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The computer programDRAWxtlproduces crystal structure drawings in the form of an interactive screen representation, as well as VRML files for use on web pages and in classroom teaching, and creates input files for the popularPersistence of Vision Raytracer(POV-Ray) rendering program for publication-quality graphics, including generation of stereo pairs.DRAWxtloutput produces the standard kinds of graphical representations: spheres, ellipsoids, bonds and polyhedra of any complexity. In addition, it can draw arrows to represent magnetic moments, show capped cones to indicate the location of lone-pair electrons and display Fourier contours in three dimensions. A unique feature of this program is the ability to plot incommensurately modulated and composite structures. This open-source program can be used with operating systems as diverse as Windows (9X, NT, 2000 and XP), Mac OS X, Linux and most other varieties of Unix.

The mineral woodruffite, Zn2+ (Mn4+1− x Mn3+ x )O2 · y H2O, x ~ 0.4 and y ~ 0.7, is the first known example of a new type of Mn oxide characterized by large tunnels that measure 3 and 4 octahedra (6.9 × 9.2 A) on a side. These tunnels are rectangular in cross-section and are the largest of any yet reported in natural or synthetic Mn oxides. The thermal stability of woodruffite is comparable to that of todorokite and other large-tunnel Mn oxide phases, breaking down at ~300 °C and eventually transforming to a spinel-type structure. The woodruffite structure may serve as a model for a new class of octahedral molecular sieves with enhanced capabilities as catalysts and selective cation-exchange agents.

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.

Effective techniques for conducting high-pressure and high-temperature single-crystal X-ray diffraction experiments were developed in the 1970s. By the end of that decade a number of papers had been published that defined optimal methods for operating diffractometers, especially for high-pressure experiments. The following decade saw the spread of the techniques from the institutions involved in the original developments into many other, mostly mineralogical, crystallography laboratories around the world. The state of the art of high-pressure diffractometry as it stood in the early 1980s was summarized in Comparative Crystal Chemistry (Hazen and Finger 1982). Since that time, advances in computing capacity and in the mechanical quality of diffractometers have been combined to increase the precision of high-pressure measurements by an order of magnitude. This chapter, while building on the work summarized in Hazen and Finger (1982), extends it on the basis of the experience gained by the authors and others in the intervening years. The aim of this chapter is to provide a crystallographer with no previous experience in high-pressure or high-temperature crystallography with the information to set up, or to convert, a diffractometer for such work as well as a detailed guide as to how such experiments should be carried out. We hope thereby to answer the questions “How do I modify my diffractometer for high-P,T experiments?” in the next section, and “How do I do the experiment?” in the following one. The last section of this chapter describes the modifications to data reduction procedures that must be made for handling data collected from crystals in diamond-anvil cells (DAC). Issues relating to DAC design and furnace design are addressed in Miletich et al. (this volume) and Yang and Peterson (this volume) respectively. Some basic familiarity with the operation of single-crystal diffractometers for conventional measurements at room conditions is assumed, although …

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.

Structural and volume compressibility data for reedmergnerite, NaBSi 3 O 8 , were obtained by single-crystal X-ray diffraction at pressures up to 4.7 GPa. The bulk modulus was determined to be 69.8(5) GPa with the pressure derivative constrained to 4. Unit-cell compression is anisotropic, as indicated by unit strain tensors. Tetrahedral bond lengths and angles remained relatively constant over the pressure interval, whereas Na-O bonds decreased systematically. T-O-T angles underwent a variety of behaviors, remaining constant or decreasing with pressure. The compression for reedmergnerite is similar to that of low-albite, wherein bending of the (Al.B)-Oco-Si angle compresses the Na-bearing zigzag channels. In contrast, micro-cline compresses by shearing the four-membered rings, which in turn compresses the K-bearing channels. At about 4 GPa, a new bond between K and Obm appears that alters the compression mechanism and explains the discontinuity in the pressure variation of crystallographic parameters observed by Allan and Angel (1997). Thus, the compression mechanism of the alkali feldspars is dominated by the compression of alkali containing channels. However, because of low symmetry, this can be accomplished in several ways. The observed variety of compression pathways resulted from T-O-T angle bending energetics that were coupled with the effects of alkali cation bonding.

