High-pressure Monoclinic Phase Research Articles

Study of the pressure effects in TiOCl by ab initio calculations

Electronic structure calculations on the low-dimensional spin−1/2 compound TiOCl were performed at several pressures in the orthorhombic phase, finding that the structure is quasi-one-dimensional. The Ti 3+ (d 1) ions have one t 2g orbital occupied ( d yz) with a large hopping integral along the b-direction of the crystal. The most important magnetic coupling is Ti–Ti along the b-axis. The transition temperature ( T c) has a linear evolution with pressure, and at about to 10 GPa this T c is close to room temperature, leading to a room temperature spin-Peierls insulator–insulator transition, with an important reduction of the charge gap in agreement with the experiment. On the high-pressure monoclinic phase, TiOCl presents two possible dimerized structures with a long or short dimerization. Long dimerized state occurs above 15 GPa, and below this pressure the short dimerized structure is the more stable phase.

Scandium tungstate above 2.5 GPa

Synchrotron x-ray powder diffraction was used to study Sc 2W 3O 12 in a diamond anvil cell up to 6.2 GPa in order to better characterize the second high pressure phase observed in earlier in-situ diffraction studies. Two crystalline-to-crystalline phase transitions were observed, in line with the earlier work. Above 2.7 GPa, Sc 2W 3O 12 adopted a monoclinic structure with probable space group P2 1/n, that is distinct from the monoclinic phase existing between 0.3 GPa and 2.7 GPa. The bulk modulous ( K 0 ) for this second high pressure phase was estimated to be 90(3) GPa using a Birch–Murnaghan equation of state with K p fixed at 4. Refining K p resulted in K 0 = 72 ( 3 ) GPa and K p = 12 ( 1 ) . The transition at ∼2.7 GPa on compression involved a unit cell volume reduction of almost 18% and was not reversible on decompression to atmospheric pressure at ambient temperature, suggesting that changes in bonding occur at the phase transition. Linear compressibility values for this high pressure monoclinic phase were estimated to be β a = 3.6 ( 1 ) × 1 0 − 3 GPa − 1 , β b = 3.7 ( 2 ) × 1 0 − 3 GPa − 1 , β c = 2.0 ( 1 ) × 1 0 − 3 GPa − 1 by straight line fits to the lattice constants over the range 0–6.2 GPa.

X-ray diffraction and infrared spectroscopy of monazite-structured CaSO 4 at high pressures: Implications for shocked anhydrite

X-ray diffraction and infrared spectroscopy of CaSO 4 are conducted to pressures of 28 and 25 GPa, respectively. A reversible phase transition to the monoclinic monazite-structure occurs gradually between 2 and ∼5 GPa with a highly pressure-dependent volume change of ∼6–8%. A second-order fit of the X-ray data to the Birch–Murnaghan equation of state yields a bulk modulus ( K) of 151.2 (±21.4) GPa for the high-pressure monoclinic phase. In the high-pressure infrared spectrum, the infrared-active asymmetric stretching and bending vibrations of the sulfate tetrahedra split at the phase transition, in accord with the results of factor group analysis. Additionally, the tetrahedral symmetric stretching vibration, which is weak in the anhydrite phase, becomes strongly resolved at the transition to the monazite structure. The infrared results indicate that the sulfate tetrahedra are more distorted in the monazite-structured phase than in anhydrite. Kinetic calculations indicate that the anhydrite to monazite transformation may generate the phase transition observed near 30 GPa under shock loading in CaSO 4. Our results indicate that the anhydrite- and monazite-structured phases may be the only phases that occur under shock loading of CaSO 4 to pressures in excess of 100 GPa.

When magnetism can stabilize the crystal structure of metals

The crystal structure of some metals is caused by magnetism, meaning spin polarization dictates atomic geometry. We show that the determining factors for magnetic stabilization of metallic crystal structures is the proximity of the border between superconducting and magnetic valence electron behavior and the energy difference between competing crystals. Recently, a high-pressure monoclinic Cm phase was found, which is stabilized by magnetism. We put these results into a broader context, showing how magnetic stabilization of metals occurs throughout the periodic table.

