,Dy and their application in sensors

We have revealed that Ca₂MgSi₂O₇:Eu micro-particles emits green light under the application of a mechanical stress, called as mechanoluminescence (ML). The ML showed a similar spectrum as photoluminescence (PL), which indicated that ML is emitted from the same center of Eu²⁺ ions as PL. Such a green light of ML emission can be seen by the naked eye when pressing the sample. Furthermore, using lab-made AFM-ML system, the ML of single micro-particle under the application of micro force (10 μN) has been investigated.     

Stoichiometric Pb(Mg1/3Nb2/3)O3 perovskite ceramics produced by reaction-sintering process

Stoichiometric lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), perovskite ceramics produced by reaction-sintering process were investigated. Without calcination, a mixture of PbO, Nb₂O₅, and Mg(NO₃)₂ was pressed and sintered directly. Stoichiometric PMN ceramics of 100% perovskite phase were obtained for 1, 2, and 4 h sintering at 1250 and 1270 °C. PMN ceramics with density 8.09 g/cm³ (99.5% of theoretical density 8.13 g/cm³) and K_max 19,900 under 1 kHz were obtained.     

Synthese sous haute pression et structure d'un nouveau pyrochlore contenant du rhenium: Ca1+xRe2O6(OH)2x (x=0,3)

A new compound Ca_1+xRe₂O₆(OH)_2x (x=0,3) with a pyrochlore-like arrangement has been synthesized at 55 kbar and 900°C. The crystal structure has been solved by conventional Fourier and least-squares methods using automated four-circle diffractometer data. The Ca²⁺ and OH^? ions are randomly distributed respectively over the 16d and 8b positions.     

Luminescent properties of long lasting phosphor Ca2MgSi2O7:Eu

Long afterglow phosphors (Ca_1−xEu_x)₂MgSi₂O₇ (0.002 ≤ x ≤ 0.02) were prepared by solid-state reactions under a weak reductive atmosphere. X-ray diffraction pattern, photoluminescence spectra, decay curve, afterglow spectra and thermoluminescence curves were investigated. The phosphors showed two emission peaks when they were excited by 343 nm, due to two types of Eu²⁺ centers existing in the Ca₂MgSi₂O₇ lattice. However, only one emission peak can be found in their afterglow spectra. Energy transfer between Eu²⁺ ions in inequivalent sites was found. A possible mechanism was presented and discussed. The afterglow decay time of Ca_1.998MgSi₂O₇:Eu_0.002 was nearly 12.5 h which means it was a good long lasting phosphor.     

High temperature creep in a 2-3-4 garnet: Ca3Ga2Ge3O12

High temperature plastic deformation in a single crystal of a 2-3-4 garnet, Ca₃Ga₂Ge₃O₁₂, was investigated. A Czochralski-grown single crystal of Ca₃Ga₂Ge₃O₁₂ was deformed in compression in air along 100 or 110 at temperatures of 1472 to 1573 K (T/T_m = 0.90–0.96). The samples show higher resistance to creep than other 3-3 garnets, namely the flow stress at the strain-rate of 4 × 10^{– 6} s^{– 1} is 200–400 MPa in this temperature range. The TEM observations of dislocation microstructures show little evidence of climb and plastic deformation in this garnet appears to occur exclusively by dislocation glide, using mostly the 1/2111{110} slip systems. Dislocations with b = 100 are frequently observed but they are interpreted as products of dislocation reactions among 1/2111. The single crystal used contained a number of precipitates that grew during annealing and also during deformation. These precipitates act as sources for dislocations but no evidence for their significant effects on creep strength is observed. The normalized flow law of Ca₃Ga₂Ge₃O₁₂ is similar to other 3-3 oxide garnets (e.g., YAG, GGG), but in contrast to 3-3 garnets, the more stable and hence less mobile dislocations have a large edge component.     

Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4

An extensive theoretical study is performed for wide bandgap crystalline oxides and nitrides, namely, SiO₂, GeO₂, Al₂O₃, Si₃N₄, and Ge₃N₄. Their important polymorphs are considered which are for SiO₂: α-quartz, α- and β-cristobalite and stishovite, for GeO₂: α-quartz, and rutile, for Al₂O₃: α-phase, for Si₃N₄ and Ge₃N₄: α- and β-phases. This work constitutes a comprehensive account of both electronic structure and the elastic properties of these important insulating oxides and nitrides obtained with high accuracy based on density functional theory within the local density approximation. Two different norm-conserving ab initio pseudopotentials have been tested which agree in all respects with the only exception arising for the elastic properties of rutile GeO₂. The agreement with experimental values, when available, are seen to be highly satisfactory. The uniformity and the well convergence of this approach enables an unbiased assessment of important physical parameters within each material and among different insulating oxide and nitrides. The computed static electric susceptibilities are observed to display a strong correlation with their mass densities. There is a marked discrepancy between the considered oxides and nitrides with the latter having sudden increase of density of states away from the respective band edges. This is expected to give rise to excessive carrier scattering which can practically preclude bulk impact ionization process in Si₃N₄ and Ge₃N₄.