O6 Polyhedra Research Articles

New polymorphs of ternary sodium tellurium oxides: hydrothermal synthesis, structure determination, and characterization of β-Na₂Te₄O₉ and Na₂Te₂O₆·1.5H₂O.

Two new sodium tellurium oxide materials, β-Na2Te4O9 and Na2Te2O6·1.5H2O, have been synthesized through hydrothermal reactions using Na2CO3, TeO2, and H2TeO4·2H2O as reagents. The structures of the novel materials have been determined by single crystal X-ray diffraction. β-Na2Te4O9 is a new polymorph of ternary tellurite that is showing a three-dimensional framework structure containing only TeO4 polyhedra. Na2Te2O6·1.5H2O reveals an anionic layered backbone composed of Te(6+)O6 octahedra and Te(4+)O5 polyhedra. Thermogravimetric analyses and powder X-ray diffractions at different temperatures suggest that the frameworks of β-Na2Te4O9 and Na2Te2O6·1.5H2O are thermally stable up to 480 and 400 °C, respectively. The Na(+) cations in between the anionic layers in Na2Te2O6·1.5H2O are completely replaced by Li(+) cations through an ion-exchange reaction. The UV-vis diffuse reflectance and infrared spectra, elemental analyses, and local dipole moment calculations are also reported.

Synthesis, crystal structures, and luminescent properties of tin(II) and lead(II) carboxyphosphonates with 3D framework structures

Two new metal carboxyphosphonates with 3D framework structures, [Sn2(HL)·2H2O] (1) and [Pb2Cl(H2L)] (2) (H5L=4-HOOCC6H4CH2N(CH2PO3H2)2), have been synthesized under hydrothermal conditions. For compound 1, every two Sn(1)O3 and Sn(2)O3 polyhedra are linked by CPO3 tetrahedra to form a 2D inorganic layer via corner-sharing in the bc-plane. Then the adjacent layers are further assembled into a 3D supramolecular structure through π–π stacking interactions. Compound 2 features a 3D open-framework structure. The Pb(1)O5Cl, Pb(2)O5 polyhedra and CPO3 tetrahedra are interconnected into a 2D inorganic layer via corner-sharing in the ac-plane. Such layers are further connected through carboxyphosphonate ligands to form a 3D open-framework structure. The luminescent properties of compounds 1 and 2 have also been studied.

The 2H and 3R polytypes of sabieite, NH4Fe3+(SO4)2, from a natural fire in an oil-bearing shale near Milan, Ohio

The mineral sabieite, NH4Fe3+(SO4)2, was found in 2011 along the banks of the Huron River near Milan, Ohio, where it formed as the result of a natural fire in an oil-bearing shale. The mineral is directly associated with pyracmonite, tschermigite, and voltaite and occurs as colorless, pale pink, tan, and yellow hexagonal tablets. The streak is white. Crystals are transparent with vitreous luster. Mohs hardness is 2½, tenacity is brittle, fracture is irregular, and cleavage is perfect on {001}. The measured density is 2.65(2) g/cm3. The mineral is optically uniaxial (-) with indices of refraction ω = 1.657(3) and ε = 1.621(5) (white light). The empirical formula (based on 2 S apfu) is [(NH4)0.73(H3O)0.22K0.04Na0.01]Σ1.00(Fe3+0.95Al0.02Mg0.01)Σ0.98(SO4)2. Powder diffraction showed crystals to be combinations of the 2H and 3R polytypes. The structure of the 2H polytype was solved and refined from single-crystal data yielding R1 = 0.0694 for 509 Fo > 4σ(F) reflections. The 2H polytype has space group P63 and cell parameters a = 4.83380(17), c = 16.4362(9) Å, V = 332.59(2) Å3, and Z = 2 and the 3R polytype has space group R3̄ and cell parameters a = 4.835(2), c = 24.496(15) Å, V = 495.9(5) Å3, and Z = 3. The sabieite polytypes (including the original sabieite from Sabie, South Africa, which is the 1T polytype) have glaserite-like structures with layers consisting of Fe3+O6 octahedra that share each of their vertices with SO4 tetrahedra. NH4 groups occupy 12-coordinated sites in the interlayer region, bonding to 6 O atoms in each of the adjacent layers. In the 1T polytype, successive layers have identical configuration and orientation, providing a one-layer repeat sequence. In the 2H polytype, alternate layers are flipped (or rotated), in a two-layer repeat sequence. In the 3R polytype, successive layers are shifted relative to one another, in a three-layer repeat sequence. The different orientations of adjacent layers in the structures of the 2H and 3R polytypes result in significant changes in the linkages between the (NH4)O12 and Fe3+O6 polyhedra.

