The growth and structure of Pb3Ga2Ge4O14 and Ba3Ga2Ge4O14 single crystals

Conditions for the flux synthesis of Pb₃Ga₂Ge₄O₁₄ and Ba₃Ga₂Ge₄O₁₄ single crystals and their solid solutions Pb_{3 − x} Ba_xGa₂Ge₄O₁₄ are studied. Structural analysis showed that the Ga³⁺-and Ge⁴⁺-cation positions in flux-grown Pb₃Ga₂Ge₄O₁₄ and Ba₃Ga₂Ge₄O₁₄ single crystals are not mixed. __________ Translated from Kristallografiya, Vol. 49, No. 2, 2004, pp. 325–328. Original Russian Text Copyright ? 2004 by Bezmaternykh, Vasil’ev, Gudim, Temerov. This work was presented at the National Conference on Crystal Growth (NCCG-2002, Moscow).     

Structure of lead germanate glasses by Raman spectroscopy

Lead germanate glasses of the xPbO(1−x)GeO₂ compositions with x ranging from 0.200 to 0.625 have been synthesised and their crystallisation behaviour and vibrational properties studied by differential thermal analysis (DTA) and Raman spectroscopy. Raman spectra of the Pb₅Ge₃O₁₁ ceramics also have been measured and compared to lead germanate glass spectra and literature data on β-PbO. Ge atoms in low lead glasses are supposed to be surrounded by four to six oxygen atoms, while the high-lead glass (x=0.625) corresponding to stoichiometry of the Pb₅Ge₃O₁₁ ferroelectrics contains predominantly fourfold coordinated germanium. Structural resemblence of high-lead germanate glasses to the Pb₅Ge₃O₁₁ crystal is proposed rather than to β-PbO. The `boson' peak range in the spectra of lead germanate glasses is discussed taking into account the similarities of the spectra of glassy and crystalline Pb₅Ge₃O₁₁.     

Gel-glass transformation in the SiO2Fe2O3 system

Gel-glass transformation has been studied by Mössbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO₂Fe₂O₃ system (A: 5 wt% Fe₂O₃, B: 10 wt% Fe₂O₃, C: 20 wt% Fe₂O₃, D: 40 wt% Fe₂O₃).The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40–1000°C.Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect.In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C.The presence of hematite was confirmed by the obtained Mössbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe³⁺ and Fe²⁺ oxidation states was also determined, together with the attribution of the probable coordination states for Fe³⁺ ions.Complex magnetic structure appeared in samples treated above 800°C.     

Formation of the Perovskite Phase Pb3NiNb2O9

Pb₃NiSNb₂O₉ was synthesized via various methods in order to investigate the mechanisms of perovskite formation. Two relatively simple routes to precursors that give single-phase perovskite after calcination at 900°C are described: a modified columbite-method and a synthesis from a freezedried complex salt solution. The results show that the formation of perovskite Pb₃NiNb₂O₉ does not start below 900 °C and that the pyrochlore formation beginning at lower temperatures has be hindered by a thermal shock calcination.     

Hydrothermal crystallization in the NaOH-GeO2-H2O system at 350°C: Mechanism of self-assembly of subpolyhedral clusters in Na4Ge

The Na,Ge^[6]-germanates of the compositions Na₄Ge ₄ ^[6] Ge ₅ ^[4] O₂₀ and Na₃HGe ₄ ^[6] Ge ₃ ^[4] O₁₆ · 4H₂O are crystallized in the NaOH-GeO₂-H₂O system under a pressure of 0.05 GPa and a temperature of 350°C. The structure of the germanates consists of open frameworks of M octahedra (Ge^[6]) and T tetrahedra (Ge^[4]). With an increase in the NaOH concentration, the crystallization fields change in the sequence R-GeO₂ (rutile structure type) ⇒ R-GeO₂ + Na₄Ge ₄ ^[6] Ge ₅ ^[4] O₂₀ ⇒ Na₄Ge ₄ ^[6] Ge ₅ ^[4] O₂₀ + Na₃HGe ₄ ^[6] O₁₆ · 4H₂O ⇒ Na₃HGe ₄ ^[6] Ge ₃ ^[4] O₁₆ · 4H₂O. The phase formation for Na,Ge^[6] germanates and Na,Ti silicates (Na₄Ti ₄ ^[6] Si ₃ ^[4] O₁₆ · 6H₂O and NaHTi ₂ ^[6] O₇ · 2H₂O) is considered based on the model of the matrix assembly of crystal structures from invariant Ge^[6],Ge^[4] subpolyhedral structural units. The homologous family of phases consisting of Na₄Ge ₄ ^[6] Ge ₅ ^[4] O₂₀ (M ₄ T ₅ framework), Na₃HGe ₄ ^[6] Ge ₃ ^[4] O₁₆ · 4H₂O, Na₄Ti ₄ ^[6] Si ₃ ^[4] O₁₆ · 6H₂O (M ₄ T ₃ framework), and NaHTi ₂ ^[6] Si^[4]O₇ · 2H₂O (M ₄ T ₂ framework) phases with topologically equivalent one-dimensional MT structures is singled out. __________ Translated from Kristallografiya, Vol. 48, No. 5, 2003, pp. 950–958. Original Russian Text Copyright ? 2003 by Ilyushin, Dem’yanets. Dedicated to the 60th Anniversary of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences     