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 structural changes associated with the incommensurate (IC)-normal (N) phase transition in akermanite have been studied with high-pressure single-crystal X-ray diffraction up to 3.79 GPa. The IC phase, stable at room pressure, transforms to the N phase at ∼1.33 GPa. The structural transformation is marked by a small but discernable change in the slopes of all unit-cell parameters as a function of pressure. It is reversible with an apparent hysteresis and is classified as a tricritical phase transition. The linear compressibility of the a and c axes are 0.00280(10) and 0.00418(6) GPa−1 for the IC phase, and 0.00299(11) and 0.00367(8) GPa−1 for the N phase, respectively. Weighted volume and pressure data, fitted to a second-order Birch-Murnaghan equation of state (K′≡4.0), yield V0=307.4(1) A3 and K0=100(3) GPa for the IC phase and V0=307.6(2) A3 and K0=90(2) GPa for the N phase. No significant discontinuities in Si–O, Mg–O and Ca–O distances were observed across the transition, except for the Ca–O1 distance, which is more compressible in the IC phase than in the N phase. From room pressure to 3.79 GP the volume of the [SiO4] tetrahedron is unchanged (2.16 A3), whereas the volumes of the [MgO4] and [CaO8] polyhedra decrease from 3.61 to 3.55(1) A3 and 32.8 to 30.9(2) A3, respectively. Intensities of satellite reflections are found to vary linearly with the isotropic displacement parametr of Ca and the librational amplitude of the [SiO4] tetrahedron. At room pressure, there is a mismatch between the size of the Ca cations and the configuration of tetrahedral sheets, which appears to be responsible for the formation of the modulated structure; as pressure increases, the misfit is diminished through the relative rotation and distortion of [MgO4] and [SiO4] tetrahedra and the differential compression of individual Ca–O distances, concurrent with a displacement of Ca along the (110) mirror plane toward the O1 atom. We regard the high-pressure normal structure as a result of the elimination of microdomains in the modulated structure.

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.

We have explored phase relations in the fundamental ternary system MgO-FeO-SiO2 at lower mantle pressure (26 GPa) via synthesis experiments on the join MgSiO3-FeSiO3. The maximum solubility of FeO in (Mg,Fe)SiO3 perovskite as a function of temperature has been determined in the multianvil apparatus in the temperature range of 1150–1740°C. Pure perovskites with 10 and 15 mol % FeSiO3 can only be synthesized above 1525°C and 2072°C, respectively, at 26 GPa. Synthesis experiments were also conducted in a laser-heated diamond anvil cell to resolve a previous discrepancy concerning the maximum solubility of FeO in (Mg,Fe)SiO3 perovskite determined by different high-pressure apparatus. Both Mossbauer spectroscopic and X ray diffraction studies revealed that laser-heated samples with more than 10 mol % FeSiO3 invariably contained magnesiowustite. The coexistence of magnesiowustite with relatively lower FeO content perovskite results from chemical disequilibrium process caused by the large temperature gradient in the laser-heated samples. Finally, we conclude that the maximum FeO content at the top of the lower mantle is about 12 mol % for a perovskite-dominated lower mantle, along a 2000-K lower mantle geotherm.

Single-crystal X-ray diffraction data have been obtained at several pressures to 5.6 GPa for synthetic LiScSi04 olivine. The bulk modulus is 118 :t I GPa, assuming K' = 4. This value is smaller than that of forsterite because compressibility of Li-O bonds in the M1 octahedral site is approximately twice that of the MP+-O bonds in other isomorphs. Compressibilities of a, b, and c orthorhombic axes are 2.70, 2.80, and 2.61 (all x 10-3 GPa-1), respectively. This nearly isotropic compression (axial compression ratios = 1.00: 1.04:0.97) contrasts with that offorsterite (1.00: 1.99:1.55), fayalite (1.00:2.83: 1.22), monticellite (1.00: 1.85:1.10), and chrysoberyl (1.00: 1.30:1.17). These differences arise from the distinctive distribution of cations of different valences and consequent differences in M 1-0 and M2-0 bond compressibilities. The Li M 1 octahedron displays a significant decrease in polyhedral distortions with pressure, a behavior not observed in other olivines.

