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1.
Raman spectra of Mg2GeO4-olivine were obtained from ambient pressure up to 34 GPa at ambient temperature. Under quasi-hydrostatic pressure conditions, the following modifications in the Raman spectra occur as pressure increases: 1) near 11 GPa, two sharp extra bands appear in the 600–700 cm?1 frequency range, and increase in intensity with respect to the olivine bands; 2) above 22 GPa, these two bands become very intense, and the number, position and relative intensity of the other vibrational bands drastically change; 3) the intensity of sharp bands progressively decreases above 25 GPa. The transformation occurs at lower pressures under non-hydrostatic conditions. During decompression to atmospheric pressure, the high-pressure phase partially reverts to olivine. These observations can be interpreted as the progressive metastable transformation from the olivine structure to a crystalline phase with four-fold coordinated Ge, in which the GeO4 tetrahedra are polymerized. We propose that the metastable high-pressure phase is a structurally disordered spinelloid close to the hypothethical ω- or ?*-phase, and forms by a shear mechanism assisted by the development of a dynamical instability in the olivine structure. Implications for the transformations undergone by olivines under far-from-equilibrium conditions (e.g. in subducting lithospheric slabs and in shocks) are discussed.  相似文献   

2.
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%.  相似文献   

3.
Infrared and Raman spectra of the quartz, rutile and amorphous forms of GeO2 have been recorded under pressure and/or temperature, in order to study the crystalline to crystalline — or amorphous — transformations of this compound in the solid state. X-ray diffraction data shown that crystalline quartz-GeO2 subjected to high pressure amorphizes. Infrared data are consistent with a gradual amorphisation of this compound at static pressures between 6 to 12 GPa at 300 K. With increasing pressure, the Ge-O distance appears to remain constant and amorphization is associated with a progressive change in the coordination of germanium atoms from fourfold to sixfold. This apparent change in coordination is not quenchable at room pressure. On decompression, the Ge in the amorphous form returns to tetrahedral coordination. The anharmonic parameters for the Raman modes of the quartz and rutile forms of GeO2, have also been estimated from pressure and temperature shifts. These data have been used to calculate heat capacities and entropies of the two polymorphs at different pressures, with the Kieffer vibrational model. The calculated heat capacities at room pressure are within 1% of the experimental values between 20 and 1500 K. The calculated entropies are used to estimate the phase boundary in the (P, T) plane. The slope of the curve at room pressure (17 bar/K) is in good agreement with experimental values.  相似文献   

4.
Enthalpies and entropies of transition for the Mg2GeO4 olivine-spinel transformation have been determined from self-consistency analyses of Dachille and Roy's (1960), Hensen's (1977) and Shiota et al.'s (1981) phase boundary studies. When all three data sets are analyzed simultaneously,ΔH 973 andΔS 973 are constrained between ?14000 to ?15300 J mol?1 and ?13.0 to ?14.1·J mol?1 K?1, respectively. High-temperature solution calorimetric experiments completed on both polymorpha yield a value of ?14046±1366 J mol?1 forΔH 973. Kieffer-type lattice vibrational models of Mg2GeO4 olivine and spinel based on newly-measured infrared and Raman spectra predict a value of ?13.3±0.6 J mol?1 K?1 forΔS 1000. The excellent agreement between these three independent determinations ofΔH andΔS suggests that the synthesis runs of Shiota et al. (1981) at high pressures and temperatures bracket equilibrium conditions. In addition, no configurational disorder of Mg and Ge was needed to obtain the consistent parameters quoted. The Raman spectrum and X-ray diffractogram show that little disorder, if any, is present in Mg2GeO4 spinel synthesized at 0.2 GPa and 973–1048 K.  相似文献   

5.
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)?Å3 and K0=100(3)?GPa for the IC phase and V0=307.6(2)?Å3 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?Å3), whereas the volumes of the [MgO4] and [CaO8] polyhedra decrease from 3.61 to 3.55(1)?Å3 and 32.8 to 30.9(2)?Å3, 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.  相似文献   

