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1.
Single crystals of Li-aegirine LiFe3+Si2O6 were synthesized at 1573?K and 3?GPa, and a polycrystalline sample suitable for neutron diffraction was produced by ceramic sintering at 1223?K. LiFe3+Si2O6 is monoclinic, space group C2/c, a=9.6641(2)?Å, b= 8.6612(3)?Å, c=5.2924(2)?Å, β=110.12(1)° at 300?K as refined from powder neutron data. At 229?K Li-aegirine undergoes a phase transition from C2/c to P21 /c. This is indicated by strong discontinuities in the temperature variation of the lattice parameters, especially for the monoclinic angle β and by the appearance of Bragg reflections (hkl) with h+k≠2n. In the low-temperature form two non-equivalent Si-sites with 〈SiA–O〉=1.622?Å and 〈SiB–O〉=1.624?Å at 100?K are present. The bridging angles of the SiO4 tetrahedra O3–O3–O3 are 192.55(8)° and 160.02(9)° at 100?K in the two independent tetrahedral chains in space group P21 /c, whereas it is 180.83(9)° at 300?K in the high-temperature C2/c phase, i.e. the chains are nearly fully expanded. Upon the phase transition the Li-coordination changes from six to five. At 100?K four Li–O bond lengths lie within 2.072(4)–2.172(3)?Å, the fifth Li–O bond length is 2.356(4)?Å, whereas the Li–O3?A bond lengths amount to 2.796(4)?Å. From 57Fe Mössbauer spectroscopic measurements between 80 and 500?K the structural phase transition is characterized by a small discontinuity of the quadrupole splitting. Temperature-dependent neutron powder diffraction experiments show first occurrence of magnetic reflections at 16.5?K in good agreement with the point of inflection in the temperature-dependent magnetization of LiFe3+Si2O6. Distinct preordering phenomena can be observed up to 35?K. At the magnetic phase transition the unit cell parameters exhibit a pronounced magneto-striction of the lattice. Below T N Li-aegirine shows a collinear antiferromagnetic structure. From our neutron powder diffraction experiments we extract a collinear antiferromagnetic spin arrangement within the a–c plane.  相似文献   

2.
High temperature creep of single crystal gadolinium gallium garnet (GGG) was studied in the temperature range of 1723–1853 K (0.86–0.94 Tm, Tm: melting temperature) and strain rate from 9 · 10?7 s?1 to 2 · 10?5 s?1. The compression tests were made along the 〈100〉 and 〈111〉 orientations. We have performed both constant strain-rate and stress-dip tests. For the 〈100〉 orientation, deformation occurs via the 〈111〉 slip systems. For the 〈111〉 orientation, both the 〈100〉 {010} and the 〈111〉 slip systems can be activated. GGG garnet is very strong under these conditions: σ/μ=(1–3)×10?3 (σ: creep strength, μ: shear modulus). The creep behavior is characterized by a power law with stress exponent n=2.9–3.3 and high activation energies E*=612–743 kJ/mol (E*～45×RTm, at zero stress which decrease with the increase of stress). Stress-dip tests suggest a small internal stress (σi/σ～0.62; σi: internal stress, σ: applied stress) compared to other materials. These results suggest that the high creep strength of GGG is mainly due to difficulty of dislocation glide rather than dislocation climb.  相似文献   