Synthetic and natural chromium-bearing spinels: an optical spectroscopy study

Four samples of synthetic chromium-bearing spinels of (Mg, Fe2+)(Cr, Fe3+)2O4 composition and four samples of natural spinels of predominantly (Mg, Fe2+)(Al, Cr)2O4 composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic end-member MgCr2O4 spinel was also studied at pressures up to ca. 10 GPa. In both synthetic and natural samples, chromium is present predominantly as octahedral Cr3+ seen in the spectra as two broad intense absorption bands in the visible range caused by the electronic spin-allowed 4A2g → 4T2g and 4A2g → 4T1g transitions (U- and Y-band, respectively). A distinct doublet structure of the Y-band in both synthetic and natural spinels is related to trigonal distortion of the octahedral site in the spinel structure. A small, if any, splitting of the U-band can only be resolved at curve-fitting analysis. In all synthetic high-chromium spinels, a couple of relatively narrow and weak bands of the spin-allowed transitions 4A2g → 2Eg and 4A2g → 2T1g of Cr3+, intensified by exchange-coupled interaction between Cr3+ and Fe3+ at neighboring octahedral sites of the structure, appear at ~14,400 and ~15,100 cm−1. A vague broad band in the range from ca. 15,000 to 12,000 cm−1 in synthetic spinels is tentatively attributed to IVCr2+ + VICr3+ → IVCr3+ + VICr2+ intervalence charge-transfer transition. Iron, mainly as octahedral Fe3+, causes intense high-energy absorption edge in near UV-range (ligand–metal charge-transfer O2− → Fe3+, Fe2+ transitions). As tetrahedral Fe2+, it appears as a strong infrared absorption band at around 4,850 cm−1 caused by electronic spin-allowed 5E → 5T2 transitions of IVFe2+. From the composition shift of the U-band in natural and synthetic MgCr2O4 spinels, the coefficient of local structural relaxation around Cr3+ in spinel MgAl2O4–MgCr2O4 system was evaluated as ~0.56(4), one of the lowest among (Al, Cr)O6 polyhedra known so far. The octahedral modulus of Cr3+ in MgCr2O4, derived from pressure-induced shift of the U-band of Cr3+, is ~313 (50) GPa, which is nearly the same as in natural low-chromium Mg, Al-spinel reported by Langer et al. (1997). Calculated from the results of the curve-fitting analysis, the Racah parameter B of Cr3+ in natural and synthetic MgCr2O4 spinels indicates that Cr–O-bonding in octahedral sites of MgCr2O4 has more covalent character than in the diluted natural samples. Within the uncertainty of determination in synthetic MgAl2O4 spinel, B does not much depend on pressure.

Synthesis, crystal structure and optical properties of Ba5V3O12F

Colorless and transparent Ba5V3O12F crystal has been synthesized by solid state reaction and characterized by single-crystal X-ray diffraction and physical properties analyses. Ba5V3O12F crystallizes in the hexagonal system with space group P63/m. The structure of apatite-like compound Ba5V3O12F can be described on the basis of a “framework” made of Ba(2)O6 polyhedra and VO4 tetrahedra, which form [001]-channels filled with Ba(1) cations and F anions. The VO4 groups presented in the sample are identified by Fourier transform infrared spectrum. UV–Vis–NIR diffuse reflectance spectrum of Ba5V3O12F shows that it has a wide transmittance range from 322 to 2600nm.