Molecular dynamics simulations of temperature- and pressure-induced solid–solid phase transitions in crystalline para-terphenyl

Molecular dynamics (MD) simulations of pressure- and temperature-induced solid–solid phase transitions in para-terphenyl have been investigated using Material Studio®. Initial simulations were performed using the COMPASS (condensed-phase optimised molecular potentials for atomistic simulation studies) force field to evaluate its ability to model the known temperature and pressure phase boundary between the triclinic and monoclinic crystal phases. Geometry optimisation using the universal COMPASS force field could not adequately model the experimental crystal structure at 113 K, and MD simulations could not adequately reproduce the known transition temperature at ambient pressure, nor yield a well-defined transition pressure at low temperature. However, a one-parameter optimisation of the torsion potential component of the polymer-consistent force field (PCFF) (incorporating COMPASS non-bond parameters) yielded MD simulations that accurately modelled the pressure–temperature boundary between the low-temperature low-pressure triclinic phase and the high-pressure high-temperature monoclinic phase of para-terphenyl.

Pressure induced structural transformation of pyrochlore Gd 2Zr 2O 7

High pressure x-ray diffraction studies on Gd 2Zr 2O 7 pyrochlore were carried out up to 45 GPa using a diamond anvil cell. A structural transition leading to monoclinic phase is initiated at a very low pressure of ∼3 GPa. Coexistence of both the parent and the daughter phase was seen up to the maximum pressure of 45 GPa. Bulk modulus of the parent phase is found to be 161.5 GPa and the lattice parameters of the high pressure monoclinic phase at 9 GPa are found to be: a=7.4839 Å, b=5.7089 Å, c=12.6498 Å and β = 97.61 . The structural phase transition is found to be reversible when quenched from 10 GPa, but it is irreversible when quenched from 45 GPa. Also, onset of amorphisation is seen above 35 GPa.

High-resolution FTIR study of the para-terphenyl phase transition at high pressure

High-resolution infrared (IR) spectroscopy has been used to investigate the pressure-induced (0–11 kbar) polymorphic phase transition of crystalline para-terphenyl at low temperature (25 K). A number of doublet bands observed in low-pressure triclinic p-terphenyl were observed to coalesce in the high-pressure monoclinic phase. The coalescing of doublet bands was attributed to changes in factor group (Davydov) splittings associated with the transition from a low-pressure triclinic phase to a high-pressure monoclinic phase. The bands that ‘disappear’ also do not correlate with frequency changes associated with changes in molecular symmetry. Molecular dynamics (MD) simulations at low temperature (20 K) yield a non-planar average molecular structure for the high-pressure monoclinic phase, in contrast to the high-temperature monoclinic phase. The MD simulations also reveal a broadening of the distribution of ring torsion angles near the triclinic–monoclinic phase transition pressure.

A comparative study of negative thermal expansion materials Sc 2(MoO 4) 3 and Al 2(WO 4) 3 crystals

Polarized Raman spectra and polycrystalline spectra of isomorphous Sc 2(MoO 4) 3 and Al 2(WO 4) 3 crystals have been measured at room temperature and ambient pressure. Some Raman spectra of Sc 2(MoO 4) 3 were taken at different values of pressures in the range of 0.0–3.0 GPa. Lattice dynamics calculations have been performed on the orthorhombic phase (ambient phase) of these materials using a rigid ion model in order to calculate the phonon frequencies at the Brillouin zone Center ( k = 0). The calculated results fit qualitatively well the Raman spectra observed in these materials at room temperature and ambient pressure, and allowed us to establish their Raman signature. A higher coupling among the modes of the Al 2(WO 4) 3 material compared with Sc 2(MoO 4) 3 material was observed. The pressure dependent studies revealed that the 362 and 433 cm −1 modes of Sc 2(MoO 4) 3 and Al 2(WO 4) 3, respectively, exhibit unusually high ∂ ω/∂ P values. The presence of these unstable modes in both materials is probably related to the mechanism of the phase transition from the monoclinic high-pressure phase to the unknown phase of lower symmetry.