3.
Molecular orbital calculations on a variety of silicate and aluminosilicate molecules have been used to explore the bonding forces that govern tetrahedral bond length variations, r(TO), in framework silicates and aluminosilicates. Not only do the calculations provide insight into the variety of structural types and the substitution limits of one tetrahedral atom for another, but they also provide an understanding of the interrelationships among r(TO) and linkage factors, bond strength sum, coordination number, and angles within and between tetrahedra. A study of these interrelationships for a theoretical data set shows that r(SiO) and r(AlO) are linearly correlated with (1) p o, the bond strength sum to a bridging oxygen, (2) f s(O), the fractional s-character of a bridging oxygen, and (3) f s (T), the fractional s-character of the T atom. In a multiple linear regression analysis of the data, 92% of the variation of r(SiO) and 99% of the variation of r(AlO) can be explained in terms of a linear dependence on p o, f s (O), and f s (T). Analogous regression analyses completed for observed r(Al, SiO) bond length data from a number of silica polymorphs and ordered aluminosilicates account for more than 75% of the bond length variation. The lower percentage of bond length variation explained is ascribed in part to the random and systematic errors in the experimental data which have a negligible effect on the theoretical data. The modeling of more than 75% of the variation of r(Al, SiO) in the framework silicates using the same model used for silicate and aluminosilicate molecules strengthens the viewpoint that the bonding forces that govern the shapes of such molecules are quite similar to the forces that govern the shapes of chemically similar groups in solids. The different regression coefficients calculated for f s (T) indicate that SiO and AlO bond length variations in framework structures should not be treated as a single population in estimating the average Al, Si content of a tetrahedral site.  相似文献   

4.
A revised model for the volume and thermal expansivity of K2O-Na2O-CaO-MgO-Al2O3-SiO2 liquids, which can be applied at crustal magmatic temperatures, has been derived from new low temperature (701–1092 K) density measurements on sixteen supercooled liquids, for which high temperature (1421–1896 K) liquid density data are available. These data were combined with similar measurements previously performed by the present author on eight sodium aluminosilicate samples, for which high temperature density measurements are also available. Compositions (in mol%) range from 37 to 75% SiO2, 0 to 27% Al2O3, 0 to 38% MgO, 0 to 43% CaO, 0 to 33% Na2O and 0 to 29% K2O. The strategy employed for the low temperature density measurements is based on the assumption that the volume of a glass is equal to that of the liquid at the limiting fictive temperature, T f ′. The volume of the glass and liquid at T f ′ was obtained from the glass density at 298 K and the glass thermal expansion coefficient from 298 K to T f ′. The low temperature volume data were combined with the existing high temperature measurements to derive a constant thermal expansivity of each liquid over a wide temperature interval (767–1127 degrees) with a fitted 1 error of 0.5 to 5.7%. Calibration of a linear model equation leads to fitted values of Vˉ i ±1 (cc/mol) at 1373 K for SiO2 (26.86 ± 0.03), Al2O3 (37.42±0.09), MgO (10.71±0.08), CaO (15.41±0.06), Na2O (26.57±0.06), K2O (42.45 ± 0.09), and fitted values of dVˉ i /dT (10−3 cc/mol-K) for MgO (3.27±0.17), CaO (3.74±0.12), Na2O (7.68±0.10) and K2O (12.08±0.20). The results indicate that neither SiO2 nor Al2O3 contribute to the thermal expansivity of the liquids, and that dV/dT liq is independent of temperature between 701 and 1896 K over a wide range of composition. Between 59 and 78% of the thermal expansivity of the experimental liquids is derived from configurational (vs vibrational) contributions. Measured volumes and thermal expansivities can be recovered with this model with a standard deviation of 0.25% and 5.7%, respectively. Received: 2 August 1996 / Accepted: 12 June 1997  相似文献   