Crystal Growth, Structure, Polarization, and Magnetic Properties of Cesium Vanadate, Cs2V3O8: A Structure–Property Study

Cesium vanadate, Cs2V3O8, a member of the fresnoite-type structure, was synthesized via a hydrothermal route and structurally characterized by single-crystal X-ray diffraction. Cs2V3O8 crystallizes in a noncentrosymmetric polar space group, P4bm, with crystal data of a = 8.9448(4) Å, c = 6.0032(3) Å, V = 480.31(4) Å(3), and Z = 2. The material exhibits a two-dimensional layered crystal structure consisting of corner-shared V(5+)O4 and V(4+)O5 polyhedra. The layers are separated by the cesium cations. The alignment of the individual polyhedra results in a macroscopic polarity for Cs2V3O8. Frequency-dependent polarization measurements indicate that the material is not ferroelectric. A pyroelectric coefficient of -2.0 μC m(-2) K(-1) was obtained from pyroelectric measurements taken as a function of the temperature. The magnetic susceptibility data were measured as a function of the temperature and yielded an effective magnetic moment of 1.78 μB for the V(4+) cation. Short-range magnetic ordering was observed around 7 K. The susceptibility data were fit to the Heisenberg square-lattice model supporting that the short-range magnetic interactions are antiferromagnetic and two-dimensional. IR and thermal properties were also characterized.

Exploration of a new compound in the M–B–O–X (M: alkali metals; X: halogen) system: Preparation, crystal and electronic structures, and optical properties of Na3B6O10Br

A new bromoborate Na3B6O10Br has been synthesized by the high temperature solution method. It crystallizes in orthorhombic space group Pnma (No. 53) with lattice constants a=12.780(8)Å, b=9.914(6)Å, c=7.518(2)Å, Z=4. The crystal structure contains an infinite three-dimensional matrix that is built from B6O13 units, Na(1)O4Br2, Na(2)O6Br, and Na(3)O6 polyhedra. Interestingly, four B6O13 units are interconnected by oxygen atoms forming hexadecagon huge pores. IR spectrum, UV–Vis–NIR diffuse reflectance spectrum, and thermogravimetric analyses for the compound are also presented, as are band structures and density of states calculations.

Calciodelrioite, Ca(VO3)2(H2O)4, the Ca analogue of delrioite, Sr(VO3)2(H2O)4

AbstractCalciodelrioite, ideally Ca(VO3)2(H2O)4, is a new mineral (IMA 2012-031) from the uraniumvanadium deposits of the eastern Colorado Plateau in the USA. The type locality is the West Sunday mine, Slick Rock district, San Miguel County, Colorado. The new mineral occurs on fracture surfaces in corvusite- and montroseite-impregnated sandstone and forms as a result of the oxidative alteration of these phases. At the West Sunday mine, calciodelrioite is associated with celestine, gypsum, huemulite, metarossite, pascoite and rossite. The mineral occurs as transparent colourless needles, bundles of tan to brown needles and star bursts of nearly black broad blades composed of tightly intergrown needles. Crystals are elongate and striated parallel to [100], exhibiting the prismatic forms {001} and {011} and having terminations possibly composed of the forms {100} and {61}. The mineral is transparent and has a white streak, subadamantine lustre, Mohs hardness of about 2½, brittle tenacity, irregular to splintery fracture, one perfect cleavage on {001} and possibly one or more additional cleavages parallel to [100]. Calciodelrioite is soluble in water. The calculated density is 2.451 g cm– 3. It is optically biaxial (+) with α = 1.733(3), β = 1.775(3), γ = 1.825(3) (white light), 2Vmeas = 87.3(9)° and 2Vcalc = 87°. The optical orientation is X = b; Z ≈ a. No pleochroism was observed. Electronmicroprobe analyses of two calciodelrioite samples and type delrioite provided the empirical formulae (Ca0.88Sr0.07Na0.04K0.01)Σ1.00(V1.00O3)2(H2.01O)4, (Ca0.76Sr0.21Na0.01)Σ0.98(V1.00O3)2(H2.01O)4 and (Sr0.67Ca0.32)Σ0.99(V1.00O3)2(H2.00O)4, respectively. Calciodelrioite is monoclinic, I2/a, with unit-cell parameters a = 14.6389(10), b = 6.9591(4), c = 17.052(2) Å, β = 102.568(9)°, V = 1695.5(3) Å3 and Z = 8. The seven strongest lines in the X-ray powder diffraction pattern [listed as dobs Å (I)(hkl)] are as follows: 6.450(100)(011); 4.350(16)(013); 3.489(18)(020); 3.215(17)(022); 3.027(50)(multiple); 2.560(28)(15,413); 1.786(18)(028). In the structure of calciodelrioite (refined to R1 = 3.14% for 1216 Fo > 4σF), V5+O5 polyhedra link by sharing edges to form a zigzag divanadate [VO3] chain along a, similar to that in the structure of rossite. The chains are linked via bonds to Ca atoms, which also bond to H2O groups, yielding CaO3(H2O)6 polyhedra. The Ca polyhedra form a chain along b. Each of the two symmetrically independent VO5 polyhedra has two short vanadyl bonds and three long equatorial bonds. Calciodelrioite and delrioite are isostructural and are the endmembers of the series Ca(VO3)2(H2O)4–Sr(VO3)2(H2O)4. Calciodelrioite is dimorphous with rossite, which has a similar structure; however, the smaller 8-coordinate Ca site in rossite does not accommodate Sr.

