Search

X-ray intensities for the “tetragonal almandine-pyrope phase” (TAPP), *(FeMg) VIII (AlMgFe) 4 [IV (SiAl)O 4 ] 3 where * refers to a capped tetrahedral site, reported by Harris et al. (1997) were recollected using the identical crystal, and the crystal structure was refined from these new data. The basic structure of the previous report is confirmed; however, some of the details regarding cation occupancies are revised. The chemical composition of this phase corresponds to a high-pressure origin; however, significant questions remain regarding the crystallization pressure of this phase.

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 crystal structure of (Mg (sub 1.54) Li (sub 0.23) Sc (sub 0.23) )Si 2 O 6 protopyroxene has been studied with single-crystal X-ray diffraction at pressures to 9.98 GPa and Raman spectroscopy to 10.4 GPa. A first-order displacive phase transformation from the Pbcn space group to P2 1 cn was observed between 2.03 and 2.50 GPa, which is characterized by a discontinuous decrease in a, c, and V by 1.1, 2.4, and 2.6%, respectively, and an increase in b by 0.9%, along with appearance of intensities of some Okl reflections with knot =2n. This is the first substantiated example of protopyroxene having the symmetry predicted by Thompson (1970). Evidence for the phase transition from Raman spectroscopy is also presented. The prominent structural changes associated with the Pbcn-to-P2 1 cn transformation involve the abrupt splitting of one type of O-rotated silicate chain in low-pressure protopyroxene into S-rotated A and O-rotated B chains in high-pressure protopyroxene, coupled with a marked decrease in the O3-O3-O3 angles and a re-configuration of O atoms around the M2 site. The kinking angle of the silicate chain in the low-pressure phase at 2.03 GPa is 165.9 degrees , whereas the angles are 147.9 degrees and 153.9 degrees for the A and B chains, respectively, in high-pressure phase at 2.50 GPa. Strikingly, the two types of silicate chains in the P2 1 cn structure alternate along the b axis in a tetrahedral layer parallel to (100). Such a mixed arrangement of two differently rotated silicate chains in a tetrahedral layer has not been observed in any other pyroxene structure. Compression anisotropy of the protopyroxene structure is affected by the phase transition. The relative linear compressibilities (beta a :beta b :beta c ) are 1.00:1.72:0.99 for low-pressure protopyroxene, but are 1.00:1.28:1.65 for high-pressure protopyroxene. The bulk moduli of low- and high-pressure phases are 130(3) and 111(1) GPa, respectively. This study concludes that the Pbcn-to-P2 1 cn phase transition results from the differential compression between SiO 4 tetrahedra and MO 6 octahedra.

Phase egg, first described by Eggleton et al. (1978), was synthesized and its composition determined to be AlSiO3OH. The crystal structure of AlSiO3OH, including the position of the hydrogen, has been solved and refined from high-resolution X-ray powder diffraction. The resulting lattice constants are a 5 7.14409(2) A ˚, b 5 4.33462(1) A ˚, c 5 6.95253(2) A ˚, and b5 98.396(1)8. The space group is P21/n; Z 5 4, V0 5 212.99(1) A ˚ 3 , and the zero