6.
Drop-calorimetry determinations of the isobaric heat capacity (CP) of Mg2GeO4, Ca2GeO4 and CaMgGeO4 have been made up to 1700 K. The thermal expansion coefficient (α) of these olivine germanates has been determined from high-temperature X-ray measurements up to 1500 K. From these measurements and available compressibility data, one calculates that the isochoric heat capacity (CV) exceeds the harmonic limit of Dulong and Petit above 1000–1200 K. Such an intrinsic anharmonic behaviour can be accounted for by introducing anharmonic parameters ai=(? ln v i/?T)V in vibrational modelling of CV. These parameters are calculated from pressure and temperature shifts of the vibrational frequencies as measured by Raman spectroscopy up to 10 GPa at room temperature and up to 1300 K at 1 bar. A comparison of the Raman spectra of the three germanates with those of natural olivines justifies once again the use of germanates as silicate analogues. Extensive Ca,Mg disordering likely takes place in CaMgGeO4, beginning at about 1100 K and leading to unusually high increases of the heat capacity and thermal expansion coefficient.  相似文献   

7.
Germanate olivines Mg2GeO4, Ca2GeO4 and CaMgGeO4 have been studied by high-pressure X-ray Diffraction and high-pressure X-ray Absorption Spectroscopy. The three compounds were compressed, in the 0–30 GPa pressure range, at room temperature in a diamond-anvil cell, silicon oil being used as the pressure transmitting medium. Values of K0 are 166 ± 15, 117 ± 15 and 152 ± 14 GPa for Mg2GeO4, Ca2GeO4 and CaMgGeO4 respectively. These olivines all exhibit compression anisotropy, the a axis being the least compressible. Crystal to crystal phase transitions have been observed in Mg2GeO4 and Ca2GeO4 above 12 GPa and 6 Gpa respectively. The nature of these structural changes remains unclear yet. The onset of amorphization has been observed in Mg2GeO4 and Ca2GeO4 at pressures above about 22 and 11 GPa respectively. These phase transitions and amorphization processes do not involve any detectable increase in the coordination number of germanium atoms. At higher pressure (P >23 GPa), we report the onset of a transition from a phase with fourfold coordinated germanium to a phase with higher germanium coordination number in CaMgGeO4.  相似文献   

8.
Spinelloid phases have been observed and characterized by powder X-ray diffraction and high-resolution electron microscopy. Mg3Ga2GeO3(III), with a narrow composition range of approximately 3 mole percent Mg2GeO4, is stable at atmospheric pressure up to about 1,420° C, and is isostructural with β-Mg2SiO4 and the spinelloid Phase III of the NiAl2O4-Ni2SiO4 system. This represents the first occurrence of a β-phase structure stable at 1 atm pressure. Above 1,420° C (1 atm) Mg3Ga2GeO8 (III) decomposes reversibly into a mixture of spinel and olivine. At high pressure (around 30 kbar at 1,100° C) it transforms into another spinelloid phase, Mg3Ga2GeO8 (IV), isostructural with Phase IV of the NiAl2O4-Ni2SiO4 system. In terms of crystal structures and phase relations therefore there exists a close analogy between the magnesium gallo-germanate and nickel alumino-silicate systems, the former being a lower-pressure analogue of the latter. Our investigation of a number of other pseudo-binary spinel-olivine oxide systems suggests that the formation of spinelloid phases can be associated with the inverse character of the spinel component.  相似文献   

9.
A two-body interatomic potential model for GeO2 polymorphs has been determined to simulate the structure change of them by semi-empirical procedure, total lattice energy minimization of GeO2 polymorphs. Based on this potential, two polymorphs of GeO2; α-quartz-type and rutile-type, have been reproduced using the molecular dynamics (MD) simulation techniques. Crystal structures, bulk moduli, volume thermal expansion coefficients and enthalpies of these polymorphs of GeO2 were simulated. In spite of the simple form of the potential, these simulated structural values, bulk moduli and thermal expansivities are in excellent agreement with the reliable experimental data in respect to both polymorphs. Using this potential, MD simulation was further used to study the structural changes of GeO2 under high pressure. We have investigated the pressure-induced amorphization. As reported in previous experimental studies, quartz-type GeO2 undergoes pressure-induced crystalline-to-amorphous transformation at room temperature, the same as other quartz compounds; SiO2, AlPO4. Under hydrostatic compression, in this study, α-quartz-type GeO2 transformed to a denser amorphous state at 7.4 GPa with change of the packing of oxygen ions and increase of germanium coordination. At higher pressure still, rutile-type GeO2 transformed to a new phase of CaCl2-type structure as a post-rutile candidate. Received: 29 July 1996 / Revised, accepted: 30 April 1997  相似文献   