A novel lithium copper iron phosphate with idealized formula Li5Cu22+Fe3+(PO4)4: crystal structure and distribution of defects

Gray–green single crystals were obtained under high-pressure, high-temperature hydro­thermal conditions. A refinement of atom occupancies gave the composition Li3.68Cu2+Fe3+(Cu0.55Li0.45)2Fe2+ 0.15(PO4)4. The structure is built from triplets of edge-sharing (Cu,Li)O5–FeO6–(Cu,Li)O5 polyhedra, CuO4 quadrilaterals and PO4 tetra­hedra. In the (Cu,Li)O5 polyhedra the Cu and Li positions are statistically occupied in a 0.551 (2):0.449 (2) ratio. Both FeO6 and CuO4 polyhedra exhibit symmetry. The positions of additional Li atoms with vacancy defects are in the inter­stices of the framework.

Structural study of nonlinear optical borates K1−xNaxSr4(BO3)3 (x≤0.5)

X-ray powder diffraction was used for the structural study of nonlinear optical borates K1−xNaxSr4(BO3)3 (x≤0.5). Results show that up to 50% K+ can be substituted by Na+ in orthorhombic K1−xNaxSr4(BO3)3. Isolated BO3 triangles in the Na-substituted compound constrict to adjust to a local distribution of alkali-metal atoms, which explains the large range of structural homogeneity. An expansion of the c axis in a unit cell with increasing Na substitution was found probably caused by the tilted BO3 triangles and asymmetric distortion of (K/Na)O8 polyhedra. As the ratio of ionic radii of alkaline-earth and alkali-metal cations decreases and the electronegative difference between alkaline-earth and alkali-metal cations increases, the crystal system of MM′4(BO3)3 borates changes from cubic to orthorhombic and then to monoclinic.

Synthesis and Crystal Structure of the First Thallium Hydrous Nesosilicate Tl4SiO4·0.5H2O

AbstractOrange prismatic crystals of the first thallium hydrous nesosilicate Tl4SiO4·0.5H2O have been obtained by evaporation from aqueous solution. There are three symmetrically independent Tl+ cations and five symmetrically independent oxygen atoms in the structure of Tl4SiO4·0.5H2O. The O(4) and O(5) atoms belong to water molecules. Coordination polyhedra of the Tl+ cations are strongly distorted because of the stereoactive behavior of lone electron pairs. The structure of Tl4SiO4·0.5H2O contains sheets of SiO4 tetrahedra and Tl coordination polyhedra. The sheets have the composition [Tl3SiO4]– and are parallel to [100]. Within the sheets, SiO4 tetrahedra link to thallium polyhedra though common corners. The sheets are linked by dimers of face‐sharing Tl(3)O5 polyhedra, thus providing interconnection of the sheets into a framework. The framework has large elliptical channels occupied by water molecules (OW2) and electron pairs of Tl+ cations.The comparison with some other M+ (M = K, Ag, Tl) silicates is given.

Hydrothermal Synthesis, Crystal Structure and Characterizations of Two New Manganese(II) Phosphonates with a 3D Framework Structure

AbstractTwo new manganese(II) phosphonates, (NH4)Mn2.5[(O3PCH(OH)CO2)2(H2O)] (1) and [NH3(CH2)4NH3]0.5Mn2.5[(O3PCH(OH)CO2)2(H2O)] (2) have been synthesized under hydrothermal conditions and structurally characterized by single‐crystal X‐ray diffraction as well as with infrared spectroscopy, elemental analysis and thermogravimetric analysis. The two isomorphous compounds feature a 3D framework structure. The Mn(1)O6 and Mn(3)O5 polyhedra are bridged by the CPO3 tetraheda into a MnII phosphonate layer in ac‐plane. Mn(2)O6 polyhedra are linked to each other by CPO3 tetraheda to form infinite chains, which are connected to layers by carboxylate groups to form a 3D framework structure with channels along the a‐ and c‐axis, respectively. The NH4+ ions or protonated 1, 4‐butylenediamine cations are located inside the channels along the a axis.