The structural changes associated with the C2/m-P2 1 /m phase transition in cummingtonite with (Fe+Mn)/(Fe+Mn+Mg) nearly equal 0.50 have been studied with single-crystal X-ray diffraction at various pressures up to 7.90 GPa and infrared spectroscopy up to 8.63 GPa. With increasing pressure, the crystal transforms from C2/m to P2 1 /m symmetry at approximately 1.21 GPa, as determined by the appearance of reflections violating the C2/m space group. Infrared spectra provide additional evidence for the phase transition: A distinct splitting of OH stretching bands results from an increase from one to two nonequivalent OH positions. The C2/m-P2 1 /m transition is of weakly displacive first-order or tricritical character with apparent slope changes in the plots of the axial ratios a/b and a/c as a function of pressure. The unit-cell compression is considerably anisotropic with the a dimension in both C2/m and P2 1 /m phases being the most compressible. Major structural changes for the C2/m-P2 1 /m transition include: (1) One crystallographically distinct silicate chain becomes two discontinuously, coupled by the splitting of the M4-05 bond, as well as M4-06, into two nonequivalent bonds, and (2) the M4-cation coordination increases from sixfold to sevenfold. More importantly, we observed a change in the sense of rotation for the A chain while the crystal structure maintains P2 1 /m symmetry: It is O rotated, as the B chain, at 1.32 GPa, but S-rotated at 2.97 GPa and higher pressures. As pressure increases from 1.32 to 7.90 GPa, there is a switching of the nearest bridging O atoms coordinated with the M4 cation: The M4-O5B distance contracts from 2.944 to 2.551 Aa, whereas the M4-O6B distance increases from 2.754 to 2.903 Aa. Compression mechanisms for the low-and high-pressure polymorphs appear to be slightly different. In the C2/m phase, the behavior of the A and M4 sites controls the compression of the structure, whereas the response of the M1, M2, and M3 octahedra to pressure also plays a role in determining the compression of the P2 1 /m structure. The phase transition is regarded as primarily driven by the differential compression between the M4 and T sites, and the symmetry breaking provides a necessary tighter coordination for the M4 site. Based on our data, the obvious changes in the hyperfine parameters of 57 Fe in grunerite between 1.0 and 3.4 GPa, observed by Zhang and Hafner (1992), are likely to result from the C2/m-P2 1 /m structural transformation.

The unit-cell dimensions and crystal structure of sillimanite at various pressures up to 5.29 GPa have been refined from single-crystal X-ray diffraction data. As pressure increases, a and b decrease linearly, whereas c decreases nonlinearly with a slightly positive curvature. The axial compression ratios at room pressure are βa:βb:βc=1.22:1.63:1.00. Sillimanite exhibits the least compressibility along c, but the least thermal expansivity along a (Skinner et al. 1961; Winter and Ghose 1979). The bulk modulus of sillimanite is 171(1) GPa with K′=4 (3), larger than that of andalusite (151 GPa), but smaller than that of kyanite (193 GPa). The bulk moduli of the [Al1O6], [Al2O4], and [SiO4] polyhedra are 162(8), 269(33), and 367(89) GPa, respectively. Comparison of high-pressure data for Al2SiO5 polymorphs reveals that the [SiO4] tetrahedra are the most rigid units in all these polymorphic structures, whereas the [AlO6] octahedra are most compressible. Furthermore, [AlO6] octahedral compressibilities decrease from kyanite to sillimanite, to andalusite, the same order as their bulk moduli, suggesting that [AlO6] octahedra control the compression of the Al2SiO5 polymorphs. The compression of the [Al1O6] octahedron in sillimanite is anisotropic with the longest Al1-OD bond shortening by ∼1.9% between room pressure and 5.29 GPa and the shortest Al1-OB bond by only 0.3%. The compression anisotropy of sillimanite is primarily a consequence of its topological anisotropy, coupled with the compression anisotropy of the Al-O bonds within the [Al1O6] octahedron.

The unit-cell parameters of quartz, SiO2, have been determined by single-crystal diffraction at 22 pressures to a maximum pressure of 8.9 GPa (at room temperature) with an average precision of 1 part in 9000. Pressure was determined by the measurement of the unit-cell volume of CaF2 fluorite included in the diamond-anvil pressure cell. The variation of quartz unit-cell parameters with pressure is described by: a −4.91300 (11) = −0.0468 (2) P + 0.00256 (7) P 2 − 0.000094 (6) P 3, c − 5.40482 (17) = − 0.03851 (2) P + 0.00305 (7) P 2 − 0.000121 (6) P 3, where P is in GPa and the cell parameters are in ångstroms. The volume–pressure data of quartz are described by a Birch–Murnaghan third-order equation of state with parameters V 0 = 112.981 (2) å3, K T0 = 37.12 (9) GPa and K′ = 5.99 (4). Refinement of K′′ in a fourth-order equation of state yielded a value not significantly different from the value implied by the third-order equation. The use of oriented quartz single crystals is proposed as an improved internal pressure standard for high-pressure single-crystal diffraction experiments in diamond-anvil cells. A measurement precision of 1 part in 10 000 in the volume of quartz leads to a precision in pressure measurement of 0.009 GPa at 9 GPa.