Crystal structure of indium monosulfide, InS, at 7.9 GPa

InS, monoclinic, Pl2i/nl (No. 14), a = 3.8630(8) Â, b = 10.7668(9) k,c = 3.8554(7) k, s = 90.880(7)°, V = 160.3 Â, Ζ = 4, wR(P) = 0.120, R(I) = 0.062, T= 295 Κ, Ρ = 7.9 GPa. Source of material The starting material were dark red pieces of InS ingots. Powdering was performed by grinding of the crystalline material at ambient conditions. tionship of tetragonal HgCl and orthorhombic as well as monoclinic InS. Detailed discussions of the crystal chemical similarities of InS, HgCl, GeS, and Til have been given earlier [1-3]. The present results [space group No. 14, standard setting P2\!c a = 386.25(1) pm,?>= 1076.6( 1 ) pm, c=5.4991(4),/? = 135.488(8)°; In (0.018(3), 0.1189(5), -0.037(2)), S (0.02(1), 0.359(1), 0.000(6))] are in disagreement with earlier work which supposes hp-InS to crystallize in the tetragonal space group lAlmmm [4]. Table 1. Data collection and handling. Powder: dark red Wavelength: synchrotron radiation (0.4654 Â) μ: 12.5 cm Diffractometer: Daresbury Station 9.1, φ scan, transmission 2dmax, stepwidth: 26°, 0.02° N(points) measured: 1001 N(hkl)measarai·· 108 N(param)ref,ned'· 9 Programs: CSD [5], ATOMS [6] Table 2. Atomic coordinates and displacement parameters (in Â). Atom Site X y ζ t/iso In 4c S 4e 0.055(2) 0.019(5) 0.1191(4) -0.038(2) 0.359(1) -0.001(6) 0.0143(3) 0.0139(5) Experimental details High pressures were generated using gasketed diamond anvil cells with a 4:1 methanol/ethanol mixture as a pressure transmitting medium. For pressure calibration we used the ruby luminescence method. Discussion The figure shows the crystal structure of a high-pressure modification of InS in a projection along the a axis. Both atom types, indium and sulphur, realise a distorted octahedron as a first coordination sphere. The crystal structure is a distorted variety of the tetragonal calomel type. It can be described as slightly distorted NaCl-type double-layers which are interconnected by homonuclear In bonds. In comparison to the orthorhombic low-pressure modification, the inclination angle between the In-dumbbells and the long axis is pronouncedly reduced in the monoclinic high-pressure phase. The selected non-standard setting P2\!n points out the metrical rela* e-mail: schwarz@cpfs.mpg.de Acknowledgments. Major parts of the work were performed at the MPI fur Festkorperforschung (Stuttgart). The author gratefully acknowledges support with fruitful discussions and beamtime at the Daresbury Synchrotron Radiation Department by K. Syassen, R. Nelmes and M. I. McMahon and is indebted to N. M. Gasanly for InS single crystals.

High pressure Raman spectroscopic study of BaFCl

Raman spectroscopic measurements at different pressures are carried out on BaFCl single crystals using diamond anvil cell up to 25.6 GPa pressure. The tetragonal structure of BaFCl at ambient transforms into a mixture of tetragonal and orthorhombic structures at 10.8 GPa. Both phases exist up to 21.1 GPa. Another phase transition to a monoclinic structure starts at 21.1 GPa. All three phases coexist up to 25.6 GPa. Mode assignment is carried out for the high pressure monoclinic phase. The mode Gruneisen parameters for the ambient tetragonal structure are obtained for four modes.

Luminescence of BaBr 2:Eu 2+ under hydrostatic pressure

Luminescence spectra of BaBr 2:Eu 2+ have been measured under pressures up to 27 GPa at room temperature. In the low-pressure range a red-shift of the 5d–4f transition of −225 cm −1/GPa is observed. From 3 to 10 GPa a phase mixture of the original orthorhombic phase and the high-pressure monoclinic phase gives rise to two luminescence bands. Above 10 GPa the crystal is completely transformed to its high-pressure phase where two different Eu 2+ sites exist of which, however, only one 5d–4f transition is detected. This transition exhibits a red-shift of −200 cm −1/GPa. The shifts are compared with other literature data.