5.
Eighteen pyrite and twelve marcasite samples which have different provenances have been investigated to determine the systematics of the influence of mineralogical and geological factors on the 57Fe Mössbauer spectra at 298 K. The following results have been obtained: there is no ambiguity in distinguishing single phase pyrite from single phase marcasite by means of 57Fe Mössbauer spectroscopy at 298 K. At 298 K the average electric quadrupole splitting, 〈ΔEQ〉, and average isomer shift, 〈δ〉, with respect to Fe metal, are 0.6110 ± 0.0030 mm s?1 and 0.313 ± 0.008 mm s?1, respectively, for the 18 pyrites; 〈ΔEQ〉 = 0.5030 ± 0.0070 mm s?1 and 〈δ〉 = 0.2770 ± 0.0020 mm s?1 for the 12 marcasites. At 77 K, ΔEQ is 0.624 mm s?1 for pyrite and 0.508 mm s?1 for marcasite. In distinguishing pyrites from marcasites, spectra obtained at 77 K are not warranted.The Mössbauer parameters of pyrite and marcasite exhibit appreciable variations, which bear no simple relationship to the geological environment in which they occur but appear to be selectively influenced by impurities, especially arsenic, in the pyrite lattice. Quantitative and qualitative determinations of pyrite/marcasite mechanical mixtures are straightforward at 298 K and 77 K but do require least-squares computer fittings and are limited to accuracies ranging from ±5 to ±15 per cent by uncertainties in the parameter values of the pure phases. The methodology and results of this investigation are directly applicable to coals for which the presence and relative amounts of pyrite and marcasite could be of considerable genetic significance.  相似文献   

6.
Oxygen fugacity (fO2) affects melting, metasomatism, speciation of C–O–H fluids and carbon-rich phases in the upper mantle. fO2 of deep off-craton mantle is poorly known because garnet-peridotite xenoliths are rare in alkali basalts. We examine the redox and thermal state of the lithospheric mantle between the Siberian and North China cratons using new Fe3+/ΣFe ratios in garnet and spinel obtained by M?ssbauer spectroscopy, major element data and P–T estimates for 22 peridotite xenoliths as well as published data for 15 xenoliths from Vitim, Russia. Shallow spinel-facies mantle is more oxidized than deep garnet peridotites (average, ?0.1 vs. ?2.5 ΔlogfO2(FMQ)). For intermediate garnet–spinel peridotites, fO2 estimates from spinel-based oxybarometers are 1.5–3.2 ΔlogfO2(FMQ) lower than those from garnet-based oxybarometers. These rocks may be out of phase and chemical inter-mineral equilibrium because the spinel–garnet reaction and concomitant changes in mineral chemistry do not keep up with P–T changes (e.g., lithospheric heating by recent volcanism) due to slow diffusion of trivalent cations and because gar-, gar-spl and spl-facies rocks may coexist on centimeter–meter scale. The spinel-based fO2 estimates may not be correct while garnet-based fO2 values provide conditions before the heating. The T (780–1,100?°C) and fO2 ranges of the Vitim xenoliths overlap those of coarse garnet and spinel cratonic peridotites. However, because of a higher geothermal gradient, the deepest Vitim garnet peridotites are more reduced (by 0.5–2.0 ΔlogfO2(FMQ)) than cratonic garnet peridotites at similar depths, and the “water maximum” conditions (>80?% H2O) in the off-craton mantle exist in a more shallow and narrow depth range (60–85?km) than in cratonic roots (100–170?km). The base of the off-craton lithospheric mantle (≥90?km) at 2.5?GPa and 1,150?°C has fO2 of ?3.0 ?logfO2(FMQ), with dominant CH4 and H2O and minor H2 in the fluid. Melting near the base of off-craton mantle lithosphere may be induced by increasing water share in migrating fluids due to oxidation of methane.  相似文献   

7.
We compare three cluster variation method (CVM) models for order-disorder in omphacite (Al0.5Mg0.5)[Na0.5Ca0.5]Si2O6: the generalized point approximation (GPA), generalized pair approximation (GPaA) and an approximation that is based on two eight-body clusters (2X8A). The same Hamiltonian (set of pairwise interactions) is used for all three approximations. Pair probabilities predicted by the GPA obey the geometric constraints of the crysal structure (the frustration constraint), but the exclusion of short-range order (SRO) leads to overestimates of the configurational internal energy (〈E〉), and the critical temperature for cation order-disorder (T c ). The GPaA violates the frustration constraint, and it therefore fails to predict a P2 1/n → C2/c order-disorder transition and yields values for 〈E〉 and SRO that are inconsistent with the omphacite crystal structure. The 2X8A predicts SRO that is consistent with the frustration constraint, and it yields improved (lower) estimates of 〈E〉 and T c relative to the GPA: 〈E〉2X8A<〈E〉GPA, and T c (2X8A)≈0.741 T c (GPA).  相似文献   