Crystal structure of britvinite [Pb7(OH)3F(BO3)2(CO3)][Mg4.5(OH)3(Si5O14)]: A new layered silicate with an original type of silicon-oxygen networks

The crystal structure of a new mineral britvinite Pb_7.1Mg_4.5(Si_4.8Al_0.2O₁₄)(BO₃)(CO₃)[(BO₃)_0.7(SiO₄)_0.3](OH, F)_6.7 from the Lángban iron-manganese skarn deposit (V?rmland, Sweden) is determined at T = 173 K using X-ray diffraction (Stoe IPDS diffractometer, λMoKα, graphite monochromator, 2θ_max = 58.43°, R = 0.052 for 6262 reflections). The main crystal data are as follows: a = 9.3409(8) ?, b = 9.3579(7) ?, c = 18.8333(14) ?, α = 80.365(6)°, β = 75.816(6)°, γ = 59.870(5)°, V = 1378.7(2) ?³, space group P1, Z = 2, and ρ_calcd = 5.42 g/cm³. The idealized structural formula of the mineral is represented as [Pb₇(OH)₃F(BO₃)₂(CO₃)][Mg_4.5(OH)₃(Si₅O₁₄)]. It is demonstrated that the mineral britvinite is a new representative of the group of mica-like layered silicates with structures in which three-layer (2: 1) “sandwiches” composed of tetrahedra and octahedra alternate with blocks of other compositions, such as oxide, oxide-carbonate, oxide-carbonate-sulfate, and other blocks. The tetrahedral networks (Si₅O₁₄)_∞∞ consisting of twelve-membered rings are fragments of the britvinite structure. Similar networks also form crystal structures of the mineral zeophyllite and the synthetic phase Rb₆Si₁₀O₂₃. In the crystal structures under consideration, the tetrahedral networks differ in the rotation of tetrahedra with respect to the layer plane. Original Russian Text ? O.V. Yakubovich, W. Massa, N.V. Chukanov, 2008, published in Kristallografiya, 2008, Vol. 53, No. 2, pp. 233–242.     

Preparation and properties of ferroelastic/ferroelectric polytypes of antimony(III) oxide iodine,Sb5O7I

Antimony oxide iodide, Sb₅O₇I (?7Sb₂O₃.SbI₃), is monoclinic and exists in at least 8 polytypic modifications. Crystals of these (transparent, pseudohexagonal prisms up to 15 × 10 × 5 mm³) were grown by vacuum sublimation and identified by optical and X-ray methods. Depending on the stacking sequence of characteristic slabs of antimony and oxygen in the (monoclinic) b-direction, centric and acentric polytypes result. The former show pure ferroelastic, the latter combined ferroelastic + ferroelectric behaviour. All polytypes transform reversibly into hexagonal, nonferroic phases at characteristic Curie temperatures between 165 and 208°C. The crystals can assume one of the three stable orientation states which may be mutually switched into each other by successive application of comprehensive stress (of the order of 10⁶ N/m²) on the three pairs of prism faces. In the ferroelectric polytypes the spontaneous polarization (about 5 × 10^-3 C/m²) is not reversible but only reorientable by ± 120° by coercive fields in the order of 2–5 kV/cm. Ferroelectricity and ferroelasticity are fully coupled.     

Enantiomorphism of the Ca3Ga2Ge4O14 compound and comparison of the Ca3Ga2Ge4O14 and Sr3Ga2Ge4O14 structures

The absolute crystal structures of two enantiomorphic forms of the Ca₃Ga₂Ge₄O₁₄ crystals (a = 8.075(1) ?, c = 4.9723(6) ?, space group P321) with the positive and negative senses of the optical activity are determined using X-ray diffraction analysis. The final R factors are as follows: R = 1.75% and R _w = 2.57% for the crystal with the positive sense of the optical activity and R = 1.86% and R _w = 2.78% for the crystal with the negative sense of the optical activity. The replacement of the Ca²⁺ ions by larger Sr²⁺ ions (with the formation of the Sr₃Ga₂Ge₄O₁₄ compound) leads to an anisotropic expansion of the crystal lattice (with a more considerable increase in the lattice parameter a as compared to the lattice parameter c), a change in the occupation of the 1a and 3f positions by the Ga³⁺ and Ge⁴⁺ ions, and symmetrization of the octahedra and tetrahedra forming the structural framework. The shape of the dodecahedron changes so that its size along the polar electric axis 2 increases significantly. This change is the main factor responsible for the increase in the piezoelectric activity of the Sr₃Ga₂Ge₄O₁₄ compound as compared to the piezoelectric activity of the Ca₃Ga₂Ge₄O₁₄ compound. Original Russian Text ? B.V. Mill, A.A. Klimenkova, B.A. Maximov, V.N. Molchanov, D.Yu. Pushcharovsky, 2007, published in Kristallografiya, 2007, Vol. 52, No. 5, pp. 816–823.     

