Structural change associated with the incommensurate-normal phase transition in akermanite, Ca2MgSi2O7, at high pressure

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)?ų and K₀=100(3)?GPa for the IC phase and V₀=307.6(2)?ų 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?ų), whereas the volumes of the [MgO₄] and [CaO₈] polyhedra decrease from 3.61 to 3.55(1)?ų and 32.8 to 30.9(2)?ų, 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.     

Structural and vibrational behaviour of fluorapatite with pressure. Part II: in situ micro-Raman spectroscopic investigation

 High-pressure Raman investigations were carried out on a synthetic fluorapatite up to about 7 GPa to analyse the behaviour of the phosphate group's internal modes and of its lattice modes. The Raman frequencies of all modes increased with pressure and a trend toward reduced splitting was observed for the PO₄-stretching modes [(ν_3a(A_g) and ν_3b(A_g); ν_3a(E_2g) and ν_3b(E_2g)] and the PO₄ out-of-plane bending modes [ν_4a(A_g) and ν_4b(A_g)]. The pressure coefficients of phosphate modes ranged from 0.0047 to 0.0052 GPa⁻¹ for ν₃, from 0.0025 to 0.0044 GPa⁻¹ for ν₄, from 0.0056 to 0.0086 GPa⁻¹ for ν₂ and 0.0046 for ν₁ GPa⁻¹, while the pressure coefficients of lattice modes ranged from 0.0106 to 0.0278 GPa⁻¹. The corresponding Grüneisen parameters varied from 0.437 to 0.474, 0.428, 0.232 to 0.409 and 0.521 to 0.800 for phosphate modes ν₃, ν₁, ν₄, ν₂, respectively, and from 0.99 to 2.59 for lattice modes. The vibrational behaviour was interpreted in view of the high-pressure structural refinement performed on the same crystal under the same experimental conditions. The reduced splitting may thus be linked to the reduced distortion of the environment around the phosphate tetrahedron rather than to the decrease of the tetrahedral distortion itself. Moreover, the amount of calcium polyhedral compression, which is about three times the compression of phosphate tetrahedra, may explain the different Grüneisen parameters. Received: 25 April 2000 / Accepted: 20 December 2000     

Structural change of GeO2 under pressure

Pressure-induced amorphization of α-quartz type GeO₂ was studied with a newly developed X-ray diffraction system which consists of a 4-circle goniometer and a curved position sensitive detector. Single-crystal diffraction was measured under pressurs up to 7.3 GPa at room temperature in order to investigate pretransitional phenomena. Diffraction intensity and line width of the diffraction profiles showed no remarkable change up to 5.9 GPa. However, no sharp diffraction line was observed at pressures over 6.5 GPa. The bulk modulus at 0.1 MPa and its pressure derivative of α-quartz type GeO₂ were determined to be K _T =32.8(3.3) GPa and K′ _T =6.0(2.0), respectively. In situ microscopic observations of the amorphization transformation was also performed. The large volume change due to amorphization was observed and estimated to be about 10%.     

Structural and vibrational behaviour of fluorapatite with pressure. Part I: in situ single-crystal X-ray diffraction investigation

Using single-crystal X-ray diffraction from a diamond anvil cell, the compressibility of a synthetic fluorapatite was determined up to about 7?GPa. The compression pattern was anisotropic, with greater change along a than c. Unit cell parameters varied linearly with β _a =3.32(8)?10^?3 and β _c =2.40(5)?10^?3 GPa^?1, giving a ratio β _a :β _c =1.38:1. Data fitted with a third-order Birch-Murnaghan EOS yielded a bulk modulus of K ₀=93(4)?GPa with K′=5.8(1.8). The evolution of the crystal structure of fluorapatite was analysed using data collected at room pressure, at 3.04 and 4.72?GPa. The bulk modulus of phosphate tetrahedron is about three times greater than the bulk modulus of calcium polyhedra. The values were 270(10), 100(4) and 86(3) GPa for P, Ca1 (nine-coordinated) and Ca2 (seven-coordinated) respectively. While the calcium polyhedra became more regular with pressure, the distortion of the phosphate tetrahedron remained unchanged. The size of the channel extending along the [001] direction represented the most compressible direction. The Ca2–Ca2 distance decreased from 3.982 to 3.897?Å on compression from 0.0001 to 4.72?GPa. The anisotropic compressional pattern may be understood in terms of the greater compressibility of the channel size over the polyhedral units. The reduction of the channel volume was measured by the evolution of the trigonal prism, having the Ca2–Ca2–Ca2 triangle as its base and the c lattice parameter as its height. This prism volume changed from 47.3?ų at room pressure to 44.78?ų at 4.72?GPa. Its relatively high bulk moduli, 86(3) GPa, indicated that the channel did not collapse with pressure and the apatite structure could remain stable at very high pressure.     