Magnetic–electronic properties of FeS and Fe7S8 studied by 57Fe Mössbauer and electrical measurements at high pressure and variable temperatures

The effect of pressure on the magnetic and electronic properties of synthetic FeS and Fe7S8 has been investigated by using 57Fe Mössbauer and electrical resistance measurements on polycrystalline samples pressurized in miniature gem anvil cells up to a pressure of ∼12 GPa in the temperature range 300–5 K. FeS in the low-pressure phases (P ≲ 7 GPa) has thermally activated charge carriers and a high-spin electronic configuration along the room temperature isotherm, whereas the high-pressure monoclinic phase (P ≳ 7 GPa) adopts a magnetically quenched low-spin state and a non-metallic behaviour associated with the filled valence band. The non-metallic behaviour observed in all pressure phases is explained in terms of electron correlation between Fe:3d electrons. In contrast, Fe7S8 is magnetic-metallic below ∼5 GPa and diamagnetic-metallic above this pressure. The metallic behaviour is ascribed to hole conduction in the S:3p band, as inferred from the temperature dependence of the Mössbauer data. The collapse of band magnetism at ∼5 GPa in Fe7S8 may be due to pressure-induced band broadening, leading to a breakdown of the Stoner criterion.

Synchrotron x-ray study of the monoclinic high-pressure structure ofAgGaS2

The structure of the monoclinic high-pressure phase of ${\mathrm{AgGaS}}_{2}$ was determined at 5.5 GPa by single-crystal x-ray diffraction using synchrotron radiation and a miniature diamond-anvil cell. The high-pressure phase has the space group symmetry of Cc with cell dimensions $a=8.031(4)\AA{},$ $b=8.014(1)\AA{},$ $c=6.221(2)\AA{},$ and $\ensuremath{\beta}=128.42(3)\ifmmode^\circ\else\textdegree\fi{}$ at 5.5 GPa. The monoclinic unit cell can be converted into a pseudotetragonal unit cell with $a=b=5.673(2)\AA{},$ $c=9.754(3)\AA{},$ $\ensuremath{\alpha}=\ensuremath{\beta}=88.75(3)\ifmmode^\circ\else\textdegree\fi{},$ and $\ensuremath{\gamma}=89.87(3)\ifmmode^\circ\else\textdegree\fi{},$ similar to the chalcopyrite-type tetragonal unit cell. The ${\mathrm{AgS}}_{4}$ tetrahedra undergo a relatively large geometrical deformation reflecting weaker force constants of Ag-S compared to those of Ga-S. On the other hand, changes in the geometrical shape of ${\mathrm{GaS}}_{4}$ are less significant and can be approximated as a rotation rather than a distortion. The two crystallographically independent ${\mathrm{S}}_{6}$ octahedral interstices in the monoclinic high-pressure phase experience discrete degrees of compression, which contributes to a further decrease of the $c/a$ ratio above the phase transition pressure of 4.2 GPa.

Study of the (P, T) phase diagram in TlD 2PO 4

This paper reports on the first structural results of a (P, T) phase diagram study of the antiferroelectric ( T c=353 K) TlD 2PO 4 by means of neutron powder diffraction. The temperature range studied was from room temperature to 400 K, and up to 8 kbar in pressure. At T> T c and pressures above 3 kbar a new phase transition takes place, which seems to have the same features of the orthorhombic to monoclinic transition observed in TlH 2PO 4 at room pressure when decreasing temperature below 357 K. The pressure-induced phase transition in TlD 2PO 4 is thus compared to the one observed in the homologue TlH 2PO 4. For T< T c the same high-pressure monoclinic phase is also observed with an intermediate phase which is likely to correspond to the room pressure high-temperature prototype phase (Pbcn).

High pressure and temperature behavior of FeS

High pressure and temperature behavior of stoichiometric FeS (troilite) was studied up to 16 GPa and 800 °C using a combination of a large volume press apparatus and a synchrotron X-ray source. A pressuretemperature phase diagram of FeS was determined from more than 200 data points. The phase diagram shows that there are four stable phases; the troilite phase, the hexagonal phase, the NiAs type phase and the high pressure phase with a monoclinic cell. In the compression curve of FeS at 300 °C, an inflection was observed at 7.5 GPa. Minimum value of cubical expansion under high pressure was also observed around 6 GPa. These results suggested an electronic transition of the hexagonal phase between 6 and 8 GPa. The transition might explain a curved boundary between the hexagonal phase and the monoclinic high pressure phase.

Structures of FeS Polymorphs at High Pressure and Temperature.

