Quantum mechanical calculations of the vibrational spectra of quartz- and rutile-type GeO2

Heat-treatment and stepwise cooling of as-delivered, water-containing quartz-type GeO₂ powder resulted in transformation into a water-free form. A rutile-type modification could be prepared by impregnation of the quartz-type phase with RbOH solutions, drying and annealing. Raman- and FTIR-absorption spectra of quartz- and rutile-type GeO₂ were measured and compared to quantum-mechanical ab initio calculations based on a hybrid functional using the Perdew–Burke–Ernzerhof correlation functional with 16.7% Hartree–Fock exchange density functional. Maximum and mean deviations between measured spectral bands and assigned vibrational modes are 14 and ±8 cm⁻¹ for the quartz-type and 30 and ±13 cm⁻¹ for the rutile-type polymorphic form. Water is incorporated into GeO₄ entities of quartz-type GeO₂; a water-free and structurally stable form can be prepared by a heating up to 1,425 K, tempering at 1,323 K and stepwise cooling. Spectral bands not explained by the calculations suggest defects and distortions in both quartz- and rutile-type structures, in case of the quartz-type one by incomplete transformation into an ideal structure after removing the water, whereas the rutile-type modification most probably incorporates Rb during its synthesis.     

High-pressure Raman spectroscopic study of Mn2GeO4, Ca2GeO4, Ca2SiO4, and CaMgGeO4 olivines

We present a Raman spectroscopic study of the structural modifications of several olivines at high pressures and ambient temperature. At high pressures, the following modifications in the Raman spectra are observed: 1)?in Mn₂GeO₄, between 6.7 and 8.6?GPa the appearance of weak bands at 560 and 860?cm^?1; between 10.6 and 23?GPa, the progressive replacement of the olivine spectrum by the spectrum of a crystalline high pressure phase; upon decompression, the inverse sequence of transformations is observed with some hysteresis in the transformation pressures; this sequence may be interpreted as the progressive transformation of the olivine to a spinelloid where Ge tetrahedra are polymerized, and then to a partially inverse spinel; 2)?in Ca₂SiO₄, the olivine transforms to larnite between 1.9 and 2.1?GPa; larnite is observed up to the maximum pressure of 24?GPa and it can partially back-transform to olivine during decompression; 3)?in Ca₂GeO₄, the olivine transforms to a new structure between 6.8 and 8?GPa; the vibrational frequencies of the new phase suggest that the phase transition involves an increase of the Ca coordination number and that Ge tetrahedra are isolated; this high pressure phase is observed up to the maximum pressure of 11?GPa; during decompression, it transforms to a disordered phase below 5?GPa; 4)?in CaMgGeO₄, no significant modification of the olivine spectrum is observed up to 15?GPa; between 16 and 26?GPa, broadening of some peaks and the appearance of a weak broad feature at 700–900?cm^?1 suggests a progressive amorphization of the structure; near 27?GPa, amorphization is complete and an amorphous phase is quenched down to ambient pressure; this unique behaviour is interpreted as the result of the incompatibilities in the high pressure behaviour of the Ca and Mg sublattices in the olivine structure.     

Pressure-induced structural modifications in Mg2GeO4-olivine: A Raman spectroscopic study

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.     

Temperature dependence of the polarized electronic absorption spectra of olivines. Part I – fayalite

 Polarized electronic absorption spectra of orthorhombic fayalite, Fe₂SiO₄, [E || a(|| Z),E || b(|| X), E || c(|| Y)], space group Pbnm, have been studied in the temperature range 293 ≤T/K ≤ 1273. The spectra were analysed into component bands originating from spin-allowed dd transitions of iron(II) at the different sites, M1 and M2, in the structure. The assignments of bands, made on the basis of the polarization dependence of the spectra and considerations of transition energies, were confirmed by the analysis of the temperature-dependent spectra. The temperature dependencies of integral intensities, half band widths and energy positions of absorptions bands caused by Fe²⁺ on the different octahedral sites, M1 and M2, were evaluated for the individual transitions. Independent of the site symmetry, absorption bands shift to lower energies and half band widths increase on rising temperature. The temperature dependence of band intensities depends on site symmetry. The integral intensities are found to increase with temperature for the transition metal ion on a centrosymmetric site, or remain constant when the site is missing an inversion centre. This is consistent with the general conclusion of Taran et al. (1994). Received: 11 October 2001 / Accepted: 17 January 2002     