Structural evolution of rutile-type and CaCl2-type germanium dioxide at high pressure

 Germanium dioxide was found to undergo a transition from the tetragonal rutile-type to the orthorhombic CaCl₂-type phase above 25 GPa. The detailed structural evolution of both phases at high pressure in a diamond anvil cell has been investigated by Rietveld refinement using angle-dispersive, X-ray powder-diffraction data. The square of the spontaneous strain (a−b)/(a+b) in the orthorhombic phase was found to be a linear function of pressure and no discontinuities in the cell constants and volume were observed, indicating that the transition is second-order and proper ferroelastic. Compression of the GeO₆ octahedra was found to be anisotropic, with the apical Ge-O distances decreasing to a greater extent than the equatorial distances and becoming shorter than the latter above 7 GPa. Above this pressure, the GeO₆ octahedron exhibits the common type of tetragonal distortion predicted by a simple ionic model and observed for most rutile-type structures such as those of the heavier group-14 dioxides and the metal difluorides. Above the phase transition, the columns of edge-sharing octahedra tilt about their two fold axes parallel to c and the rotation angle reaches 10.2(5)° by 36(1) GPa so as to yield a hexagonal close-packed oxygen sublattice. The compressibility increases at the phase change as is expected for a second-order transition at which an additional compression mechanism becomes available.     

Solid solution between lead fluorapatite and lead fluorvanadate apatite: mixing behavior,Raman feature and thermal expansivity

The solid solution between lead fluorapatite and lead fluorvanadate apatite, Pb₁₀[(PO₄)_6-x (VO₄) _x ]F₂ with x equal to 0, 1, 2, 3, 4, 5 and 6, was synthesized by solid-state reaction at 1 atm and 700°C for 72 h and characterized by scanning electronic microprobe, electronic microprobe analysis, micro-Raman spectroscopy, and powder X-ray diffraction. The volume-composition relationship at ambient temperature does not show significant deviation from the Vegard’s Law. The Raman spectrum data suggest that both P and V are identical on a C _s site and both end-members show no apparent factor-group effect. The Raman frequency shift of the symmetric stretching vibration is linearly dependent on the composition. High temperature X-ray diffraction data, up to 600°C, suggest that the thermal expansion coefficients α _a , α _c , and α _V also vary linearly with the compositions of the apatites.     

Diamond stability in silicate-halogenide melts at high pressure

Chlorides of alkali metals of mantle origin and with the possibility to influence the preservation of natural diamonds were found in kimberlites of deep horizons of Udachnaya pipe (Yakutia).     

Density of peridotite melts at high pressure

Densities of ultramafic melts were determined up to 22 GPa by relative buoyancy experiments. Olivine and diamond were used as buoyancy markers. We confirmed that the density crossover of PHN 1611 melt and its equilibrium olivine (Fo94) occurs at around 13.5 GPa and 2030 °C and that olivine floats from deeper regions in the magma ocean of the primordial terrestrial mantle. The comparison of the compression curves of basic and ultrabasic melts implies that the basic melt is more compressible. This can be explained by the difference in the amount of compressible linkage of SiO_n and AlO_n polyhedra. The interstitial melt trapped by the density crossover can be the cause of the impedance anomaly of the seismic wave in the deep upper mantle.     