Powder x-ray diffraction data for FeS polymorphs were collected at high pressure and temperature, using monochromatic synchrotron x-radiation and image plate. The diffraction data collected at 10. 95 GPa and 300 K revealed a monoclinic high-pressure phase (FeS III) with cell parameters, a = 8. 025 (4) A, b = 5. 614 (2) A, c = 6. 414 (4) A, and β=93. 01 (4) o. The space group of this monoclinic phase is P21/a . We also collected diffraction data for the high PT phase (FeS IV) at 5. 2 GPa and 9. 0 GPa and at 530 K and for the high-T phase (FeS V) at 3. 4 GPa and 650 K. The FeS IV phase has a hexagonal (2a, c) unit cell (space group P63mc) and the FeS V phase has a simple NiAs-type structure (space group P63/mmc). The structure parameters were determined by Rietveld refinements.

Metastable phases synthesized by inert-gas-condensation

Using the inert-gas condensation method we have produced metastable phases of Ta, W and Y 2O 3. The formation of these phases results from both the undercooling achieved during inert-gas condensation as well as the surface-tension-induced increase in pressure with decreasing particle size. For example, we have found that Y 2O 3 can be synthesized in the equilibrium cubic phase (α-Y 2O 3) as well as in the high-pressure monoclinic phase (γ-Y 2O 3) by varying the preparation conditions. The resulting crystal structure is found to be determined by the crystallite size 〈D〉. Based on these observations it is shown that by utilizing-inert gas condensation one can prepare metastable structures and alloys that can hardly be synthesized by classical metallurgical processes.

Pressure-induced semiconductor-metal transition in Te-Se mixtures

X-ray diffraction patterns, electrical resistance and optical reflectivity for amorphous and crystalline Te 1 − x Se x have been measured under high pressure. It is found that the high pressure monoclinic phase with x ~ 0.6, which has a puckered layer structure containing planar zigzag chains with period 3, can be quenched to atmospheric pressure. The correlation between the chain structure and the electronic states in Te-Se mixtures is discussed.

High-pressure phase of magnetite.

M\"ossbauer spectroscopy and x-ray diffraction were employed to investigate the magnetic and structural properties of the high-pressure monoclinic phase of magnetite. Measurements were performed to 66 GPa at 300 K using diamond anvil cells. Based primarily upon the M\"ossbauer parameters, the following features of the high-pressure phase were deduced. With increasing pressure the high-pressure phase evolves at P\ensuremath{\ge}25 GPa and its abundance increases monotonically at the expense of the low-pressure cubic phase. The high-pressure phase is not magnetic at 300 K and its monoclinic structural features resemble that of the low-pressure phase. The tetrahedral and octahedral sites characteristic of the inverse spinel structure, albeit distorted, remain the building blocks of the high-pressure phase. The fast electron hopping between ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$ at the octahedral sites prevails to the highest pressure. It is suggested that the cubic\ensuremath{\rightarrow}monoclinic\ensuremath{\rightarrow}cubic hysteretic cycle involves a mild displacive phase transition not affecting the coordination number of any of the iron cations.

Formation and stability of {113} defects in oxygen implanted silicon-on-insulator material

High dose oxygen implantation into Si (SIMOX – Separation by Implantation of Oxygen) is a viable technique for producing silicon-on-insulator structures. The {113} defect was first observed in SIMOX by van Ommen. It has received considerable study in bulk Si. Bergholz et al. proposed that {113} defects are coesite, a high pressure monoclinic phase of silica, and are a possible precursor to amorphous silicon oxide. Recently Bourrel proposed a self-interstitial model for the formation of hexagonal silicon in the {113} planes in the diamond cubic lattice which was an extension of Tan's model and suggested that oxygen is generally not required for the creation of {113} defects. In this paper we have examined the formation and stability of {113} defects in as-implanted and annealed SIMOX.The first set of samples studied was implanted to 2.4×1018 cm-2 at 200 keV from 400°C to 600°C. The second set was implanted at 150 keV to 2.0×1018 cm-2 at 500°C. Five equivalent parts of the wafer were annealed from 650°C to 1050°C in steps of l00°C for two hours in Ar. Cross-section samples were made by gluing, grinding, dimpling and ion milling of the wafer chips.