The unit-cell dimensions and crystal structure of kyanite at various pressures up to 4.56 GPa were refined from single-crystal X-ray diffraction data. The bulk modulus is 193(1) GPa, assuming K9 = 4.0. Calculated unit-strain tensors show that kyanite exhibits more isotropic compressibility than andalusite or sillimanite. The most and least compressible directions in the kyanite structure correspond approximately to the most and the least thermally expandable directions. The analysis of the distortion of the closest packing in kyanite indicates that the most compressible direction of the structure (along [012]) corresponds to the direction along which the closest-packed O monolayers are stacked. The bulk moduli for the A11, A12, A13, and A14 octahedra are 274(43), 207(14), 224(26), and 281(24) GPa, respectively, and those for the Si1 and Si2 tetrahedra are 322(80) and 400(95) GPa, respectively. Four A1O 6 octahedra that all become less distorted at higher pressures do not display clearly dominant compression directions. The average unshared O-O distance for each octahedron is considerably more compressible than the shared O-O distance. The high-pressure behaviors of the A11 and A14 octahedra are very similar but different from those of the A12 and A13 octahedra. Bulk moduli for the three Al 2 SiO 5 polymorphs (kyanite, sillimanite, and andalusite), as well as those for the AlO 6 octahedra in their structures, appear to decrease linearly as their volumes increase. The significantly larger bulk modulus and more isotropic compressibility for kyanite than for andalusite or sillimanite are a consequence of the nearly cubic close-packed arrangement of O atoms and the complex edge-sharing among four distinct AlO 6 octahedra in the kyanite structure.

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.

Unit-cell parameters of RbTi2(PO4)3, a framework structure (space group R3c) with alternating corner-linked Ti octahedra and P tetrahedra that define six-coordinated alkali cation sites, were determined at eight pressures from 0.0 to 6.2 GPa, and crystal structure refinements were obtained at 0.0 and 6.2 GPa. The structure remains dimensionally rhombohedral at all pressures, with the a axis more than four times more compressible than c. Compressibilities of both axes increase abruptly above about 1.7 GPa; the bulk modulus decreases from 104(3) GPa (K'=4) to approximately 60 GPa. Furthermore, the appearance of reflections that violate the c glide-plane symmetry above 1.7 GPa signals the occurrence of a reversible phase transition involving distortion of the framework. From the pressure dependence of the intensities of critical X-ray reflections, the phase transition is revealed as being continuous and close to second order in character. Structure refinements of the 6.2 GPa data reveal R3 symmetry, an acentric space group, for the high-pressure form. Average Ti-O-P angles decrease from 151.5 to 140 degrees between room pressure and 6.2 GPa. These changes result in a coordination change for Rb from one centric six-coordinated site (Rb-O=2.855 AA) at room pressure, to two symmetrically distinct acentric 12-coordinated sites (mean Rb-O approximately=3.01 AA) at 6.2 GPa.

High-pressure optical and x-ray studies of ${\mathrm{H}}_{2}$-${\mathrm{H}}_{2}$O mixtures have revealed the formation of the first hydrogen clathrate hydrates. A rhombohedral hydrate with a ${\mathrm{H}}_{2}$O sublattice similar to ice II and a ${\mathrm{H}}_{2}$:${\mathrm{H}}_{2}$O ratio of 1:6 is stable between 0.75 and 3.1 GPa (295 K). Above 2.3 GPa, a novel hydrate forms with the ${\mathrm{H}}_{2}$O molecules in a cubic diamond structure and with a very high ${\mathrm{H}}_{2}$:${\mathrm{H}}_{2}$O stoichiometry of 1:1. The ${\mathrm{H}}_{2}$ molecules occupy voids in the ${\mathrm{H}}_{2}$O framework, thus improving the packing efficiency and stabilizing this hydrate to very high pressures of at least 30 GPa.

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.

The effect of pressure on chemical interactions in molecular mixtures is important for problems spanning fundamental chemistry, planetary science, and materials science. Diamond-anvil cell studies reveal pressure-induced chemistry in the CH4-H2 system. The system, which has no known compounds at ambient conditions, formed four molecular compounds, CH4(H2)2, (CH4)2H2, CH4(H2)4, and CH4H2, at pressures up to 10 gigapascals. These have been characterized by synchrotron single-crystal x-ray diffraction, polycrystalline x-ray diffraction, Raman spectroscopy, and visual observation. Although CH4(H2)2 crystallizes in the MgZn2-type, hexagonal Laves phase structure, (CH4)H2 has a body-centered tetragonal structure that is similar to that of Al2Cu. The 1:1 and 1:2 compounds are stable to at least 30 gigapascals.