ABSTRACf Pressure-volume X-ray diffraction data were collected at eight pressures to 17.2 GPa for synthetic single-crystalforsterite, Mg2Si04.Birch-Murnaghanequation-of-state parameters were determined to be Ko = 125(2) GPa and K~ = 4.0(4). To ensure hydrostatic conditions, we monitored the diffraction-peak shapes while the sample was in different pressure media: 4:1 methanol-ethanol, He, and Ar. The diffraction peaks for forsterite in 4:I methanol-ethanol at 12.2 GPa were broadened to such an extent that we believe that any data collected at similar pressures and in this medium may be suspect and should be examined critically. The diffraction-peak profiles for the sample in He were sharp and well-formed, suggestingHe provides an excellent medium at pressures ~ 17.2 GPa. However, upon decompression, the diffraction peaks of the sample in He also showed significant broadening. This broadening suggeststhat with increasing pressure He diffused into the crystal, but with the release of pressure the He was unable to diffuse out quickly. The diffraction peaks regained their sharpness after several months.

Single crystals of lead aluminosilicate hollandite, with composition Pbo.sA11.6Si2.40s, have been synthesized at 16.5 GPa and 1450 0c. Crystals are tetragonal [space group 14, Z = 2, a = 9.414(3), c = 2.750(3) A, V = 243.7(3) A3]. Si and Al are disordered on the octahedral site. Pb is best modeled by two atom sites, one on the fourfold axis and the other split, lying off the fourfold axis. This feature was predicted by Post and Burnham (1986) but not previously observed.

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.

Analysis of a crystal structure using the Rietveld profile technique requires a suitable description of the shape of the peaks. In general, modern refinement codes include accurate formulations for most effects; however, the functions used for peak asymmetry are semi-empirical and take very little account of diffraction optics. The deficiencies in these methods are most obvious for high-resolution instruments. This study describes the implementation of powder diffraction peak profile formulations devised by van Laar & Yelon [J. Appl. Cryst. (1984), 17, 47–54]. This formalism, which describes the asymmetry due to axial divergence in terms of finite sample and detector sizes, does not require any free parameters and contains intrinsic corrections for the angular dependence of the peak shape. The method results in an accurate description of the observed profiles for a variety of geometries, including conventional X-ray diffractometers, synchrotron instruments with or without crystal analyzers and neutron diffractometers.

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.

The pressure dependence of the structure of the pure fullerene C60 has been studied by high-resolution synchrotron X-ray powder diffraction at 300 K in a diamond-anvil cell with fluid helium as a pressure-transmitting medium. Reflections corresponding to the simple cubic (s.c.) phase of C60 are identified at 2.5 GPa. The powder data agree well with calculated profiles for an ordered structure based on Pa symmetry and are consistent with a long-range orientational ordering transition under pressure identical to that occurring at 255 K and ambient pressure; the onset of the transition occurs below 0.5 GPa.

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.

SINGLE, a program that runs under Windows operating systems, can be used to control a variety of four-circle Eulerian-cradle single-crystal diffractometers including Huber instruments equipped with Huber motor controllers, the Stoe Stadi-4 and the Siemens P4. The software can be configured,viavariables set by the user, to operate any of these diffractometers with a variety of environmental devices including furnaces and diamond-anvil pressure cells. It is specifically designed to determine unit-cell parameters to a precision of 1 part in 30 000.

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 framework topology of the novel zeolite SSZ-42 is determined from single crystal data (15 × 15 × 35 µm) and refined from powder diffraction data; the crystalline architecture is characterized by an undulating, one-dimensional 12-membered ring channel system.

We report results of high-pressure experiments with a new diamond-anvil cell in a monochromatic, high-resolution x-ray scattering geometry with alinear position-sensitive detector. The experiments make possible the study of factors controlling line widths of diffraction profiles at pressures in the 100 GPa range, and demonstrate the potential for the use of line profile analysis and Rietveld refinement techniques with high-pressure powder diffraction data. Combined data for various materials indicate that relative contributions to linewidths due to particle size, intrinsic material strength, pressure and state of stress in the sample can be resolved. With light rare-gas solids as pressure-transmitting media, measured FWHMs of the order 0.03∘ 2 θ corresponding to resolution Δd/d of 2.5 × 10−3 for 2θ∼10-15∘ are reported. Formation of a high pressure phase appears to involve growth of submicron domains, judging from substantially broadened diffraction peaks under quasihydrostatic conditions. Detaile...