Viscosity of albite melt at high pressure and high temperature

 The viscosity of albite (NaAlSi₃O₈) melt was measured at high pressure by the in situ falling-sphere method using a high-resolution X-ray CCD camera and a large-volume multianvil apparatus installed at SPring-8. This system enabled us to conduct in situ viscosity measurements more accurately than that using the conventional technique at pressures of up to several gigapascals and viscosity in the order of 10⁰ Pa s. The viscosity of albite melt is 5.8 Pa s at 2.6 GPa and 2.2 Pa s at 5.3 GPa and 1973 K. Experiments at 1873 and 1973 K show that the decrease in viscosity continues to 5.3 GPa. The activation energy for viscosity is estimated to be 316(8) kJ mol⁻¹ at 3.3 GPa. Molecular dynamics simulations suggest that a gradual decrease in viscosity of albite melt at high pressure may be explained by structural changes such as an increase in the coordination number of aluminum in the melt. Received: 6 January 2001 / Accepted: 27 August 2001     

Electrical conductivity of K-feldspar at high temperature and high pressure

Electrical conductivity measurements on dry polycrystalline K-feldspar were performed at 1.0 to 3.0 GPa and 873 to 1,173 K with a multi-anvil high-pressure apparatus and the Solartron-1260 Impedance/Gain Phase Analyzer in the frequency range of 10^?1 to 10⁶ Hz. At each temperature the complex impedance displays a perfect semi-circular arc that represents the grain-interior conduction. Under the experimental conditions, electrical conductivity exponentially increases with increasing temperature and slightly decreases with increasing pressure; however, the effect of pressure on the conductivity is less pronounced than that of temperature. The activation enthalpy decreases slightly from 0.99 to 1.02 eV with increasing pressure, and the activation energy and activation volume for K-feldspar are 0.98 eV and 1.46?±?0.17 cm³/mol, respectively. According to these Arrhenius parameters, ionic conduction is proposed to be the dominant conduction mechanism in K-feldspar at high temperatures and pressures, and potassium ions are the charge carriers transporting by an interstitial mechanism. The diffusion coefficient of potassium at high temperatures was calculated from our conductivity data on K-feldspar using Nernst–Einstein equation, and the results were compared with the previous experimental results.     

Friction in faulted rock at high temperature and pressure

Two hundred observations of frictional behavior of seven low-porosity silicate rocks were made at temperatures to 700°C and pressures from 2.5 to 6 kbar. For all rocks except one, peridotite, stick-slip occurred at low temperature and gave way to stable sliding at some high temperature, different for each rock. These differences could be related to the presence or absence of minerals such as amphibole, mica, or serpentine. Up to some temperature, depending on rock type, the friction stress was relatively unaffected by temperature. The shear stress decreased at higher temperature, and in some cases such decrease was related to the coincidence of fracture and friction strength. While somewhat dependent on rock type, the friction stress for the seven rocks studied was about the same, within 10–15%. Up to 265°C, water had little effect on the frictional behavior of faulted granite at 3 kbar effective pressure. The frictional stresses measured in the laboratory were significantly higher than estimated for natural faults. This difference could be accounted for by high pore pressure or weak alteration materials in the natural fault zone.     

Chromium solubility in MgSiO3 ilmenite at high pressure

The crystal structure and chemical composition of crystals of (Mg_1?x Cr _x )(Si_1?x Cr _x )O₃ ilmenite (with x = 0.015, 0.023 and 0.038) synthesized in the model system Mg₃Cr₂Si₃O₁₂–Mg₄Si₄O₁₂ at 18–19 GPa and 1,600 °C have been investigated. Chromium was found as substitute for both Mg at the octahedral X site and Si at the octahedral Y site, according to the reaction Mg²⁺ + Si⁴⁺ = 2Cr³⁺. Such substitutions cause a shortening of the <X–O> and a lengthening of the <Y–O> distances with respect to the values typically observed for pure MgSiO₃ ilmenite and eskolaite Cr₂O₃. Although no high Cr contents are considered in the pyrolite model, Cr-bearing ilmenite may be the host for chromium in the Earth’s transition zone. The successful synthesis of ilmenite with high Cr contents and its structural characterization are of key importance because the study of its thermodynamic constants combined with the data on phase relations in the lower-mantle systems can help in the understanding of the seismic velocity and density profiles of the transition zone and the constraining composition and mineralogy of pyrolite in this area of the Earth.     