The unique cation-disordered crystal structures of two samples of phase E, a non-stoichiometric, hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions above 13 GPa and 1000° C, have been solved and refined in space group $$\bar 3$$ . The compositions and unit cells for the two materials, assuming six oxygens per cell, are Mg2.08Si1.16H3.20O6, a=2.9701(1) A, c=13.882(1) A V = 106.05(4) A3 for sample 1, and Mg2.17Si1.01H3.62O6, a=2.9853(6) A, c=13.9482(7) A, V= 107.65(4) A3 for sample 2. The structure contains layers with many features of brucite-type units, with the layers stacked in a rhombohedral arrangement. The layers are cross linked by silicon in tetrahedral coordination and magnesium in octahedral coordination, as well as hydrogen bonds. Interlay er octahedra share edges with intralayer octahedra. Interlayer tetrahedra would share faces with intralayer octahedra. To avoid this situation, there are vacancies within the layers. There is, however, no long-range order in the occupation of these sites, as indicated by the lack of a superstructure. Selected-area electron diffraction patterns show walls of diffuse intensity similar in geometry and magnitude to those observed in short-range-ordered alloys and Hagg phases. Phase E thus appears to represent a new class of disordered silicates, which may be thermodynamically metastable.

Single crystals of five wadsleyite compositions, β-(Mg,Fe)2SiO4 with Fe/(Fe+Mg)=0.00, 0.08, 0.16, 0.25 and 0.40, have been synthesized at high temperature and pressure in a uniaxial, split-sphere apparatus. Crystal structures of these samples, determined by x-ray diffraction techniques, reveal that iron is significantly ordered: Fe is depleted in the M2 octahedron, while it is enriched in M1 and M3. The most iron-rich synthetic sample, which falls well outside previous estimates of wadsleyite stability, raises questions regarding published Mg2SiO4-Fe2SiO4 phase diagrams at transition zone conditions.

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.

The crystal structure of BiVO4, which has a monoclinic, distorted scheelite structure at ambient conditions, has been determined above the high-pressure transition at 1.6 and 4.3 GPa, and 23°C. At these pressures BiVO4 has the tetragonal scheelite structure, which is also observed at high temperature above 250°C at room pressure. Vanadium tetrahedra remain rigid with no significant change in size at high pressure. Reduction of unit-cell volume is principally a result of Bi-O polyhedral compression. This behavior is similar to that observed for other scheelite-type compounds, in which tetrahedra are rigid and compression occurs in the 8-coordinated polyhedron.

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+.

Crystal structures and lattice constants have been determined for ruby at hydrostatic pressures up to 46 kbar using a gasketed opposed‐anvil diamond cell on a four‐circle diffractometer. The measured compressibility is slightly anisotropic, having a value of 1.36±0.03×10−4 kbar−1 parallel to c and 1.22±0.03×10−4 kbar−1 perpendicular to c. If a Birch‐Murnaghan equation of state is used and K′0 is assumed to be 4, the isothermal bulk modulus is 2.57±0.06 Mbar. Refined atomic coordinates do not change with pressure; therefore, the structure compresses in a uniform manner. This study demonstrates that crystal structures may be determined at high pressure on single crystals with a precision approaching that of room‐pressure results.

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.

The crystal structure of synthetic BeO, bromellite, has been determined at several pressures to 5.0 GPa and several temperatures to 1183 K. The single variable atomic‐positional parameter, z of oxygen, does not vary significantly with pressure, but does undergo a small increase with temperature. The hexagonal axial ratio c/a is constant with pressure, but decreases slightly from 1.624±0.001 to 1.623±0.001 between 300 and 1183 K. The observed crystal bulk modulus is 212±3 GPa, if K’ is assumed to be 4. The bulk modulus of the beryllium tetrahedron in BeO is 210 GPa, identical to that of the crystal, and close to the value observed for beryllium tetrahedra in other beryllium minerals. The average volume thermal expansivity of the BeO beryllium tetrahedron between 298 and 1183 K is 2.5±0.2×10−5 K−1, compared to 2.66±0.10×10−5 K−1 for the crystal.