Raman spectra of silicate garnets

The single-crystal polarized Raman spectra of four natural silicate garnets with compositions close to end-members almandine, grossular, andradite, and uvarovite, and two synthetic end-members spessartine and pyrope, were measured, along with the powder spectra of synthetic pyrope-grossular and almandine-spessartine solid solutions. Mode assignments were made based on a comparison of the different end-member garnet spectra and, in the case of pyrope, based on measurements made on additional crystals synthesized with ²⁶Mg. A general order of mode frequencies, i.e. R(SiO₄)>T(metal cation)>T(SiO₄), is observed, which should also hold for most orthosilicates. The main factors controlling the changes in mode frequencies as a function of composition are intracrystalline pressure (i.e. oxygen-oxygen repulsion) for the internal SiO₄-vibrational modes and kinematic coupling of vibrations for the external modes. Low frequency vibrations of the X-site cations reflect their weak bonding and dynamic disorder in the large dodecahedral site, especially in the case of pyrope. Two mode behavior is observed for X-site cation vibrations along the pyrope-grossular binary, but not along the almandine-spessartine join. Received: 3 December 1996 / Revised, accepted: 13 April 1997     

Phonon spectra of 2∶1 mullite in infrared and Raman experiments

Single crystal Raman and infrared phonon spectra of mullite were measured. The number of modes, their frequencies and line profiles are analysed. The local structural changes associated with the formation of oxygen vacancies contribute to the appearence of modes in both the Raman and infrared spectra of mullite. The number of modes is increased compared with the degrees of freedom predicted by group theory for the mullite structure without oxygen vacancies.     

Raman spectra of high-pressure polymorphs of SiO2 at various temperatures

Raman spectra of the two high-pressure polymorphs of SiO₂ (coesite and stishovite) were investigated in the temperature range 105–875 K at atmospheric pressure. Coesite remained intact after the highest temperature run, but stishovite became amorphous at temperatures above about 842～872 K. Most Raman modes exhibit a negative frequency shift with temperature for these polymorphs, but positive trends were also observed for some modes. Except for some weak modes, nonlinear temperature variation were established for these polymorphs within the experimental uncertainty and temperature range spanned. The slopes of the variation (δv_i/δT)_P for these polymorphs were compared with the published values. When compared with quartz and stishovite, the four-membered rings of SiO₄-tetrahedra in coesite exhibit very little change with both temperature and pressure. It is also suggested that temperature and pressure should have opposite effects on the Raman shift of each vibrational mode.     

Polarized Raman spectra of beryl and bazzite

The polarized Raman spectra of four different beryl crystals were studied at room temperature in the range from 30 to 4000 cm^-1. The spectra show significant differences between the samples studied, and corrections are proposed for the reference Raman spectra of beryl previously reported by Adams and Gardner (1974). Type II water is observed in two crystals; the corresponding symmetric Raman stretching band at 3595 cm^-1 is extremely strong for an impurity (about 20% of the strongest beryl lattice mode). Another, sharper, band of similar intensity at 3605 cm^-1 could possibly originate from a hydroxyl stretching mode. Additional weaker bands are observed around 1600 cm^-1 and 3600–3750 cm^-1. The first polarized Raman spectra of bazzite are presented and discussed.     

Comparison of the raman microprobe spectra of (Mg,Fe)2SiO4 and Mg2GeO4 with olivine and spinel structures

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.     