8.
Annealing at 400?T?600 °C (40?P?60 MPa and HM buffer-controlled f(O2)) of tourmalines synthesised at the same T and P with NNO buffer induces an oxidation of Fe2+ into Fe3+ in some Y sites only (Mössbauer Spectroscopy data). Annealing in the same conditions of natural tourmalines is consistent with these results. FTIR spectroscopy shows that oxidation of Fe2+ into Fe3+ is charge-balanced by deprotonation of the external OH(3) groups. To cite this article: Y. Fuchs et al., C. R. Geoscience 334 (2002) 245–249.  相似文献   

9.
Electron transport properties of single crystal and polycrystalline natural mineral galena (PbS) samples from the Trep?a mine, Yugoslavia, were determined using the photoacoustic frequency transmission technique. Their thermal diffusivity (D T≈0.16 × 10?5 m2 s?1), the coefficient of diffusion (D between 0.15×10?2 0.16×10?2 m2 s?1) and lifetime of the excess carrier (τ≈35 μs and the front and rear recombination velocity (s g≈65.5 m s?1 and s b≈66.4 m s?1, respectively), were calculated by comparing the experimental results and the theoretical photoacoustic amplitude and phase signals. The lattice parameter obtained by X-ray work was a?=5.936?Å. The free carrier concentration of these single-crystal samples was measured using the Hall method (N?=?3×1018 cm?3). Measurements of the optical reflectivity of the same samples, as a function of wavelength, in the infrared and far infrared ranges, were performed. In the far infrared range a free electron plasma frequency was observed and numerically analyzed, using the least-squares fitting procedure. The values of optical parameters were calculated and the value of the free carrier concentration obtained by the Hall method was confirmed.  相似文献   

10.
The Gladstone-Dale law (specific refraction) and the Drude law (molecular refraction) for silica polymorphs, at “sodium light” (λ D =0.5893 μm), are derived from simple atomic properties of SiO2 complex (atomic weight, first ionization potential). The considerations are based on the Lorentz electron theory of solids. The characteristic frequency (or eigenfrequency) v 0 of elementary electron oscillators (in energy units, hv) is identified with the band gap E G of a solid; on the other hand, this E G -gap is identified with the single ionization potential \(\tilde U\) of non-free atoms. For \(\tilde U\) =E G =10.2 eV (energy gap of quartz, see Nitsan and Shankland 1976b) the Gladstone-Dale law, or specific refraction, is (n?1)/ρ=0.208 cm3/g, where n and ρ are the refractive index and the density of medium, respectively. According to empirical data, the average value of the specific refraction of pure SiO2 polymorphs (except stishovite-high density phase of silica) is (〈n〉?1)/ρ=0.207±0.001 (〈n〉 denotes the mean refractive index of crystal). For stishovite the Drude law (n 2?1)/ρ=0.542 cm3/g is valid under an assumption that the first ionization potential \(\tilde U\) =E G ≈9 eV; this result is good agreement with the empirical value (〈n〉2?1)/ρ=0.536 cm3/g.  相似文献   

