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The crystal structure of Ba4Ru3O10 has been determined by single-crystal X-ray diffraction at room pressure. From refinements to R=0.0203 at room temperature and ambient pressure, the material is orthorhombic with space group Cmca (space group No. 64) and has lattice parameters of a=5.7762(15) A, b=13.271(4) A, and c=13.083(3) A. The unit cell thus has a volume of V=1002.9(8) A3 and contains four formula units (Z=4). Ba4Ru3O10 is therefore of higher symmetry than the previously reported monoclinic structure based on powder X-ray data. It is isostructural with the quaternary oxides Ba4(Ti, Pt)3O10 and Ba4Ir2AlO10 and the ternary fluorides Cs4M3F10 (M=Mg, Co, Ni, Zn). Kinked chains of RuO6 octahedra run along the c direction, consisting of sets of three face-sharing units joined at the corners of the end units to additional similar sets. The two distinct Ba sites show 10-fold and 11-fold coordination. Compressibilities and bulk modulus have been determined from lattice parameter variations at pressures up to 5.4 GPa. No phase transition was observed up to this pressure. Compressibility is greatest along the c axis and the bulk modulus obtained from a weighted fit to a Vinet equation of state is 113.3(47) GPa.

The crust and upper part of the mantle of the Earth are dominated by phases with silicon in tetrahedral coordination with oxygen, whereas, in the lower part of the mantle, silicon is in octahedral, or higher, coordination. The goal of high-pressure mineralogy is to study the transitions that accomplish this transformation, and the properties of the phases that result. Since any density discontinuity in the Earth will give rise to a seismic signal, several such boundaries have been located. Particularly large changes occur at the crust-mantle and mantle-core boundaries; however, additional density contrasts are found at 400 and 670 km, which define the transition zone between the upper and lower mantle. From high-temperature, high-pressure experiments and estimates of the pressure-temperature gradient in the planet, the upper boundary of this zone corresponds to the breakdown of olivine, and the lower boundary is formed by the spinel to silicate perovskite transformation. Many of the effects associated with pressure increase, such as bond compression and transformations to higher coordination numbers, are not surprising; however, other effects that seem to be associated with ordering are unexpected. Among these are the crystallization of very complicated structures with simple chemistry. Apparent violations of some of the crystal-chemical rules formulated at low pressure are also observed in some phases.

The previously observed pressure-induced amorphization transition at 11 kbar [A.J. Schultz, U. Geiser, H.H. Wang, J.M. Williams, L.W. Finger and B.M. Hazen, Physica C 208 (1993) 277] was avoided and single-crystal X-ray data up to 28 kbar were obtained by replacing mineral oil with a hydrostatic alcohol mixture as the pressure medium in diamond anvil cell. The linear compressibilities of the crystallographic axes are remarkably isotropic given the two-dimensional nature of the structure. A full structure determination was performed at room temperature and 28 kbar with the use of X-ray data. It is noteworthy that, a novel structure feature, short interlayer H…H contacts through a hole in the anion layer, may account for the isotropic compressibilities. Electronic band structures calculated for the crystal structures of κ-(ET) 2 Cu[N(CN) 2 ]Cl at 1 bar, 3 kbar and 28 kbar show that this salt is a two-dimensional metal, and that the density of states at the Fermi level decreases gradually with increasing pressure.

Crystal structures of HgBa2CaCu2O6+δ (Hg-1212) and HgBa2Ca2Cu3O8+δ (Hg-1223) have been determined on single crystals at room temperature conditions by X-ray diffraction techniques. Both tetragonal specimens are isomorphous with compounds in the Tl-Ba-Ca-Cu-O system. We observe excess oxygen in both samples on two sites near the z=0 plane: at ( 1 2 00) and close to ( 1 2 , 0.4, 0) - but no oxygen at ( 1 2 1 2 0) as reported in earlier studies. Refined Hg occupancies for both samples indicate a substitution of Cu for Hg of more than ten percent. If the substitution of Cu for Hg and excess O at ( 1 2 00) are coupled, then the effect is to insert a perovskite-like six-coordinated copper site at (000). No evidence was found for a split atom at the barium sites in these compounds.

A reversible symmetry-lowering structural phase transition is observed by X-ray diffraction techniques from single crystals of κ-(ET) 2 Cu[N(CN) 2 ]Cl (ET, or BEDT-TTF, is bis(ethylenedithio) tetrathiafulvalene, C 10 H 8 S 8 ) under applied pressure. At room temperature, by use of a diamond anvil cell, a transition above 8 kbar characterized by peak broadening begins to occur. The transition is also observed by cooling crystals coated with Apiezon N vacuum grease, which is a technique that is also used to produce the high pressure superconducting state ( T c =12.8 K, P =0.3 kbar). Given the anisotropic nature of the structure, the ambient pressure linear compressibilities are surprisingly isotropic, with values of β a =2.39(8)×10 -3 , β b =2.47(7)×10 -3 , β c =2.29(9)×10 -3 kbar -1 . At 3 kbar and room temperature, from a complete structure analysis it is shown that the ET molecule conformation is the same as that at ambient pressure although S⋯S contacts are reduced by as much as 0.05 A. A second transition, characterized by reversible disappearance of Bragg intensities consistent with pressure-induced amorphization, was observed between 11.4 and 11.8 kbar.

Undoped La 2 CuO 4 oxide ceramic single crystal and polycrystalline materials were studied up to 25.0 GPa in a diamond-anvil cell (DAC) apparatus. La 2 CuO 4 undergoes a phase transformation from an orthorhombic to a tetragonal structure at about 3.4 GPa, and this result is different from the data previously published by others. Compression of this material is only 9% at 25.0 GPa.