A survey of high-pressure silicates reveals 12 distinct high-density structural topologies with octahedral Si. Seven of these structure types - stishovite, perovskite, ilmenite, hollandite, calcium ferrite, pyrochlore and K2NiF4 type - contain only six-coordinated silicon. Other high-pressure silicates, including those with the garnet, pyroxene, wadeite, anhydrous phase B and phase B structures, contain both tetrahedral and octahedral Si. Five systematic trends among these dozen structures suggest the existence of other, as yet unobserved, possible mantle Si phases. The criteria are: (1) structures like rutile, hollandite and calcium ferrite formed from edge-sharing chains of silicon octahedra; (2) germanates synthesized at room pressure with octahedral Ge; (3) isomorphs of roompressure oxides with 3+ or 4+ transition-metal cations; (4) high-pressure magnesium silicates related to room-pressure aluminates by the substitution 2AI Mg+ Si; and (5) the homologous structures in system Mg-Si-O-H that includes phase B and anhydrous phase B. Each of these criteria can be used to predict other potential octahedral Si phases. Of special interest are predicted structure types that fulfill more than one criterion: diaspore-type (MgSi)O2(OH)2, aerugite-type Mgl0Si3Oi6 , sphene-type CaSi2Os, benitoite-type BaSi409, gibbsite-type MgSi(OH)6 and pseudobrookite-type Fe2SiOs.

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.

Superconductivity up to about 110 K in the CrTlSrCaCuO system was observed by resistance and ac susceptibility measurements. Powder X-ray diffraction analyses showed that a 1212-type phase, Cr -substituted TlSr 2 CaCu 2 O 7, is responsible for the observed superconductivity. This is the first time that single-element substitution for TlSr 2 CaCu 2 O 7 increases Tc of the 1212 phase to above 100 K. The Cr -substituted TlSr 2 CaCu 2 O 7 is easily prepared and has very good quality suitable for practical applications.

Metallic filaments with sub-micrometer diameters have recently been fabricated using novel materials fabrication techniques at NRL. The specimens are enshrouded in a glass sheath and all efforts to obtain structural information from these samples with conventional x-ray sources have been negative. By using synchrotron radiation on a wiggler beam line, x-ray diffraction data have been obtained from samples with diameters of 0.22, 0.09, 0.07, and 0.04 μm. The two thicker samples were found to be single crystals with a structure consistent with that of normal Bi. Single crystal diffraction peaks obtained from the 0.07 μm sample are incompatible with the Bi-1 or any other known structure of Bi. We have provisionally identified this as Bi-X.

Pressure-volume equations of state of solid normal hydrogen and deuterium are determined from single-crystal x-ray diffraction measurements to maximum pressures of 26.5 GPa (265 kbar) at 300 K. Experimental data for deuterium are presented and used with the previously reported results for hydrogen. The measurements for deuterium indicate that the structure remains hexagonal closed packed to at least 14.2 GPa at 300 K. The P-V data are analyzed with both phenomenological equation-of-state formalisms and pair-potential models. The thermal contributions are calculated from a Mie-Gruneisen model and from lattice-dynamics calculations using effective potentials. A P-V equation of state is determined that accurately describes both the high-pressure diffraction data (to V/${\mathit{V}}_{0}$=0.2) and previous low-pressure compression results (V/${\mathit{V}}_{0}$=1.0 to 0.4). There is a significant softening of the equation of state relative to the predictions of lattice-dynamics calculations using previously reported pair potentials, and several effective potentials are examined. Possible corrections to the equation of state at higher pressures, above the range of the present experiments, include effects associated with order-disorder transitions, vibron softening, and band overlap.

SOLID hydrogen is predicted1,2 to become metallic at high pressures. Although metallization was recently reported in high-pressure shock-wave compression experiments using liquid hydrogen3, efforts to understand the high-pressure behaviour of the solid phase have relied mainly on spectroscopic studies in the diamond-anvil cell4–6 and on ab initio calculations7–10. Central to these studies is the high-pressure crystal structure—something that is difficult to determine in the diamond-anvil cell—and its pressure dependence. Here we report X-ray diffraction measure-ments of the structure of single-crystal molecular hydrogen at pressures of up to 109 GPa for H2 and 119 GPa for D2. From these measurements we deduce the high-pressure equation of state (EOS: the volume–pressure relation). We find that solid hydro-gen is more compressible than previously thought, that the crystal becomes increasingly anisotropic with pressure, and that the difference in EOS between H2 and D2 is unexpectedly small. The 'softening' of the EOS relative to previous estimates4 increases by about 25% the expected transition pressure to the atomic phase. Our EOS differs significantly from that predicted by ab initio calculations7,9, indicating that theoretical understand-ing of the behaviour of dense hydrogen remains incomplete.