Two superconducting phases, ${\mathrm{Tl}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10+\mathrm{\ensuremath{\delta}}}$ (2223) and ${\mathrm{Tl}}_{2}$${\mathrm{Ca}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{2}$${)}_{8+\mathrm{\ensuremath{\delta}}}$ (2122), both with onset ${\mathrm{T}}_{\mathrm{c}}$ near 120 K and zero resistivity at 100 K, have been isolated from samples in the Tl-Ca-Ba-Cu-O system. The new 2223 superconductor has a 5.40\ifmmode\times\else\texttimes\fi{}5.40\ifmmode\times\else\texttimes\fi{}36.25-A${\mathrm{\r{}}}^{3}$ pseudotetragonal unit cell. The 2122 superconductor, which appears to be structurally related to ${\mathrm{Bi}}_{2}$${\mathrm{CaSr}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$, has a 5.44\ifmmode\times\else\texttimes\fi{}5.44\ifmmode\times\else\texttimes\fi{}29.55-A${\mathrm{\r{}}}^{3}$ pseudotetragonal subcell. The 2223 phase is probably related to 2122 by the addition of extra calcium and copper layers.

X-ray diffraction from a single crystal of solid $n$-${\mathrm{H}}_{2}$ has been obtained at high pressure in a diamond-anvil cell, and the crystal structure has been determined. Nineteen diffraction maxima, corresponding to the hexagonal-close-packed structure ($\frac{P{6}_{3}}{\mathrm{mmc}}$, $Z=2$), were observed. Unit-cell dimensions at 5.40\ifmmode\pm\else\textpm\fi{}0.03 GPa and 300\ifmmode\pm\else\textpm\fi{}1 K are $a=2.659\ifmmode\pm\else\textpm\fi{}0.002$ \AA{}, $c=4.334\ifmmode\pm\else\textpm\fi{}0.003$ \AA{}, and $V=26.55\ifmmode\pm\else\textpm\fi{}0.05$ ${\mathrm{\AA{}}}^{3}$. The structure determination and volume measurement provide the most accurate checks to date on the equation of state and phase diagram of hydrogen in this pressure range.

Argon and neon crystallize in the face‐centered cubic structure (Fm3m, z = 4) at 11.5±0.5 and 47.4±0.5 kbar, respectively, at 293 K. Single‐crystal cell dimensions were obtained at 82 kbar for argon and 144 kbar for neon with high‐pressure, x‐ray diffraction techniques. The data were corrected to 0 K and fitted to a second‐order Murnaghan equation of state with V0 and K0 constrained to results obtained in low‐temperature experiments. No solid‐solid phase transitions are observed to the highest pressures studied.

Single‐crystal x‐ray diffraction data from DyBa2Cu4O8 are used to confirm the structure of RBa2Cu4O8 (R=Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, and Tm), all of which have been synthesized in bulk and display substantial Meissner diamagnetism. The structure is orthorhombic, space group Ammm, with a=3.846 3(3), b=3.872 6(3), and c=27.237(2) A. Copper‐oxygen layers parallel to (001) are dimpled as in the 1‐2‐3 structure. Unique copper‐oxygen strips parallel to (100) have copper in distorted square‐planar coordination. Magnitudes of oxygen anisotropic thermal vibrations are significantly less than in the 1‐2‐3 structure; thus the oxygen content of 1‐2‐4 is expected to be significantly more stable.

WATER locked in mineral phases within the Earth's mantle may be as significant for the development of life as water now found at the planet's surface. The Earth's original hydrosphere probably would not have survived a collision with a Mars-sized object, such as may have formed the Moon1. The water now on the surface could have been replenished by further impacts of smaller planetesimals; however, at least a portion could have been stored in minerals deep in the mantle being released gradually through volcanic eruptions. This mechanism requires a stable high-pressure phase able to store water under mantle conditions. Of the several hydrous phases studied in the past, the material known as phase B has the highest density and is the only known hydrous form stable at pressures corresponding to depths of 400–500 km (refs 2, 3). Although phase B has been known for over twenty years, its crystal structure and crystal chemistry have remained an unsolved problem. Here we describe the crystal structures of phase B (Mg12Si4O19(OH) 2) and a chemically and structurally similar anhydrous form, AnhB (Mg14Si5O24). These structures contain silicon in both fourfold and sixfold coordination; the silicon octahedra share all twelve edges with magnesium octahedra in a unique thirteen-cation cluster. Description of the structures of phases AnhB and B requires 18 and 40 atoms, respectively, demonstrating that high-pressure phases can have very complicated crystal structures. The multitude of octahedral sites in these phases could lead to rather complicated fractionation behaviour during solid–solid or solid–liquid transitions in the presence of other octahedrally coordinated ions such as Al, Fe, Ti and Mn.