11.
The heat capacity of natural chamosite (XFe=0.889) and clinochlore (XFe=0.116) were measured by differential scanning calorimetry (DSC). The samples were characterised by X-ray diffraction, microprobe analysis and Mössbauer spectroscopy. DSC measurements between 143 and 623?K were made following the procedure of Bosenick et?al. (1996). The fitted data for natural chamosite (CA) in J?mol?1?K?1 give: C p,CA = 1224.3–10.685?×?103?×?T ??0.5???6.4389?× 106?×T ??2?+?8.0279?×?108?×?T ??3 and for the natural clinochlore (CE): C p,CE = 1200.5–10.908?×?103?×T ??0.5?? 5.6941?×?106?×?T ??2?+?7.1166?×?108?×?T ??3. The corrected C p-polynomial for pure end-member chamosite (Fe5Al)[Si3AlO10](OH)8 is C p,CAcor = 1248.3–11.116?× 103?×?T ??0.5???5.1623?×?106?×?T ??2?+?7.1867?×?108×T ??3 and the corrected C p-polynomial for pure end-member clinochlore (Mg5Al)[Si3AlO10](OH)8 is C p,CEcor = 1191.3–10.665?×?103?×?T ??0.5???6.5136?×?106?×?T ??2?+ 7.7206?×?108?×?T ??3. The corrected C p-polynomial for clinochlore is in excellent agreement with that in the internally consistent data sets of Berman (1988) and Holland and Powell (1998). The derived C p-polynomial for chamosite (C p,CAcor) leads to a 4.4% higher heat capacity, at 300?K, compared to that estimated by Holland and Powell (1998) based on a summation method. The corrected C p-polynomial (C p,CAcor) is, however, in excellent agreement with the computed C p-polynomial given by Saccocia and Seyfried (1993), thus supporting the reliability of Berman and Brown's (1985) estimation method of heat capacities.  相似文献   

12.
Extended Hückel molecular orbital theory (EHT) and simple, approximate Self-Consistent-Field MO methods are employed to explain the geometries of nontransition metal bearing minerals and inorganic compounds. The spectra of such minerals and the electronic structure of transition metal oxidic minerals are explained using the Self-Consistent-Field X α MO method. EHT provides an objective algorithm for rationalizing and correlating bond length and angle data for insular and polymerized TO 4 ?n tetrahedral oxyanions where T=Be, B, Al, Si, P, S, Ge, As and Se. Calculated bond overlap populations n(T-O), correlate linearly with the observed T-O bond lengths with shorter bonds tending to involve larger n(T-O) values. Such calculations show that n(T-O) is strongly dependent upon the average of the three O-T-O angles associated with a common bond, larger n(T-O) values involving wider angles. Calculations of n(T-O) as a function of the T-O-T angles in T 2O 7 ?n ions, indicate that the n(T-O) values for the bonds to the bridging oxygen atoms increase nonlinearly with increasing T-O-T angle whereas those the nonbridging oxygens decrease slightly as the angle widens. In agreement with the experimental data, these results predict that shorter T-O bonds should involve wider O-T-O and T-O-T angles. The SCF-X α MO cluster model is then applied to silica and FeO. The calculations yield a satisfactory interpretation of the visible, UV and X-ray emission and X-ray photoelectron spectra of these materials. Theoretical and empirical MO diagrams are constructed and the electronic structures of the materials are discussed.  相似文献   

13.
The occurrence and binding energies of the U6+, U5+ and U4+ bands in the U 4f7/2 peak of 19 uranyl minerals of different composition and structure were measured by XPS. The results suggest that these minerals can be divided into the following four groups: (1) Uranyl-hydroxy-hydrate compounds with no or monovalent interstitial cations; (2) Uranyl-hydroxy-hydrate minerals with divalent interstitial cations; (3) Uranyl-oxysalt minerals with (TOn) groups (T = Si, P, and C) in which all equatorial O-atoms of the uranyl-polyhedra are shared with (TOn) groups; (4) Uranyl-oxysalt minerals with (TOn) groups (T = S and Se), in which some equatorial O-atoms are shared only between uranyl polyhedra. The average binding energies of the U6+and U4+ bands shift to lower values with (1) incorporation of divalent cations and (2) increase in the Lewis basicity of the anion group bonded to U. The first observation is a consequence of an increase in the bond-valence transfer from the interstitial species (cations, H2O) groups to the O-atoms of the uranyl-groups, which results in an electron transfer from O to U6+. The second trend correlates with an increase in the covalency of the UO bonds with increase in Lewis basicity of the anion group, which results in a shift of the electron density from O to U. The presence of U4+ on the surface of uranyl minerals can be detected by the shape of the U 4f7/2 peak, and the occurrence of the U 5f peak and satellite peaks belonging to the U 4f5/2 peak. The presence of U4+ in some of the uranyl minerals and synthetics examined may be related to the conditions during their formation. A charge-balance mechanism is proposed for the incorporation of lower-valence U in the structure of uranyl minerals. Exposure of a Na-substituted metaschoepite crystal in air and to Ultra-High Vacuum results in dehydration of its surface structure associated with a shift of the U6+ bands to higher binding energies. The latter observation indicates a shift in electron density from U to O, which must be related to structural changes inside the upper surface layers of Na-substituted metaschoepite.  相似文献   