10.
Raman microprobe (RMP) spectra were produced for each of the olivine and spinel structured phases of Mg2GeO4 and (Mg, Fe)2SiO4. The assembled data show that bands due to the tetrahedra in silicate and germanate olivines shift in a way that indicates a dominant mass effect. This correspondence is difficult to make in spinels due to differences in structural type. Differences in Fe/Mg content of olivine shift the tetrahedral vibration bands only slightly, but their linear shifts could be used to indicate the composition of the phase.  相似文献   

11.
Synthetic polycrystals of α-Mg2GeO4 (with the olivine structure) and γ-Mg2GeO4 (with the spinel structure) deformed at high temperature and pressure in their respective stability fields were investigated by analytical transmission electron microscopy. Specimens with a mean grain size of 20–30 µm deform by dislocation glide and/or climb. The predominance of glide versus climb depends on stress and grain orientation. The defect microstructures of both polymorphs are very similar to those observed in their respective silicate analogues, α- and γ-(Mg,Fe)2SiO4, and, in the case of the spinel phase, very similar to those observed in magnesium aluminate spinels. These observations suggest that Mg2GeO4 is a good rheological analogue for the Earth’s upper mantle. A spinel specimen deformed under the same conditions of temperature and strain rate as an olivine specimen was approximately three times stronger than olivine. In specimens of both phases deformed at or above 1400 K, a thin amorphous film composed of Mg, Ge, and O was detected along some grain boundaries. Grains ≤10 µm diameter surrounded by a film of amorphous phase (>10 nm thick) exhibited low dislocation densities, and deformation appeared to have occurred by grain boundary sliding.  相似文献   

12.
In order to investigate compression mechanism and the pressure-induced amorphization of portlandite, Ca(OH)2, the crystal structure has been refined up to 9.7?GPa using Rietveld analysis. Angular-dispersive synchrotron X-ray powder diffraction experiments were performed using a diamond anvil cell and an imaging plate at BL-18C in the Photon Factory at KEK, Japan. Compression behavior is highly anisotropic and the c axis is approximately 2.5 times as compressible as the a axis (βa=0.004, βc=0.011?GPa?1). Because the refined fractional coordinate, z, of the O atom increases linearly with pressure, compression along the c axis is due to the shortening of the interlayer spacing. The compression mechanism shows no change up to the amorphization pressure and is basically the same as that of brucite, Mg(OH)2, observed below 10 GPa. The octahedral regularity of CaO6 approaches a regular configuration with pressure. The interlayer O…O distance is expected to be about 2.75 Å at the amorphization pressure and should affect hydrogen bonding.  相似文献   

13.
Shock-loading experiments are performed on single-crystal specimens of BaZnGeO4 (BZG) between 13 and 51 GPa using a propellant gun and a two-stage light gas gun. Observation by an optical microscope reveals that all the shocked specimens are made of fragmented blocks of 10 ～ 100 μm in size. X-ray analyses indicate that the samples shocked to pressures above 41 GPa became a polycrystalline state of BZG with a weakly textured structure. A small amount of dissociation products of BaGeO3 (pyroxene type) and ZnO (wurtzite type) are detected in this pressure range. Between 27 and 19 GPa, a new phase which is thought to be produced by considerable deformations in the ring structure consisting of ZnO4 and GeO4 tetrahedra is observed. Below 19 GPa, shocked specimens are brought into a well aligned mosaic structure, but no significant change in the crystal structure is detected.  相似文献   