X-ray diffraction study of magnesite at high pressure and high temperature

 P–V–T measurements on magnesite MgCO₃ have been carried out at high pressure and high temperature up to 8.6 GPa and 1285 K, using a DIA-type, cubic-anvil apparatus (SAM-85) in conjunction with in situ synchrotron X-ray powder diffraction. Precise volumes are obtained by the use of data collected above 873 K on heating and in the entire cooling cycle to minimize non-hydrostatic stress. From these data, the equation-of-state parameters are derived from various approaches based on the Birch-Murnaghan equation of state and on the relevant thermodynamic relations. With K′₀ fixed at 4, we obtain K₀=103(1) GPa, α(K⁻¹)=3.15(17)×10⁻⁵ +2.32(28)×10⁻⁸ T, (∂K_T/∂T)_P=−0.021(2) GPaK⁻¹, (dα/∂P)_T=−1.81×10⁻⁶ GPa⁻¹K⁻¹ and (∂K_T/∂T)_V= −0.007(1) GPaK⁻¹; whereas the third-order Birch-Murnaghan equation of state with K′₀ as an adjustable parameter yields the following values: K₀=108(3) GPa, K′₀=2.33(94), α(K⁻¹)=3.08(16)×10⁻⁵+2.05(27) ×10⁻⁸ T, (∂K_T/∂T)_P=−0.017(1) GPaK⁻¹, (dα/∂P)_T= −1.41×10⁻⁶ GPa⁻¹K⁻¹ and (∂K_T/∂T)_V=−0.008(1) GPaK⁻¹. Within the investigated P–T range, thermal pressure for magnesite increases linearly with temperature and is pressure (or volume) dependent. The present measurements of room-temperature bulk modulus, of its pressure derivative, and of the extrapolated zero-pressure volumes at high temperatures, are in agreement with previous single-crystal study and ultrasonic measurements, whereas (∂K_T/∂T)_P, (∂α/∂P)_T and (∂K_T/∂T)_V are determined for the first time in this compound. Using this new equation of state, thermodynamic calculations for the reactions (1) magnesite=periclase+CO₂ and (2) magnesite+enstatite=forsterite+CO₂ are consistent with existing experimental phase equilibrium data. Received September 28, 1995/Revised, accepted May 22, 1996     

The Electrical Conductivity of Gabbro at High Temperature and High Pressure

The electric conductivity of gabbro has been measured at 1.0 - 2.0 GPa and 320-700℃, and the conduction mechanism has been analyzed in terms of the impedance spectra.Experimental results indicated that the electric conductivity depends on the frequency of alternative current. Impedance arcs representing the conduction mechanism of grain interiors are displayed in the complex impedance plane, and the mechanism is dominated at high pressure.These arcs occur over the range of 102 - k× 105 Hz (k is the positive integer from 1 to 9). On the basis of our results and previous work, it is concluded that gabbro cannot form any high conductivity layer (HCL) in the middle-lower crust.     

X-ray diffraction study of arsenopyrite at high pressure

The high-pressure X-ray diffraction study of a natural arsenopyrite was investigated up to 28.2 GPa using in situ angle-dispersive X-ray diffraction and a diamond anvil cell at National Synchrotron Light Source, Brookhaven National Laboratory. The 16:3:1 methanol–ethanol–water mixture was used as a pressure-transmitting medium. Pressures were measured using the ruby-fluorescence method. No phase change has been observed up to 28.2 GPa. The isothermal equation of state (EOS) was determined. The values of K ₀, and K′ ₀ refined with a third-order Birch–Murnaghan EOS are K ₀ = 123(9) GPa, and K′ ₀ = 5.2(8). Furthermore, we confirm that the linear compressibilities (β) along a, b and c directions of arsenopyrite is elastically isotropic (β _a  = 6.82 × 10⁻⁴, β _b  = 6.17 × 10⁻⁴ and β _c  = 6.57 × 10⁻⁴ GPa⁻¹).     

Chemical diffusion of fluorine in jadeite melt at high pressure

The chemical diffusion of fluorine in jadeite melt has been investigated from 10 to 15 kbars and 1200 to 1400°C using diffusion couples of Jadeite melt and fluorine-bearing jadeite melt (6.3 wt.% F). The diffusion profile data indicate that the diffusion process is concentration-independent, binary, F-O interdiffusion. The F-O interdiffusion coefficient ranges from 1.3 × 10^?7 to 7.1 × 10^?7 cm²/sec and is much larger than those obtained by Kushiro (1983) for Si-Ge and Al-Ga interdimision in jadeitic melts. The Arrhenius activation energy of diffusion is in the range of 36 to 39 kcal/mole as compared with 19 kcal/mole for fluorine tracer diffusion in a lime-aluminosilicate melt. The diffusivity and activation energy of F-O interdiffusion vary slightly with pressure, but the pressure dependence of F-O, Al-Ga and Si-Ge interdiffusion may be related to the relative volumes of the interdiffusing species for each pair. The magnitude of chemical diffusivity of fluorine is comparable to that of the chemical diffusivity of water in obsidian melts. The diffusivities of various cations are significantly increased by the addition of fluorine or water to a silicate melt. This fact, combined with the high diffusivity of fluorine, suggests that the F^? ion is the principal diffusing species in dry aluminosilicate melts and that dissolved fluorine will accelerate chemical equilibration in dry igneous melts.     