Optical spectroscopic study of synthetic NaScSi2O6–CaNiSi2O6 pyroxenes at normal and high pressures

Six synthetic NaScSi2O6–CaNiSi2O6 pyroxenes were studied by optical absorption spectroscopy. Five of them of intermediate (Na1−x , Ca x )(Sc1−x , Ni x )Si2O6 compositions show spectra typical of Ni2+ in octahedral coordination, more precise Ni2+ at the M1 site of the pyroxene structure. The common feature of all spectra is three broad absorption bands with maxima around 8,000, 13,000 and 24,000 cm−1 assigned to 3 A 2g → 3 T 2g, 3 A 2g → 3 T 1g and →3 T 1g (3 P) electronic spin-allowed transitions of VINi2+. A weak narrow peak at ∼14,400 cm−1 is assigned to the spin-forbidden 3 A 2g → 1 T 2g (1 D) transition of Ni2+. Under pressure the spin-allowed bands shift to higher energies and change in intensity. The octahedral compression modulus, $$ k^{{{\text{loc}}}}_{{{\text{Ni}}{\left( {{\text{M}}1} \right)}}} , $$ calculated from the shift of the 3 A 2g → 3 T 2g band in the (Na0.7Ca0.3)(Sc0.7Ni0.3)Si2O6 pyroxene is evaluated as 85±20 GPa. The Racah parameter B of Ni2+(M1) is found gradually changing from ∼919 cm−1 at ambient pressure to ∼890 cm−1 at 6.18 GPa. The Ni end-member pyroxene [(Ca0.93 Ni0.07)NiSi2O6] has a spectrum different from all others. In addition to the above mentioned bands of Ni2+(M1) it displays several new relatively intense and broad extra bands, which were attributed to electronic transitions of Ni2+ at the M2 site. In difference to CaO8 polyhedron geometry of an eightfold coordination, Ni2+(M2)O8 polyhedra are assumed to be relatively large distorted octahedra. Due to different distortions and different compressibilities of the M1 and M2 sites the Ni2+(M1)- and Ni2+(M2)-bands display rather different pressure-induced behaviors, becoming more resolved in the high-pressure spectra than in that measured at atmospheric pressure. The octahedral compression modulus of Ni2+(M1) in this end-member pyroxene is evaluated as 150 ± 25 GPa, which is noticeably larger than in Ni0.3 pyroxene. This is due to a smaller size and, thus, a stiffer character of Ni2+(M1)O6 octahedron in the (Ca0.93Ni0.07)NiSi2O6 pyroxene compared to (Na0.7Ca0.3)(Sc0.7Ni0.3)Si2O6.

Hydrothermal synthesis and structure of [C2N2H10][La2(H2O)4(SO4)4]·2H2O, a new organically templated rare earth sulfate with a layer structure

Employing ethylenediamine (en) as the template, a new two-dimensional (2D) layered lanthanum sulfate [C2N2H10][La2(H2O)4(SO4)4]·2H2O has been hydrothermally synthesized for the first time. Its structure, solved by single-crystal X-ray diffraction analysis, comprises a network of La(1)O10, La(2)O8 polyhedra and SO4 tetrahedra forming one-dimensional ladder-like chains. The chains, in turn, are linked via oxygen atoms forming macroanionic inorganic zigzag layers. The diprotonated amine molecules and the water molecules are located in the apertures of zigzag layers and interact with the inorganic framework through hydrogen bonding.

