Nano-polycrystalline diamond synthesized through the decomposition of stearic acid

ABSTRACT

Here we report a novel route for synthesizing nano-polycrystalline diamond (NPD) using stearic acid (C₁₈H₃₆O₂) as a starting material under high pressure and high temperature. The obtained NPD shows a transparent dark-yellowish color similar to the standard NPD synthesized from graphite and consists of extremely fine diamond grains (～10?nm). The temperature required for the present synthesis of pure transparent NPD is as low as 1000°C at 13 and 17?GPa, which is surprisingly lower than that for conventional NPD synthesis (1800–2000°C). The amorphous-like, extremely poorly crystalline graphite produced by the thermal decomposition of stearic acid likely provides preferential nucleation sites for diamond and significantly lower the activation energy. The removal of volatile components such as H₂O generated through the decomposition from the system is a key to obtain pore-free transparent NPD. Magnesite, MgCO₃ and periclase, MgO can be used as an efficient H₂O remover through the hydration reaction.     

In situ X-ray observation of decomposition of hydrous aluminum silicate AlSiO3OH and aluminum oxide hydroxide d-AlOOH at high pressure and temperature

We clarified the stability limit of phase Egg, AlSiO₃OH, a candidate for water reservoir in the siliceous sediment of slabs in the transition zone conditions by in situ X-ray observation using high energy X-ray from synchrotron radiation source of SPring-8. Phase Egg is stable at least up to 1625 °C at 17 GPa. We observed decomposition of phase Egg into δ-AlOOH and stishovite at pressures greater than 23 GPa at temperatures below 1200 °C. No water release occurs associated with the decomposition. At temperatures above 1200 °C at 23 GPa, we observed decomposition of phase Egg into corundum+stishovite+fluid. We also determined the phase boundary of the decomposition reaction of δ-AlOOH to corumdum+fluid based on the in situ X-ray diffraction at high pressure and temperature.     

X-ray diffraction study of potassium metal under high pressure

Abstract

The equation of state of potassium metal at ～25°C was measured to 6.1 GPa using energy-dispersive synchrotron radiation diffraction and a Drickamer-type cell. Previous piston-cylinder, shock wave and x-ray measurements disagree and this study is intended to help resolve these disagreements. Our P-V data fit closely to a Murnaghan equation of state and least-squares refinement gives B_o = 3.35 GPa and B_o′ = 3.28 in good agreement with recent theoretical calculations based on accurate experimental results.     

The baddeleyite-type high pressure phase of Ca(OH)2

Abstract

A phase transition from Ca(OH)₂ I (portlandite) to Ca(OH)₂ II at high pressure and temperature has been confirmed, using in situ x-ray diffraction in a multianvil high pressure device (DIA). The structure was determined at 9.5 GPa and room temperature from data collected after heating the sample at 300°C at 7.2 GPa in a diamond anvil cell. Both the Le Bail fit and preliminary Rietveld refinement suggest that the new phase, which reverts to Ca(OH), I during pressure release, has a structure related to that of baddeleyite (ZrO₁); it is monoclinic (P2₁/c) with a= 4.887(2), b= 5.834(2), c = 5.587(2), β = 99.74(2)°. The coordination number of Ca increases from six to seven (5 + 2) across the transition. At 500°C, the phase boundary is bracketed at 5.7 ± 0.4 GPa by reversal experiments performed in the DIA.     

Effect of pressure on the Yb valency in YbS and YbTe

Abstract

We have measured L_III-edge x-ray absorption spectra (L_III-XA) of Yb in YbS and YbTe and optical reflectivity spectra of YbTe at pressures up to 34 GPa. In both materials the Yb ion undergoes a continuous valence change. In the case of YbS, the pressure-induced variation of the Yb mean valence v(I_III) agrees well with previous optical and lattice parameter studies. For YbTe, however, the XA data indicate the onset of a valence transition at a lower pressure (～10 GPa) than observed in optical spectra (～16 GPa). This behavior is explained by larger hybridization effects between the chalcogen p-bands and 4f states in YbTe as compared to YbS.     