Unit-cell parameters and crystal structures of CaWO4(scheelite) and CaMoO4 (powellite) have been determined at several pressures to 5 8 GPa, and unit-cell parameters of PbMoO4 (wulfemte), PbWO4 (stolzite) and CdMoO4 have been measured at pressures to 6 0 GPa All five tetragonal scheelite-type compounds compress anisotropically, with the (itc) axis 1 2 to 1 9 times more compressible than a. In both CaWO4 and CaMoO4 the cation tetrahedra (with W6+ or Mo6+) behave as rigid structural elements with no observed cation-oxygen compression (W-O and Mo-O bond compression < 0001 GPa−1) Compression of the eight-coordinated calcium polyhedron, on the other hand, is comparable to bulk compression of the compounds (Ca-O bond compression = 0005 ± 0 001 GPa−1) Amsotropies in the pressure response of the calcium polyhedron, which is more compressible parallel to c than perpendicular to c, result in the amsotropic unit-cell compression Bulk moduli of the five compounds (with K' assumed to be 4) are CaWO4 (68 ± 9 GPa), CaMoO4 (81 5 ± 0 7 GPa), PbWO4 (64 ± 2 GPa), PbMoO4 (64 ± 2 GPa), and CdMoO4 (104 ± 2 GPa) No reversible transitions to the monoclinic (fergusomte) distortion of scheelite were observed in these compounds Pressure-volume data for PbWO4, however, display strong positive curvature (K'calc) = 23 ± 2) up to about 5 GPa, at which pressure crystals appear to undergo a first-order phase transition The relatively large curvature may be a premonitory effect pnor to a reconstructive transition Structural changes in these compounds with increasing pressure are qualitatively similar to changes that result from isobanc cooling or substitution of a smaller cation in the eight-coordinated site.

Single-crystal X-ray diffraction experiments on α-quartz at pressures to 15 GPa reveal structural instabilities that result in its gradual transition to an amorphous state. With increasing pressure the average SiO distance (≈ 1.61 ± 0.01 A) and SiO4 tetrahedral volume (≈ 2.14 ± 0.02 A3 remain constant. Compression of α-quartz results from a dramatic decrease in SiOSi angle and corresponding decrease in inter-tetrahedral (i.e., OO) distances. The onset of amorphization coincides with bending of all SiOSi angles to less than 120° and severe distortion of SiO4 tetrahedra, as oxygens approach a close-packed configuration.

The crystal structure of fassaite from the Angra dos Reis meteorite has been determined by least-squares refinement of three-dimensional X-ray data to anR value of 3.3%. The pyroxene is monoclinic, space groupC2/c, with unit-cell dimensionsa = 9.738(1),b = 8.874(2),c = 5.2827(5)A, β = 105.89(1)°, andV = 439.1(1)A3. Average bond lengths are (Si,Al)-O = 1.651, M1-O = 2.061, and M2-O = 2.489A. The distribution of iron and magnesium between M1 and M2 suggests a temperature of equilibration greater than 1000°C. Electron microprobe analysis of several fassaite grains reveals small but statistically significant variations of (Mg + Si) versus (Al - Ti). The range of fassaite composition may be represented byEn3Hd22TiCpx6(Di53±2CaTs16∓2) whereEn=Mg2Si2O6,Hd=CaFeSi2O6,TiCpx=CaTiAl2O6,Di=CaMgSi2O6,CaTs=CaAl2SiO6. Most fassaite analyses calculated on the basis of four cations yielded greater than six anions, suggesting that part of the titanium or chromium might be reduced to Ti3+ or Cr2+.

Unit-cell parameters and crystal structures of five rutile-type compounds, including TiO 2 ; SnO 2 , GeO 2 , RuO 2 ; and MnF 2 ;, have been determined at pressures to 50 kbar at 20°C. All five compounds compress anisotropically with the a axis approximately twice as compressible as c . The one variable positional parameter, x of oxygen or fluorine, changes little with pressure. The uniform behavior of these RX 2 compounds at high pressure contrasts with their highly variable structural changes at high-temperature. Rutile-type oxides are, therefore, unlike most oxides and silicates, in which structural variations at high pressure mirror those at high temperature.

Mcbirneyite, Cu3(VO4)2, a new copper vanadate mineral, has been discovered in the summit crater fumaroles of Izalco volcano, El Salvador. The mineral formed as a sublimate from the volcanic gases and is associated with stoiberite, fingerite, ziesite, euchlorine and thenardite. Mcbirneyite is the naturally occurring analog of synthetic Cu3V2O8. Mcbirneyite is triclinic, P 1 , a=5.3418(9) A, b=6.5100(8) A, c=5.1798(7) A, α=88.61(1)°, β=68.11(1)°, γ=69.22(1)°. The five most intense lines in the diffraction pattern are 4.01 (80; 01 1 ), 3.12 (100; 120), 2.82 (100; 10 1 ), 2.641 (80; 211) and 2.428 A (80; 021). The crystal structure of the mineral has been determined; the independent structure solution demonstrates that mcbirneyite is isostructural with stranskiite, CuZn2 (AsO4)2, and a synthetic Cu phosphate, Cu3(PO4)2. The structure is constructed of (1 1 0) layers of Cu atoms in square-planar and square-based pyramidal coordination with oxygen; adjacent layers are linked by isolated VO4 tetrahedra. Mcbirneyite is black with a metallic luster. The calculated density (Z=1) is 4.50 g cm−3. The mineral is opaque to transmitted light; in reflected light in air the mineral is medium-dark gray and nonpleochroic, with reflectance values as follows: 470 nm, 18.5%; 546 nm, 17.5%; 589 nm, 18.7%; 650 nm, 20.6%. The mineral is named in honor of Alexander R. McBirney, Editor-in-Chief, Journal of Volcanology and Geothermal Research.