Solid solutions between lead fluorapatite and lead fluorvanadate apatite: compressibility determined by using a diamond-anvil cell coupled with synchrotron X-ray diffraction

The synthetic solid solutions between lead fluorapatite and lead fluorvanadate apatite, Pb₁₀[(PO₄)_6−x (VO₄) _x ]F₂ with x equal to 0, 1, 2, 3, 4, 5, and 6, were compressed up to about 9 GPa at ambient temperature by using a diamond-anvil cell coupled with synchrotron X-ray radiation. A second-order Birch–Murnaghan equation of state was used to fit the data. As the substitution of the PO₄ ³⁻ cations by the VO₄ ³⁻ cations progresses, the isothermal bulk modulus steadily decreases, with a maximum reduction of about 16% (from 68.4(16) GPa for Pb₁₀(PO₄)₆F₂ to 57.2(28) GPa for Pb₁₀(VO₄)₆F₂). For the entire composition range, the a-axis dimension remains more compressible than the c-axis dimension, with the ratio of the axial bulk moduli (K _T−c :K _T−a ) larger than 1. The ratio of K _T−c to K _T−a increases from about 1.04(4) to 1.23(14) as the composition parameter x increases from 0 to 6, suggesting that the apatite solid solutions Pb₁₀[(PO₄)_6−x (VO₄) _x ]F₂ become more elastically anisotropic.     

Thermodynamic properties of San Carlos olivine at high temperature and high pressure

In this study, the thermal expansion and heat capacity of San Carlos olivine under high temperature and high pressure are reported. Combining accurate sound velocity data under different P–T conditions with density and heat capacity data at ambient pressure, the density, adiabatic bulk modulus, shear modulus, and most importantly, thermal expansion and heat capacity, of San Carlos are extracted to 14 GPa by a numerical procedure using classic thermodynamic relationships. These data are in agreement with published findings. To estimate the temperature gradient in the upper mantle, we also report the fitting equations of thermal expansion and heat capacity of San Carlos olivine as a function of both temperature and pressure to the P–T condition of the 410 km discontinuity, which provide the thermodynamic properties with increasing depth in the Earth’s interior.     

Stability and band gap engineering of silica-confined lead halide perovskite nanocrystals under high pressure

SiO₂is the major mineral substance in the upper mantle of the earth.Therefore,studies of the silica-coated materials under high-pressure are essential to explore the physical and chemical properties of the upper mantle.The silica-confined CsPbBr₃nanocrystals(NCs)have recently attracted much attention because of the improved photoluminescence(PL)quantum yield,owing to the protection of silica shell.However,it remains considerable interest to further explore the relationship between optical properties and the structure of CsPbBr₃@SiO₂NCs.We systemically studied the structural and optical properties of the CsPbBr₃@SiO₂NCs under high pressure by using diamond anvil cell(DAC).The discontinuous changes of PL and absorption spectra occurred at~1.40 GPa.Synchrotron X-ray diffraction(XRD)studies of CsPbBr₃@SiO₂NCs under high pressure indicated an isostructural phase transformation at about 1.36 GPa,owing to the pressure-induced tilting of the Pb-Br octahedra.The isothermal bulk moduli for two phases are estimated about 60.0 GPa and 19.2 GPa by fitting the equation of state.Besides,the transition pressure point of CsPbBr₃@SiO₂NCs is slightly higher than that of pristine CsPbBr₃NCs,which attributed to the buffer effect of coating silica shell.The results indicate that silica shell is able to enhance the stabilization without changing the relationship between optical properties and structure of CsPbBr₃NCs.Our results were fascinated to model the rock metasomatism in the upper mantle and provided a new‘lithoprobe’for detecting the upper mantle.