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.

A detailed diamond anvil-cell study using synchrotron X-ray diffraction, Raman scattering, and optical microscopy has been conducted for the He-N system, with a view to the weakly-bound van der Waals molecule interactions that can be formed in the gas phase. High pressure is found to stabilize the formation of a stoichiometric, solid van der Waals compound of He(N2)11 composition which may exemplify a novel class of compounds found at high pressures in the interiors of the outer planets and their satellites.

Metallic filaments with submicrometer diametere have been fabricated. Standard diffraction techniques with conventional x-ray sources were unsuccessful in identifying the structure of these materials. However, with the use of synchrotron radiation produced on a wiggler beam line, diffraction data were obtained in measurement periods as short as 10 milliseconds. Two cylindrical single crystals of bismuth were studied, each with a diameter of 0.22 +/- 0.02 micrometer. The volume of sample illuminated for these measurements was 0.38 cubic micrometer, less than 0.5 femtoliter. The crystals are grown in glass capillaries, and, because bismuth expands on solidification, they are under a residual hoop stress. The crystallographic data indicate the presence of a linear compressive strain of about 2 percent, which is assumed to be the result of a residual stress of about 2 gigapascals.

An iron-sulfur compound (Fe 3 S 2 ) was synthesized at pressures greater than 14 gigapascals in the system Fe-FeS. The formation of Fe 3 S 2 changed the melting relations from a simple binary eutectic system to a binary system with an intermediate compound that melted incongruently. The eutectic temperature in the system at 14 gigapascals was about 400°C lower than that extrapolated from Usselman's data, implying that previous thermal models of Fe-rich planetary cores could overestimate core temperature. If it is found in a meteorite, the Fe 3 S 2 phase could also be used to infer the minimum size of a parent body.

We have carried out Raman spectroscopy and X-ray diffraction to 60 GPa of the 1:1 H2O:2 clathrate hydrate, which has water molecules in an arrangement similar to pure ices Ic and VII. The pressure dependence of both the νOH frequency and the intermolecular distance dOHO is significantly larger than in pure ice: unexpectedly, both dependencies can be mapped onto those of pure ice by doubling the pressure. Nevertheless, the correlation between νOH and dOHO is the same as in pure ice, making the clathrate a model system for ice at pressures reduced by a factor 2. Thus, the transition to a phase with symmetric hydrogen bonds probably occurs in the range 30–40 GPa, which is significantly lower than that for H2O.

Evidence of micron-sized structural inhomogeneities in several high transition temperature (Tc) superconductors is presented. By illuminating samples with high energy, highly collimated x rays produced on a synchrotron wiggler, small changes in the lattice were detected over a spatial scale as small as 10 micrometer. In the YBa2Cu3O7-(delta ) crystals, these changes are interpreted as evidence of variations in the oxygen content and in the Nd2-xCexCuO4-y crystal, as a variation in the cerium content. Each type of inhomogeneity can affect the superconducting properties.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Relative compressibilities of five silicate garnets were determined by single-crystal x-ray diffraction on crystals grouped in the same high-pressure mount. The specimens include a natural pyrope [(Mg2.84Fe0.10Ca0,06) Al2Si3O12], and four synthetic specimens with octahedrally-coordinated silicon: majorite [Mg3(MgSi)Si3O12], calcium-bearing majorite [(Ca0.49Mg2.51)(MgSi)Si3012], sodium majorite [(Na1.88Mgp0.12)(Mg0.06Si1.94)Si3O12], and an intermediate composition [(Na0.37Mg2.48)(Mg0.13Al1.07 Si080) Si3O12]. Small differences in the compressibilities of these crystals are revealed because they are subjected simultaneously to the same pressure. Bulk-moduli of the garnets range from 164.8 ± 2.3 GPa for calcium majorite to 191.5 ± 2.5 GPa for sodium majorite, assuming K′=4. Two factors, molar volume and octahedral cation valence, appear to control garnet compression.

The phase diagram of the binary system hydrogen‐water has been studied up to 30 GPa and 450 K. In this system, two new clathrate hydrate phases have recently been found, giving rise to at least five three‐phase lines and two quadruple points. Furthermore, several instances of metastability or slow transformations of phases have been identified. The phase behavior of the low‐pressure clathrate is in qualitative agreement with expectations based on simple thermodynamic arguments, whereas the high‐pressure clathrate is stable to unexpectedly high pressures.

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.