Mossbauer resonance: A well adapted characterization method of unusual electronic configurations or oxidation states of iron stabilized under high oxygen pressure conditions

Abstract

During these last twenty years high oxygen pressure techniques have led to the stabilization of unusual electronic configurations or oxidation states of d-elements. Due to the specific equipment the quantity of material elaborated in high pressure-high temperature conditions are small(100-500 mg).In such a case a non-destructive characterization is important.

For iron oxides, Mö ssbauer resonance has allowed to point out the stabilization of the high spin state for Fe(IV) in the (A, A')₂M'_0,50Fe_05,0O₄ oxides(A =Ca, Sr, Ba; A' = La; M'=Li, Mg, Zn prepared under oxygen gas pressure (0, 05 < P < 0, 4 GPa).

Another illustration is the characterization, for the first time, of six-coordinated Fe(V) in the ordered perovskite La Li₂FeO₆elaborated in a belt apparatus (7GPa, 900° C) using the "in situ" decomposition of KClO₃ as oxygen source.     

New polymorphs of alumina: Part II μ and λ alumina

Abstract

We had reported observation of polymorphs of alumina having rare earth sesquioxide type structures in xerogel γ alumina (XGA) at high pressures and temperatures (High Pressure Research, 1998). In this paper we report observation of two additional phases at somewhat different pressures and temperatures. The XGA quenched from 5.2 GPa and 1450°C showed, besides α Al₂O₃, a new hexagonal metastable polymorph μAl₂O₃. Over a period of 10 to 12 weeks μ A1₂O₃ transforms to a stable hexagonal phase λ Al₂O₃. The cell parameters of λ Al₂O₃ are comparable with those of k Al₂O₃ found by Saalfield on dehydration of gibbsite A1₂O₃3H₂O in air at 800°C. The XGA containing 1 wt% of Cr₂O₃ yielded similar results at 4.56GPa and 1100°C. Further the XGA containing Cr showed complete transformation to n Al₂O₃ at 5.2 GPa and 1520°C.     

Distribution of light alkanes in the reaction of graphite hydrogenation at pressure of 0.1–7.8 GPa and temperatures of 1000–1350°C

ABSTRACT

We studied the effect of pressure and temperature on the hydrocarbon (HC) chain length distribution and total amount of HCs in the reaction of direct graphite hydrogenation at pressures of 0.1–7.8?GPa and temperatures of 1000–1350°C. An increase in pressure was found to lead both to an increase in the absolute yield of HCs due to direct graphite hydrogenation and to chain elongation of HC products. Light alkanes predominate among HCs in the entire studied range of P–T parameters. However, their concentration in quenched fluids increases as pressure is elevated, from less than 10?rel.% at 0.1?GPa to more than 40–50?rel.% at P?≥?3.8?GPa. Methane is actually the only light alkane among reaction products at 0.1?GPa and 1000°C, while it is a minor component at 7.8?GPa and 1350°C. The most stable alkane at pressures above 3.8?GPa is ethane (C₂H₆).     

Pressure and temperature induced phase transitions in LiInSe2

Abstract

At 4.1 GPa LiInSe₂ transforms from the β-NaFeO₂ - type structure to the NaCl-type structure LiInSe₂-hpI (cubic; Fm3m; a=546.4(3)pm, Z=2, D _x =5.75g/cm³; 4.1GPa) which remains metastable at normal conditions. Heating to 210°C at 1.8 GPa causes ordering of the cations and a phase transition from LiInSe₂-hpI to the α-NaFeO₂ - type structure LiInSe₂-hpII (rhombohedral; R3m; a=393.4pm, c=1919.7pm, Z=3, D _x =5.53g/cm³; 1.8GPa). Heating to 210°C at 0.27 GPa results in a phase transformation from LiInSe₂-hpII to the chalcopyrite-type phase LiInSe₂-hpIII (tetragonal; 142d; a=580.7(8)pm, c=1181.0(31)pm, Z=4, D _x =4.66g/cm³; 0.27 GPa).     