INSIGHTS INTO ASTROPHYLLITE-GROUP MINERALS. II. CRYSTAL CHEMISTRY

The crystal chemistry of 20 specimens of astrophyllite-group minerals from seven localities has been studied using a combination of single-crystal X-ray analysis of the structure and electron-microprobe analyses. Members of the astrophyllite group are triclinic ( P 1) and monoclinic ( C2/c or A 2) heterophyllosilicates with the general formula A 2 BC 7 D 2 T 8O26(OH)4 X 0–1, where A = [10]–[13]K, Rb, Cs, Na, H3O+, H2O, or □; B = [10]Na or Ca; C = [6]Mn, Fe2+, Fe3+, Na, Mg, or Zn; D = [6]Ti, Nb, or Zr; T = Si and Al, and X = F, OH, O, or □. The structure consists of two composite sheets, a heterogeneous ( H ) sheet and a sheet of octahedra ( O ), stacked along [001] in a 2:1 ratio. These 2:1 composite layers are bonded via a shared apical D ϕ6 anion through an interlayer containing an ordered array of K and Na, resulting in a layered HOH structure reminiscent of the TOT structure in 2:1 phyllosilicates. There are four crystallographically distinct M O6 octahedra in the O sheet; they host Mn, Fe2+, Fe3+, Na, Mg and Zn. The M (1) octahedron, the largest and least distorted within the O sheet, is host predominantly to Mn and Fe2+, with significant Na (up to 0.274 apfu ). The M (2) octahedron is highly distorted and contains predominantly Mn and Fe2+, whereas the M (3) and M (4) octahedra are the smallest and are occupied by Mn, Fe2+, Fe3+, Mg and Zn. The H sheet consists of open-branched zweier [100] single [Si4O12]8− chains cross-linked by corner-sharing D ϕ6 octahedra in triclinic species and kupletskite- Ma 2 b 2 c , and D O5 polyhedra in magnesium astrophyllite. Distortion of the D ϕ6 octahedron is minimized by (Nb,Zr) ⇔ Ti substitution, resulting in displacement of the cation toward the center of the octahedron. The composition of the [Si4O12]8− chains does not vary; there is essentially no Al, and Mossbauer spectroscopy did not indicate the presence of [4]Fe3+. Lateral fit of the O and H sheets is accomplished primarily by: (1) flattening of O -sheet octahedra, (2) tilting of the H -sheet tetrahedra out of the (001) plane, and (3) elongation or thickening of the tetrahedra along [001].

A Mixed α/β Superstructure in NASICON Ionic Conductors: Neutron Diffraction Study of Li2FeTi(PO4)3 and Li2FeZr(PO4)3

The lithium conductors Li2FeTi(PO4)3 and Li2FeZr(PO4)3, synthesized by solid-state reaction and characterized by X-ray powder diffractometry, were studied structurally at room temperature by neutron powder diffraction at high resolution (HRPD, ISIS Facility, U.K.). By trial-and-error and Rietveld refinements (Rp=0.111, R(F2)=0.112), the first compound (orthorhombic Pbca, Z=8; a=8.5515(1), b=8.6229(1), c=23.9116(3) Å) was shown to have a complex superstructure sharing features of both the α and β NASICON-type phases of LiZr2(PO4)3. Four (001) layers of PO4 and (Fe, Ti)O6 polyhedra are present per unit-cell, and they are related both by 1 inversion centers (α structure) and by a glide planes (β structure). Ti4+ and Fe3+ order in the two interlayer regions, respectively. Owing to the structure complexity, only half of the lithium atoms could be refined in tetrahedral coordination with 〈Li–O〉=1.99 Å. Li2FeZr(PO4)3 (orthorhombic Pbna, Z=4; a=8.70559(8), b=8.78572(9), c=12.2202(1) Å) proved to be similar to β-LiZr2(PO4)3; however, by Fourier synthesis and Rietveld refinement (Rp=0.0618, R(F2)=0.0574) Li was located in a fully ordered tetrahedral configuration with 〈Li–O〉=2.01 Å, instead of being disordered as in the β phase of LiZr2(PO4)3.

A zinc phosphate oxalate with phosphate layers pillared by the oxalate units

An open-framework phosphate oxalate of zinc, of composition [NH3(CH2)3NH3][Zn6(PO4)4(C2O4)], 1, has been synthesized, for the first time, employing hydrothermal methods in the presence of ethylenediamine. The structure comprises a network of Zn(1)O5, Zn(2)O4, Zn(3)O5 polyhedra and PO4 tetrahedra connected through their vertices forming neutral layers bridged by the oxalate units, giving one-dimensional channels. The presence of Zn in tetrahedral, trigonal-bipyramidal and square-pyramidal coordinations, and the formation of neutral layers, are noteworthy features.