14.
Mössbauer spectroscopy has been used widely to characterize the ferric (Fe3+) and ferrous (Fe2+) proportions and coordination of solid materials. To obtain these accurately, the recoilless fraction is indispensible. The recoilless fractions (f) of iron-bearing minerals, including oxides, oxyhydroxides, silicates, carbonates, phosphates and dichalcogenides, and silicate glasses were evaluated from the temperature dependence of their center shifts or absorption area with the Debye model approximation. Generally, the resolved Debye temperature (θD) of ferric iron in minerals, except dichalcogenides, through their center shifts ranging from 400 to 550 K, is significantly larger than ferrous iron ranging from 300 to 400 K, which is consistent with the conclusion from previous work. The resolved f (Fe3+)RT with the center shift model (CSM) ranges from 0.825 to 0.925, which is larger than that obtained for f(Fe2+)RT, which ranges from 0.675 to 0.750. Meanwhile, the θD and f resolved from temperature-dependence of absorption are generally lower than from center shifts, especially for ferric iron. The significant difference between f(Fe3+) and f(Fe2+) indicates the necessity of recoilless fraction correction on the Fe3+/(Fe3++Fe2+) resolved from Mössbauer spectra.  相似文献   

15.
In order to elucidate the solution behavior of carbon and hydrogen in iron-bearing magmatic melts in equilibrium with a metallic iron phase and graphite at oxygen fugacity (fO2) values 2–5 orders of magnitude below the iron-wustite buffer equilibrium, fO2 (IW), experiments were carried out at 4 GPa and 1550°C with melts of FeO-Na2O-SiO2-Al2O3 compositions. Melt reduction in response to an fO2 decrease was accompanied by a decrease in FeO content. The values of fO2 in the experiments were determined on the basis of equilibrium between Fe-C-Si alloy and silicate liquid. Infrared and Raman spectroscopy showed that carbon compounds are formed in FeO-Na2O-SiO2-Al2O3 melts: CH4 molecules, CH3 complexes (Si-O-CH3), and complexes with double C=O bonds. The content of CO2 molecules and carbonate ions (CO 3 2? ) is very low. In addition to carbon-bearing compounds, dissolved hydrogen occurs in melt as H2 and H2O molecules and OH? groups. The spectral characteristics of FeO-Na2O-SiO2-Al2O3 glasses indicate the occurrence of redox reactions in the melt, which are accompanied at decreasing fO2 by a significant decrease in H2O and OH?, a slight decrease in H2, and a significant concomitant increase in CH4 content. The content of species with the double C=O bond increases considerably at decreasing fO2 and reaches a maximum at ΔlogfO2(IW) = ?3. According to the obtained IR spectra, the total water content (OH? + H2O) in the glasses is 1.2–5.8 wt % and decreases with decreasing fO2. The high H2O contents are due largely to oxygen release related to FeO reduction in the melt. The total carbon content at high H2O (4.9–5.8 wt %) is approximately 0.4 wt %. The carbon content in liquid iron alloys depends on silicon content and, probably, oxygen solubility and ranges from 0.3 to 3.65 wt %. Low carbon contents were observed at a significant increase in Si content in liquid iron alloy, which may be as high as ～13 wt % at fO2 values 4–5 orders of magnitude below fO2(IW).  相似文献   