Studies of crystallization process of silica and germania glasses at high pressure

Abstract

The process of crystallization of SiO₂ and GeO₂ glasses was studied using samples retrieved after heat treatment at high pressures up to 12GPa. Two different samples of fused quartz and silica gel were studied in order to compare SiO₂ glass structure. Upon heating beyond 400°C at fixed pressures under which stishovite, rutile-type phase, is thermodynamically stable, the SiO₂ glasses underwent crystallization into coesite and stishovite, without quartz. In the course of heating GeO₂ glass in the stability field of rutile-type phase, low-quartz-type phase appeared as an intermediate at pressures below 7GPa, whereas only rutile-type phase was observed at 12GPa. The crystallization sequences are discussed in terms of coordination numbers in the glass and crystalline states.     

High-pressure X-ray diffraction study of dimedone

Abstract

Dimedone (i.e. 5.5-dimethyl-I,3-cyclohexanedione) crystals, C₈HI₁₂O₂, have been studied at high pressures by X-ray diffraction using a Merrill-Bassett diamond-anvil cell. The unit-cell dimensions have been measured to 1.20(5) GPa and the structure has been determined at 0.95(5) GPa. The crystal compressibility is strongly anisotropic and non-linear, relatively strong compressibility of the crystals is observed along the helices of the hydrogen-bonded molecules. Small anomalous changes of the unit-cell dimensions are observed between 0.1 and 50 MPa. The main structural changes are compression of intermolecular contacts, but also an alongation of the O=C bond—accompanied with the compression of the hydrogen bond involving the carbonyl oxygen atom—has been observed. This elongation is consistent with similar effects reported on compression of the hydrogen bonds in 1,3-cyclohexanedione and 2-methyl-1,3-cyclopentanedione. Crystal data for the dimedone structure at 0.95 GPa: monoclinic, P2₁/c, a=9.909(6), b= 6.505(3), c=12.313(6) Å, β=14.51°, V=722.1(5) Å, Z=4, R=0.139 for 336 independent reflections.     

Absorption of shocked benzene

Abstract

We used absorption spectroscopy to observe decomposition of benzene (C₆H₆) subjected to the passage of strong shock waves generated by projectile impact. These measurements were made using a recently developed double-beam, double-pass, fiber-optic-coupled apparatus. Near 13 GPa, we observe absorption throughout most of the visible region, with strong absorption occurring for wavelengths below 400 nm. The absorption is most likely due to a combination of molecular absorption and Mie scattering from carbon particles formed as a result of shock decomposition.     

Electrical resistivity and critical temperature of Bi-based High-TC superconductors to 1 GPa

Abstract

We have measured the electrical resistance R of a sintered, two-phase, high-T_C superconductor with the nominal composition BiSrCaCu₂O_x, as a function of T and p. We find d(lnR)/dp ? -0.06 GPa^?1 at 295 K, while dT_C/dp is 2.5 K/GPa for the phase with T_c ? 76 and 2 K/GPa for that with T_C ? 106 K.     

Effects of pressure on the two polymorphs of [Co(NH3)5NO2]I2: The anisotropy of lattice distortion and a phase transition

Abstract

The effect of pressure on the two polymorphs of [CO(NH₃)₅NO₂]I₂ (phase I-orthorhombic, S.G. Pnma; phase II-monoclinic, S.G. C2/m) was studied by X-ray powder diffraction in a diamond anvil cell (DAC). In the presence of the ethanol-methanol-water mixture used as a pressure-transmitting liquid polymorph I was shown to undergo a phase transition at pressures between 0.45 GPa and 0.65 GPa. The diffraction pattern of the high-pressure phase (phase III) could be indexed as tetragonal with lattice parameters similar to those, which were previously reported for polymorph II in a 'pseudotetragonal setting'. The lattice distortions of phases II and III were studied at pressures up to 3.2 GPa and 3.7 GPa, correspondingly, and were shown to be very similar. Phases II and III were supposed to be very closely related. If poly(chlortrifluorethylen)-oil was used as a pressure-transmitting medium, no phase transitions were observed in phase I of [CO(NH₃)₅NO₂I₂ at least up to 1.8 GPa (the point when poly(chlortrifluorethylen)-oil becomes solid), and the anisotropy of lattice distortion could be measured.     

High pressure raman study of PbMoO4

Abstract

Raman spectra of PbMoO₄ have been measured up to 31 GPa in a diamond anvil cell (DAC). Two new phases were found at 10 and 16 GPa pressures at room temperature.     