Temperature dependence on the electron paramagnetic resonance spectra of natural jasper from Taroko Gorge (Taiwan)

Structural properties of natural jasper from Taroko Gorge (Taiwan) have been investigated by means of powder X-ray diffraction, electron paramagnetic resonance (EPR) and Fourier transform infrared spectroscopic techniques. The EPR spectrum at room temperature exhibits a sharp resonance signal at g = 2.007 and two more resonance signals centered at g ≈ 4.3 and 14.0. The resonance signal at g = 2.007 has been attributed to the E′ center and is related to a natural radiation-induced paramagnetic defect. Two more resonance signals centered at g ≈ 4.3 and 14.0 are characteristic of Fe³⁺ ions. The EPR spectra recorded at room temperature of jasper samples, heat-treated at temperatures ranging from 473 to 1,473 K exhibit marked temperature dependence. The resonance signal corresponding to E′ center disappears at elevated temperatures. A broad, intense resonance signal centered at g ≈ 2.0 appears at elevated temperatures. This resonance signal is a characteristic of Fe³⁺ ions, which are present as hematite in the jasper sample. The intensity of the resonance signal becomes dominant at elevated temperatures at ≥873 K, masking g ≈ 4.3 and g ≈ 14.0 resonance signals. The EPR spectra of jasper heat-treated at 673 K have been recorded at temperatures between 123 and 296 K. The population of spin levels (N) has been calculated for the broad g ≈ 2.0 resonance signal. It is found that N decreases with decreasing temperature. The linewidth (ΔH) of g ≈ 2.0 resonance signal of the heat-treated jasper is found to increase with decreasing temperature. This has been attributed to spin–spin interaction of the Fe³⁺ ions present in the form of hematite in the studied jasper sample.     

Raman spectra of gypsum under pressure

Raman sprectra of a gypsum crystal were made at pressures between 0.001 and 7 kbar using He gas as the pressure medium. \(\frac{{{\text{d}}v}}{{dP}}\) values for bands in the range 3,600–100 cm^?1 were obtained. Comparison of results with \(\frac{{{\text{d}}v}}{{{\text{d}}T}}\) from the literature for temperatures of 77 and 300° K. shows that the internal modes of the SO₄ units are more sensitive to pressure than to temperature. The effect is small. Coupled H₂O-SO₄ translational modes are greatly affected by both pressure and temperature while coupled Ca-SO₄ mode are less so. It was found that stretching vibrations of water molecules were affected differently under pressure. The band at 3,500 cm^?1 is more greatly displaced by pressure \(\left( {\frac{{{\text{d}}v}}{{{\text{d}}P}} = {\text{2}}{\text{.11cm}}^{{\text{ - 1}}} /{\text{kbar}}} \right)\) than the band at 3,400 cm^?1 \(\left( {\frac{{{\text{d}}v}}{{{\text{d}}P}} \simeq {\text{2}}{\text{.11cm}}^{{\text{ - 1}}} /{\text{kbar}}} \right)\) . Assuming two different hydrogen bond intensities for the water molecules, one can attribute this difference in behavior of stretching modes to and increase in hydrogen bonding of one of the hydrogens which is exterior to the double H₂O planes in the gypsum structure. The great variety of pressure derivatives for the different types of vibrational modes observed indicates that each molecular unit readjusts internally to pressure induced volume changes and the some of the chemical bonds between the units are significantly affected.     

Raman spectra of analcime under pressure

Raman spectra of natural analcime have been recorded at atmospheric pressure and up to 9.4 kbar. The basic Si, Al-O network vibrations are little affected by pressure even though significant volume changes and a minor phase transition take place. However, the 3,557 cm^?1 OH-stretch mode is modified in that band splitting takes place indicating at least two O-OH hydrogen bond distances. Thus there are at least three sites of hydrogen bonding in analcime. The bonded water (H₂O) in analcime appears to remain in the mineral at high pressure. The bulk volume change, determined previously by cell dimension measurements, can be traced to reduction of the size of the “voids” in the structure. This is deduced from the fact that Si-Al-O vibrations are little affected by pressure but O-H vibrations of water molecules found in the voids are strongly pressure-dependent.     

Anharmonicity and high-temperature heat capacity of crystals: the examples of Ca2GeO4, Mg2GeO4 and CaMgGeO4 olivines

Drop-calorimetry determinations of the isobaric heat capacity (C_P) 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 (C_V) 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 a_i=(? ln v _i/?T)_V in vibrational modelling of C_V. 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.     