Structural investigation of platinum solubility in silicate glasses

The coordination environment of 20-200 ppm Pt in yellowish glasses from the CaO-Al 2O3-SiO2 (CAS) ternary was studied using X-ray absorption fine structure spectroscopy at the Pt-L III edge. Analysis of the Pt-L III edge region suggests that Pt in these glasses is mainly tetravalent and sixfoldcoordinated by O (with a mean Pt-O distance of 2.08 ± 0.02 A). No evidence for Pt 2+ or Pt 6+ was found in any of the glasses studied, suggesting that one can not derive valence information easily from solubility data. No second-neighbor contribution was observed around Pt 4+ O6 polyhedra. However, bond-valence modeling suggest that these polyhedra are likely to bond mostly to [VI] Ca 2+ , which should promote high positional disorder of second -neighbor cations around Pt. This particular bonding arrangement may explain the relatively high solubility of Pt in these relatively depolymerized melts, as CaPtO3-type units.

Structural study of the trigonal Bi2.34U0.33La0.33O5 oxide ion conductor: Rietveld refinement of X-ray and neutron powder diffraction data

The structure of Bi2.34U0.33La0.33O5 has been investigated by Rietveld refinement of powder X-ray and neutron diffraction data. The compound is trigonal, space group P, Z = 1, and lattice parameters aH = 4.01395(8), cH = 9.5423(2) A. X-Ray diffraction data are in favour of a model in which U and La atoms are situated in different crystallographic positions: i.e. 1a and 2d of the P space group respectively, forming mixed Bi/U and Bi/La layers which alternate along the c axis. The structure of this bismuth-based mixed oxide can be described as built up of interlocked edge-sharing (Bi/U)O8 and (Bi/La)O8 polyhedra. The co-ordination polyhedron around the Bi/U atoms is a hexagonal bipyramid, the equatorial plane being a puckered hexagon. The Bi/La atoms are co-ordinated to eight oxygen atoms forming a slightly distorted cube. Electron diffraction studies showed that the structure can be described as a fluorite-type superstructure, the relation to the fluorite sub-cell is a*H ≈ ⅓[4]a*C, b*H ≈ ⅓[ 22]a*C, c*H ≈ ⅓[111]a*C. This compound is a good oxide ion conductor (σ300 °C = 2.5 × 10–5 S cm–1). On the basis of the anion vacancies distribution determined from neutron diffraction refinement, a pathway for the oxide ion conduction is proposed.

Structural control of polyhedral compression in synthetic braunite, Mn 2+ Mn 3+ 6 O 8 SiO 4

The compression of synthetic braunite, Mn2+Mn3+6O8SiO4, was studied by high-pressure single-crystal X-ray diffraction carried out in a diamond-anvil cell. The equation of state at room temperature (third-order Birch-Murnaghan equation of state: V0=1661.15(8) A3, K0,298=180.7±0.9 GPa, K′=6.5±0.3) was determined from unit-cell volume data to 9.18 GPa. Crystal structures were determined at 6 different pressures to 7.69 GPa. Compression of the structure (space group I41/acd) was found to be slightly anisotropic (a0=9.4262(4) A, Ka=499±4 GPa, Ka′=19.7±0.9; c0=18.6964(6) A, Kc=657±6 GPa, Kc′=15.7±1.4) which can be attributed to the fact that the Mn3+-O bonds, which are the most compressible bonds, are aligned closer to the (001) plane than to the c axis. The large bulk modulus is the result of the structural topology in which 2/3 and 1/2 of the edges of the Mn2+O8 and Mn3+O6 polyhedra share edges with other polyhedra. The Mn2+O8 polyhedra were found to compress isotropically, whereas anisotropic compressional behaviour was observed for all three Mn3+O6 octahedra. Although the polyhedral geometry of all three crystallographically independent Mn3+ sites shows the same type of uniaxially elongated distortion, the compression of the individual octahedral configurations was found to be strongly dependent upon both the geometry of the polyhedron itself and the types of, and the connectivity to, the neighbouring polyhedra. The differences in the configuration of the different oxygen atoms, and therefore the structural topology, is one of the major factors determining the type and degree of the pressure-induced distortion, while the Jahn-Teller effect plays a subordinate role.