Application of the model-free approach to the study of non-isothermal decomposition of un-irradiated and γ-irradiated hydrated gadolinium acetylacetonate

The non-isothermal decomposition of unirradiated and γ-irradiated hydrated gadolinium acetylacetone with 10² kGy γ-ray absorbed dose was carried out in air and in nitrogen atmospheres and in the temperature range of 25–1000°C. The results indicate that gadolinium acetylacetonate decomposes through four main decomposition steps leading to the formation of intermediate products whose chemical structure is independent of the gas atmosphere applied and on the investigated absorbed dose. The final product at 820°C was found to be Gd₂O₃ irrespective of the gas atmosphere and the irradiation conditions. The non-isothermal data were analyzed using linear Flynn–Wall–Ozawa and non-linear Vyazovkin (VYZ) iso-conversional methods. The results of the application of these free models on the present kinetic data showed that the activation energy, E_a is independent of α in a very wide conversion range (0.1–0.9) indicating that the decomposition process is controlled by a unique kinetic model. The results of the model-fitting analysis showed that the decomposition course of the four decomposition steps of hydrated gadolinium acetylacetone was controlled by the D₃ Jander diffusion model. Pure phase of Gd₂O₃ nanoparticles was obtained by thermal oxidation of γ-irradiated GdAcAc.3 H₂O at 800°C for 6 h. X-ray diffraction, transmission electron microscopy (TEM) and atomic force microscopy (AFM) techniques were employed for characterization of the as-synthesized nanoparticles. This is the first attempt to prepare Gd₂O₃ nanoparticles by solid-state thermal decomposition of γ-irradiated hydrated gadolinium acetylacetone.     

Volume dependence of the raman frequencies in Y Ba2Cu3Ox single crystals with different oxygen content

Abstract

The Raman spectra of Y B _a2C_u3O_x, single crystals (x=6.25; 6.75; 7.0) were measured at pressures up to 22 GPa (35 GPa for x=6.25) at room temperature in nearly hydrostatic conditions. The frequency-volume curves for most of the Raman-active fundamental vibrations were derived from the present data making use of a previous high-pressure study of the equations of state of Y Ba₂ Cu ₃ O _x compounds.     

High-temperature/high-pressure X-ray diffraction: Recent developments

Abstract

We have developed two Merrill-Bassett diamond-anvil cells for specialized high-temperature uses. The first is constructed largely of rhenium to provide uniform, constant P and T on the order of 20 GPa at 1200 K for extended periods. The second is for single-crystal x-ray diffraction, but can be heated to 630 K at 20 GPa to grow single-crystal samples which cannot be produced at room temperature. With this cell, the crystal structure of ?-O₂ was shown to be monoclinic with a = 3.649 A, b = 5.493 A, c = 7.701 A, and β = 116.11° at 19.7 GPa.     

X-ray study of SnO2 under high pressure and temperature generated with sintered diamond anvils

Abstract

In-situ X-ray diffraction technique using synchrotron radiation was applied for polymorphic transitions in SnO₂ under high pressure and temperature generated with 6–8 type double-stage multianvils made of sintered diamond. At 23.6 GPa, the mixed phases of rutile-type structure (R-SnO₂) and columbite-type (c-SnO₂) were heated: at 700°C peaks of fluorite-type structure (F-SnO₂) began to appear, and at 1000°C, C-SnO₂ and 5m later R-SnO₂ disappeared completely and a single phase of F-SnO₂ was recognized, indicating a direct transition from R-SnO₂ to F-SnO₂. The volume reduction of 5.3% was obtained for C-SnO₂→F-SnO₂ transition.     

High-pressure crystal structure of thorium disulfide and diselenide and uranium disulfide

Abstract

The crystal structure of ThS₂, ThSe₂ and US₂ has been investigated for pressure up to 60GPa using x-ray powder diffraction. The bulk moduli are 175(10), 155(10) and 155(20) GPa, respectively. A pressure-induced phase transformation occurs at about 40 GPa for ThS₂, 30 GPa for ThSe₂ and 15 GPa for US₂. The results for ThSe₂ indicate that its high-pressure phase has a monoclinic structure. The same structure is compatible with the observed high-pressure spectra of ThS₂ and US₂. However, the crystal system assignment is less certain for these compounds.