Raman spectra from the Al2SiO5 polymorphs at high pressures and room temperature

Raman spectra of the three Al₂SiO₅ polymorphs; andalusite, sillimanite and kyanite were recorded as a function of pressure at room temperature. All the Raman active bands which could be observed from the high-pressure cell showed a linear pressure dependence for each of the three Al₂SiO₅ polymorphs and no phase changes were observed over the pressure ranges used in this study. In andalusite and to a lesser extent in sillimanite, vibrations which could be correlated with internal motions of the SiO₄ tetrahedra were generally well separated from the lattice modes and showed a greater pressure dependence than that observed for other modes. The distinct pressure dependence of the internal SiO₄ modes is less evident in kyanite, probably due to the lack of continuous tetrahedral chains and the fact that the rigid SiO₄ tetrahedra now form an integral part of the structural network. At ambient pressure, kyanite also exhibits two fluorescence bands at 705 and 706.2 nm which are due to small amounts of Cr³⁺ in the kyanite crystals. These fluorescence bands showed a non-linear frequency shift as pressure was increased.     

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%.     

Raman spectra of hydrous γ -Mg2SiO4 at various pressures and temperatures

 One well-defined OH Raman band at 3651 ± 1 cm⁻¹ and one weak feature near 3700 ± 5 cm⁻¹ are recognized for the hydrous γ-phase of Mg₂SiO₄. Like the hydrous β-phase, the H₂O content in the γ-phase shifts most of the corresponding silicate modes towards lower frequencies. Variations in Raman spectra of the hydrous γ-phase were investigated up to about 200 kbar at room temperature and in the range 81–873 K at atmospheric pressure. Unlike the anhydrous γ-phase, which remains intact up to at least 873 K, the hydrous γ-phase sometimes converts to a defective forsterite structure above 800 K. Although the hydrous γ-phase remains intact up to at least 800 K, Raman signals of the OH bands disappear completely above 423 K. The Raman frequency of the well-defined OH band decreases linearly with increasing temperature between 81 and 423 K. In the region of the silicate vibrations, the Raman frequencies of the two most intense bands increase nonlinearly with increasing pressure, and decrease with increasing temperature. The frequencies for all other weak bands, however, decreased linearly with increasing temperature. The latter most likely reflects the larger scatter of the data for the weak bands. Received: 27 April 2001 / Accepted: 12 September 2001     

Raman spectroscopy of ferritungstite: Experimental and model spectra

Raman spectra of ferritungstite are interpreted on the basis of model calculations. The presence of two broad vibrational bands in the spectral range 600–1000 cm^?1 is explained through the combination of two factors. Mainly, two principle bands in the spectral ranges 600–750 and 850–1000 cm^?1 are characteristic of the pyrochlore-type structure attained by ferritungstite and correspond to symmetric and asymmetric stretching of the W-O bonds, respectively. Also, broadening of the Raman lines results from structural disorder caused by the presence of iron atoms.     

A note on the Raman spectra of water-bearing albite glasses

The Raman spectra of albite glasses with 4.5 and 6.6 weight percent water have been obtained, and are compared with that of a dry sample. The hydrous glasses show bands near 3600 cm^?1 due to O-H stretching, and a previously unreported weak band near 1600 cm^?1 due to bending of molecular H₂O. Other weak spectral features are discussed, and the effect of dissolved water on the aluminosilicate framework vibrations is considered.     

原位低温拉曼光谱技术在人工合成CaCl_2-H_2O和MgCl_2-H_2O体系流体包裹体分析中的应用Ⅰ:低温拉曼光谱研究

利用原位低温拉曼光谱分析技术,对在设定的实验条件下合成的CaCl2-H2O体系和MgCl2-H2O体系的流体包裹体进行分析研究,结果表明低温下CaCl2水溶液和MgCl2水溶液形成的盐水合物具有各自不同的特征拉曼光谱,通过测定特征光谱,能够简易直接的鉴定这些物质,进而确定盐水体系包裹体的成分.因此,原位低温拉曼光谱技术,能够有效地测定Ca2+和Mg2+这两种盐水体系包裹体流体中常见,但显微测温过程难以观察和判别的二价阳离子.实践表明,原位低温拉曼光谱分析技术是对传统的流体包裹体显微测温技术的一个十分有效的补充,在包裹体研究领域会得到越来越广泛的应用.     

A vibrational study of phase transitions among the GeO2 polymorphs

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.