Solid neon was compressed under static conditions at 300 K to pressures in the 100 GPa (megabar) range using diamond-anvil cell techniques. The crystal structure and /ital P/-/ital V/ equation of state were determined by energy-dispersive x-ray diffraction with microcollimated synchrotron radiation. Pressures were determined from ruby fluorescence spectra and from x-ray diffraction of tungsten powder contained within the sample. Solid neon remains an insulator with the fcc structure to the maximum pressure of 110 GPa at 300 K, where the compression /ital V///ital V//sub 0/ is 0.28. The 300-K /ital P/-/ital V/ isotherm measured at high pressure is in excellent agreement with the results of electronic structure calculations but is incorrectly described by pure pair potentials recently developed for neon. These results indicate that there is a significant softening of the material by many-body interactions at high pressures. Finally, the measurements of ruby fluorescence and tungsten x-ray diffraction in the neon medium obtained in this study provide an extension of the quasihydrostatic ruby pressure scale above 100 GPa.

Single-crystal x-ray techniques were used to determine two structures that comprise the Y-Ba-Cu-O superconductor. A green phase has the orthorhombic ${\mathrm{Y}}_{2}$${\mathrm{BaCuO}}_{5}$ structure. An opaque phase (${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{x}}$, 0\ensuremath{\le}x\ensuremath{\le}1.0) is tetragonal (a=3.859 A\r{}, c=11.71 A\r{}; space group P4\ifmmode\bar\else\textasciimacron\fi{}m2;Z=1). The structure is related to a 1:1:3 AB${\mathrm{O}}_{3}$ perovskite: Ba and Y order in A sites and Cu is in B sites. Systematic oxygen vacancies between adjacent copper atoms lead to a fivefold-coordinate Cu in proximity to Y and a twofold-coordinate Cu in the vicinity of the Ba site.

X-ray diffraction measurements have been carried out on cesium iodide (CsI) to 302 gigapascals with a platinum pressure standard. The results indicate that above 200 gigapascals CsI at 300 K has a hexagonal close-packed crystal structure with the ideal c / a ratio of 1.63 ± 0.01. The crystal structure and pressure-volume relations converge at high pressure with those of solid xenon, which is isoelectronic with CsI. The results indicate a significant loss of ionic bonding in the hexagonal close-packed metallic phase of CsI at ultrahigh pressure.

Transformations involving a change from tetrahedrally coordinated to octahedrally coordinated silicon ((IV)Si --(VI)Si) are observed to occur at high pressure when the mean (IV)Si-O bond compresses to approximately 1.59 angstroms based on known room-pressure crystal structures, Si-O bond compressibilities, and pressures of (IV)Si --(VI)Si transformations. The lower two-thirds of the mantle transition zone of high-density gradient (500 to 900 kilometers) corresponds to the predicted range of (IV)Si --(VI)Si transformations. The 10 percent density increase of this zone at zero pressure is attributed primarily to the density increase associated with the change in silicon coordination. Below 900 kilometers all silicon is predicted to be in octahedral or greater coordination. The concept of cation polyhedral stability fields is defined.

Crystal structures of several of the corundum‐type oxides have been determined at pressures to 50 kbars. All materials have linear compression within the pressure range and precision of the techniques used. Compression of Cr2O3 and Al2O3 is essentially isotropic (c/a remains constant), Fe2O3 has a slightly anisotropic compression, with c/a decreasing slightly with pressure, and V2O3 is very anisotropic, with the a axis nearly three times more compressible than c. Similar differences are observed in the structural parameters. Aluminum, iron, and chromium sesquioxides simply scale, whereas atomic positions in V2O3 approach an ideal HCP arrangement with increasing pressure. The differences in structural variation with pressure for these ’’isostructural’’ compounds emphasize the difficulty in using simple bonding parameters to predict details of crystal structures under nonambient conditions.