14.
The phase transition of Mg2GeO4 from the olivine structure (α-phase) to the spinel structure (γ-phase) was determined by complex electrical resistivity measurements in the frequency range 10 Hz up to 100 kHz. The stability fields of the α- and the γ-phase were confirmed up to 2.05 GPa in the temperature range 845° C–1400° C. Based on volume resistivity data, a decrease of about a factor of 5 was found at the α-γ phase transition. Activation energies of electrical conduction E a at 1.05 GPa and 2.05 GPa were calculated using the volume resistivities (?) and the relaxation times (τ). The values range from 1.98 eV up to 2.78 eV. The relative dielectric permeability increases with increasing temperature. This is is due to crystal defects and charge transport mechanisms.  相似文献   

15.
 In-situ X-ray diffraction measurements of CaGeO3-wollastonite at high pressure at room temperature have been performed using a diamond anvil cell with an X-ray source. A new structural modification of CaGeO3-wollastonite is observed at about 6GPa and the characteristic reflections of the high pressure form are preserved on decompression to an ambient pressure. A rhodonite-like structure is proposed as a high pressure form from the crystal chemical consideration. The rhodonite-like phase is further transformed into a perovskite-form at about 15 GPa. The rhodonite-like-form of CaGeO3 seems not to be a stable phase from the heating experiments under high pressures. The metastable transition path from the wollastonite to the perovskite polymorph through the rhodonite-like structure is kinetically favored under room temperature pressurization. No pressure-induced amorphization is observed during the transition into the perovskite-form, although the transition is accompanied by the coordination change of Ge atoms from fourfold to sixfold. Received: July 19, 1995 / Revised, accepted: August 1996  相似文献   

16.
The temperature dependence (at ambient pressure) of the Raman spectra of both the quartz- and rutile-types of GeO2 has been studied from 109 to 874?K. All spectra were corrected for the effects of temperature and are presented in their reduced form to allow a direct comparison of intensities at all temperatures. In the quartz-type GeO2, the Raman bands above 400?cm?1 exhibited relatively larger temperature dependences and at least four of the bands in this region vary nonlinearly with increasing temperature. Deconvolution of the most intense Raman band at 700?cm?1 in the rutile-type GeO2 revealed the presence of a previously unreported band at 684?cm?1 at 298?K which may arise from splitting of the A1g mode. A nonlinear temperature dependence was observed for all the Raman bands above 600?cm?1 in the rutile-type GeO2 with the new band at 684?cm?1 exhibiting the largest curvature. In common with previous studies of rutile-type oxides, the B1g mode at 171?cm?1 showed anomalous behaviour by increasing linearly in frequency with increasing temperature. In a separate experiment, the oxidation of metallic germanium in air demonstrated that the quartz-type GeO2 is the preferred form of germanium oxide at temperatures above 745?K at atmospheric pressure. Thermodynamic calculations predict that the rutile-form of GeO2 should be the stable species under these conditions. This suggests that atmospheric gases may have a marked effect on the kinetics and stability of the quartz and rutile forms of GeO2.  相似文献   

17.
An in situ high pressure powder diffraction study, using high-brilliance synchrotron radiation, on lead feldspar (PbAl2Si2O8) was performed. Two samples, with Q od=0.68 and 0.76, were loaded in a diamond anvil cell and were compressed up to 11 GPa. Up to P=7.1 GPa the only phase present is lead feldspar. In the range 7.1–9.4 GPa sudden changes in the position of the reflections suggest the transformation of lead feldspar to a new phase (probably feldspar-like). The absence of split that would be compatible with triclinic symmetry rules out the monoclinic-triclinic transition, that was reported for the structurally similar strontium feldspar. At P>9.4 GPa some new extra reflections not indexable in the feldspar cell are present as well. During decompression the lead feldspar was the only phase present at P<6 GPa. Peak enlargement was observed with pressure, probably preliminary to amorphization. However almost complete amorphization was observed only after fortuitous shock compression at ∼18 GPa; the crystallinity was recovered at room pressure after decompression. The bulk modulus for lead feldspar was K=71.0(9) and 67.6(1.2) GPa for the two samples, in the range reported for feldspars. The cell parameters show a compression pattern which is similar to that observed in anorthite, with Δa/a 0>Δc/c 0>Δb/b 0; comparison with the high temperature behaviour shows that for lead feldspar the strain tensor with pressure is more isotropic and the deformation along a is less prominent. A turnover in the behaviour of the β angle with pressure suggests a change in the compression behaviour at P∼2 GPa. Rietveld refinement of the Pb coordinates was performed in a series of spectra with pressure ranging from 0.6 to 6.5 GPa. The combined analysis of cell parameters and Pb coordinates with pressure showed that the compression of the structure is mainly achieved by an approach of Pb atoms along a *. Received: 21 July 1998 / Revised, accepted: 13 October 1998  相似文献   