Si modified Ge10 cluster: the preparation and characterization of single crystal Na4(Ge,Si)9O20

A Si modified Ge₁₀ cluster with structure Na₄(Ge,Si)₉O₂₀ (denoted as HUT‐1) was synthesized by hydrothermal synthesis at 160 °C with a sodium silicate source. The compound was characterized by single crystal, powder X‐ray diffraction and TGA‐DSC analysis. HUT‐1 crystallizes in space group I4₁ (80) with calculated unit cell (a=14.966(5) Å, c=7.343(2) Å, V=1644.8(9) ų), which has the same structure as Na₄Ge₉O₂₀. HUT‐1 has a high Si/Ge ratio with an approximate formula of Na₄Ge_7.68Si_1.32O₂₀. Single crystal X‐ray structure refinements together with results from X‐ray powder diffraction (XRPD) confirm the occupancy of Si at two tetrahedral sites. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Glass forming ability and crystallization in the PbO-Al2O3 system

The formation of glass by the splat-quenching of PbO-Al₂O₃ melts was confirmed in the range 67-95 mol.% PbO. The crystallization of the glasses obtained in this way was examined by thermal analysis, dark field microscopy, X-ray diffraction and IR spectroscopy. This crystallization is generally a two-step process: orthorhombic PbO crystallizes first around 280-36-°C depending on the composition, then the aluminate PbO · Al₂O₃ crystallizes around the rather constant temperature of 600°C.     

Ion exchange between glass melts of the system Na2ORb2OSiO2

Ion exchange between glass melts of the quasi-binary system Na₂O · 3SiO₂Rb₂O₂ was investigated at 700—1300°C by means of a special capillary method. Concentration profiles were obtained by electron microprobe analysis and were evaluated for a concentration-dependent quasi-binary interdiffusion coefficient $\text{tilde}\text{D}$ using a modified Boltzmann-Matano method. At 700–1000°C interdiffusion could be obtained in pure form with $\text{tilde}\text{D}$ values ranging from about 10^?7 ?5 × 10^?6 cm² s^?1. Above 1000°C convection processes superimposed interdiffusion, making a further evaluation impossible. The data are compared with those from a 0g rocket experiment and are discussed with respect to a mixed-alkali effect and in terms of the Nernst-Planck diffusion model.     

Variation of optical basicity with probe ion,for Cs2O + B2O3 and Li2O + B2O3 glasses

Optical basicities (Γ) for Cs₂O + B₂O₃ and Li₂O + B₂O₃ glasses have been measured as a function of glass composition, using Tl⁺, Pb²⁺ and Bi³⁺ probe ions. The three probe ions register different values of Γ for glasses of given composition (and also for pure B₂O₃ glass and water). The divergence decreases as the alkali metal ion size decreases.For the Li₂O + B₂O₃ glasses, ideal (calculated) optical basicities agree within experimental precision with experimental values registered by Pb²⁺ (Γ_Pb²⁺) up to about 15 mol% Li₂O. For higher Li₂O contents, and for the Cs₂O + B₂O₃ glasses, ideal optical basicities agree less well with Γ_Pb²⁺, but show similar trends with composition to those shown by Γ_Pb²⁺.     

Phase equilibria and growth of β-BaB2O4 crystals in the BaB2O4-Ba2Na3[B3O6]2F system

A complex investigation of the BaB₂O₄-Ba₂Na₃[B₃O₆]₂F cut, which belongs to the ternary mutual Ba, Na//BO₂, F (BaB₂O₄-(NaBO₂)₂-(NaF)₂-BaF2) system, has been performed. The cut is quasibinary, with the following eutectics coordinates: 810 ± 5°C, 85 mol % Ba₂Na₃[B₃O₆]₂F, 15 mol % BaB₂O₄. It is shown that this system can be used to grow bulk β-BaB₂O₄ crystals.     

Small angle X-ray scattering study of spinodal decomposition in B2O3---PbO---Al2O3 glass

SAXS measurement for 80B₂O₃·15PbO·5Al₂O₃ (wt%) glass can be performed with high accuracy. SAXS spectra were obtained for glasses which had been heat-treated for different lengths of time at 380°C and 400°C. A converging point k′_c and a common curve for k >k′_c were observed in the SAXS spectra of glasses subjected to appropriate aging. This suggests the validity of Cook's modification of Cahn's theory. At the very early stage of the decomposition a relaxation takes place of $\text{?″}\text{and}\text{D}\text{?}$ and the spectra of Cahn-Cook's amplification factor show a typical aspect of the early stage of decomposition after the relaxation. Plots of k³I(k) versus k³ suggest that the interface between the two phases is diffuse and the demixing process is in the early stage of the spinodal decomposition.     

Glasses and glass-ceramics in the oxyfluoride ternary system Pb(PO3)2-WO3-PbF2

Glasses were prepared by the melt-quenching method in the ternary system Pb(PO₃)₂-WO₃-PbF₂ and doped with Er³⁺ in order to prepare luminescent transparent glass-ceramics. This work focused on thermal and structural characterization of tungsten lead-phosphate glasses and crystallization study for preparing transparent glass-ceramics. Thermal properties such as thermal stability and crystallization behavior upon heating were investigated by DSC in function of PbF₂ content. For low PbF₂ concentrations, only one crystallization peak due to Pb₃(PO₄)₂ is observed whereas samples containing more than 15% of PbF₂ present another exothermic event at lower temperatures related with precipitation of PbF₂, Pb₂P₂O₇ and Pb₂OF₂. Structural investigations by Raman spectroscopy suggest that PbF₂ modifies the tungsten-phosphate network through the formation of P―F and P―O―Pb bonds but the average network connectivity remains almost constant. A crystallization study has been performed by DSC to investigate the dominant crystallization mechanisms in these glasses and it has been established that Pb₃(PO₄)₂ is nucleated on the surface whereas PbF₂, Pb₂P₂O₇ and Pb₂OF₂ crystallize dominantly from the glassy bulk. Transparent glass-ceramics containing nanosized PbF₂ crystallites were also prepared by suitable heat-treatment on the glass sample containing 20% of PbF₂ and Raman microscopy of these glass-ceramics supports the crystallization mechanisms determined by DSC.     

Change in the crystal structure of germanium-containing lithiophosphate upon heating

The crystal structure of lithiophosphate with phosphorus atoms partially replaced by germanium, Li_3.17(P_0.69Ge_0.24Mo_0.07)O₄, at temperatures of 25, 150, 300, 450, and 600°C has been refined by the Rietveld method using powder data. New additional Bragg reflections are observed at T = 600°C, which indicate a change in the crystal structure of this compound.     

A Novel Three Dimensional Organic–Inorganic Hybrid Based Porous Phase: Synthesis and Characterization of Reduced Oxovanadium pyromellitate, [VIV2O2(H2O)2(C6H2(COO)4)]

The hydrothermal reaction of a mixture of VOSO₄ · xH₂O, 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid) and 0.1 M H₂SO₄ for 72 h at 160 °C gives blue needle like crystals of [V^IV ₂O₂(H₂O)₂(C₆H₂(COO)₄)] in 30% yield. The compound has a porous 3-D extended network structure having a rigid architecture which is held together by the multidentate functionalities of 1,2,4,5-benzenetetracarboxylate ligand. Crystal data for the compound: monoclinic space group C 2/c (No:15), a = 11.756(5) ?, b = 9.645(3) ?, c = 11.822(7) ?, β = 107.10(4)°, Z = 8. The compound constitutes the first example of a fully reduced oxovanadium based solid incorporating the organic ligand. This article consists of synthesis, crystal structure and characterization of [V^IV ₂O₂(H₂O)₂(C₆H₂(COO)₄)].     

Glass formation in the GexTe100−x binary system: Synthesis by twin roller quenching and co-thermal evaporation techniques

The Ge–Te system exhibits one main composition domain where glasses can be easily prepared by melt quenching technique; this domain is centered on the eutectic composition Ge₁₅Te₈₅. In this work, bulk flakes and films of composition Ge_xTe_100−x with x  50 at.% were prepared by two different quenching techniques: (i) the twin roller quenching for bulk flakes and, (ii) the co-thermal evaporation for films (with thickness comprised between 2 and 14 μm). Electron Probe Micro-Analysis was used to check the composition of the materials while X-ray diffraction allowed identifying the amorphous state and/or the crystalline phases present in the Ge_xTe_100−x samples. Thermal properties for both types of materials were investigated by differential scanning calorimetry. The glass-forming regions were: 11.7–22.0 at.% Ge for bulk flakes and 10.2–35.9 at.% Ge for films. A similar thermal behavior of bulk flakes and thick films was highlighted by Differential Scanning Calorimetry.