18.
Ca2SiO4 dicalcium silicate has been transformed at high pressure in a diamond-anvil cell (DAC) coupled with a YAG laser heater, in order to study the high-pressure modifications of this compound. Starting material was the olivine form of Ca2SiO4 (γ-polymorph). Several samples have been synthesized at loading pressures of 4.5, 10 and 15 GPa respectively, at room temperature. Other samples have been obtained at pressures ranging between 4.5 and 45 GPa and temperatures estimated to be about 2500 °C. The study of the quenched high pressure and/or high temperature phases has been performed using analytical transmission electron microscopy (ATEM) and X-ray diffraction (XRD). All the polymorphs of Ca2SiO4 usually produced with high temperatures, including α-Ca2SiO4, have been observed in the samples recovered from the high-pressure experiments. The α′-Ca2SiO4 and α-Ca2SiO4 polymorphs have been obtained at ambient conditions for the first time without stabilizing impurities. A new modification of α′-Ca2SiO4 has also been synthesized. Finally, the breakdown at high-pressure and temperature of Ca2SiO4 into CaSiO3 and CaO is reported.  相似文献   

19.
The high-pressure behaviour of chromous orthosilicate, Cr2SiO4, has been studied by means of single-crystal X-ray diffraction and electronic absorption spectroscopy. X-ray diffraction data show that the structure remains orthorhombic to the highest pressure reached of 9.22?GPa. The compressibility of the unit-cell is strongly anisotropic with the c axis approximately six times more compressible than the a and b axes. A third-order Birch-Murnaghan equation of state fitted to the volume-pressure data yields V 0?=?610.10(3)?Å3, K = 94.7(4)?GPa, K′?=?8.32(14). Cr2SiO4 is therefore more compressible than the isostructural Cd analogue, even though its molar volume is smaller. This unusual behaviour can be attributed to the fact that the Cr atom is too small for the six-coordinated site that it occupies, and the site is therefore strongly distorted. Structure refinements indicate that under high pressures the Cr atom remains strongly displaced from the central position of the octahedron. Polarized and unpolarized electronic absorption spectra include a strong absorption band occuring at 18.300 cm?1 for E//c (which is parallel to the shortest Cr-Cr vector in the structure) which has an unusually large half width (5000?cm?1), indicative of electronic interaction between metal centres. Deconvolution of unpolarized high-pressure spectra show that the relative integrated intensity of this component increases linearly from 40% at 1?bar to 60% at 11.2?GPa. Both the structural changes and the absorption spectra at high pressures suggest that pairs of adjacent Cr atoms in chromous orthosilicate form chromium dimers with a weak metal-metal bond, which is consistent with the diamagnetic response found at ambient pressure.  相似文献   

20.
The enthalpies of solution in molten 2PbO · B2O3 of phases synthesized at one atmosphere in the system MgGa2O4-Mg2GeO4 have been measured. A spinel solid solution, which is stable at 1400 °C from the MgGa2O4 end-member to 27 mole percent Mg2GeO4, shows endothermic heats of mixing of up to 10 kJ/mole at the solubility limit. The spinelloid phase, Mg3Ga2GeO8, is energetically less stable than a mixture of terminal spinel solid solutions (0.73 MgGa2O4·0.27 Mg2GeO4(sp)+Mg2GeO4(sp)), by 3.63±3.64 kJ/mole. This indicates that the spinelloid is a high-entropy phase.The volume of the spinel solid solution, MgGa2O4-Mg2GeO4, shows a positive deviation from Vegard's Law. Modeling of the cation distribution in the solid solution indicates that this V is due to a change in the spinel type from inverse towards normal as the Mg2GeO4 content increases.  相似文献   

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