16.
Intrinsic oxygen-fugacity (fO2) measurements were made on five ordinary chondrites, a carbonaceous chondrite, an enstatite chondrite, a pallasite, and a tektite. Results are of the form of linear log fO2 ? 1T plots. Except for the enstatite chondrite, measured results agree well with calculated estimates by others.The tektite produced fO2 values well below the range measured for terrestrial and lunar rocks. The lowpressure atmospheric regime that is reported to follow large terrestrial explosions, coupled with a very high temperature, could produce glass with fO2 in the range measured.The meteorite Salta (pallasite) has low fO2 and lies close to Hvittis (E6). Unlike the other samples, results for Salta do not parallel the iron-wüstite buffer, but are close to the fayalite-quartz-iron buffer in slope.Minor reduction by graphite appears to have taken place during metamorphism of ordinary chondrites. fO2 values of unequilibrated chondrites show large scatter during early heating suggesting that the constituent phases were exposed to a range of fO2 conditions. The samples equilibrated with respect to fO2 in relatively short time on heating. Equilibration with respect to fO2 in ordinary chondrites takes place between grades 3 and 4 of metamorphism. Application of P ? T ? fO2 relations in the system C-CO-CO2 indicates that the ordinary chondrites were metamorphosed at pressures of 3–20 bars, as it appears that they lay on the graphite surface.A steep positive thermal gradient in a meteorite parent body lying at the graphite surface will produce thin reduced exterior, an oxidized near-surface layer, and an interior that is increasingly reduced with depth; a shallow thermal gradient will produce the reverse. A body heated by accretion on the outside will have a reduced exterior and oxidized interior. Meteorites from the same parent body clearly are not required to have similar redox states.  相似文献   

17.
Nuclear magnetic resonance spectroscopic data are presented for the cristobalite polymorphs of AlPO4 and SiO2 from RT to 770 K, through their respective α-β transitions. The nuclear magnetic resonance (NMR) data include chemical shifts for 31P, 27Al, and 29Si, 27Al quadrupole coupling parameters, and 31P and 27Al spin-lattice relaxation rates. Also presented are electron diffraction patterns of β-cristobalite AlPO4 that show diffuse scattering similar to that reported previously for SiO2. For the α-phases of both AlPO4 and SiO2, the chemical shifts decrease approximately linearly with increasing temperature from RT to Tc and discontinuously by -2 to -3 ppm from α to β. This result is consistent with a small, continuous increase in the mean T-O-T angle (〈θ〉) of the α-phases with increasing T and an increase of 〈θ〉 by about 4° across the α-β transition for both cristobalite and its AlPO4 analogue. Based on the 29Si chemical shifts, the mean Si-O-Si angle for β-cristobalite is 152.7±1° near Tc. For AlPO4-cristobalite, the 27Al nuclear quadrupole coupling constant (CQ) decreases approximately linearly from 1.2 MHz at RT to 0.94 MHz near Tc (493±10 K). At the α-β transition the 27Al CQ approaches zero, in agreement with the cubic average structure observed by diffraction. The satellite transitions retain a small frequency distribution above the α-β transition from electric field gradients attributed to defects. The short-range cubic symmetry of the Al-site and non-linear Al-O-P angle support a dynamically disordered model of the β-cristobalite structure. Complete averaging of the 27Al quadrupole coupling in the β-phase indicates that the lifetime of any short-range ordered domains must be shorter than about 1 μs.  相似文献   

18.
The integrated radio luminosities of 311 long-period (P > 0.1 s) and 27 short-period (P < 0.1 s) pulsars have been calculated using a new compilation of radio spectra. The luminosities are in the range 1027 ? 1030 erg/s for 88% of the long-period pulsars and 1028 ? 1031 erg/s for 88% of the short-period pulsars. We find a high correlation between the luminosity L and the estimate L 1 = S 400 d 2 from the catalog of Taylor et al. The factor η for the transformation of the rotational energy of the neutron star into radio emission increases-decreases with increasing period for long-period and short-period pulsars. The mean value of η is ?3.73 for the long-period and ?4.85 for short-period pulsars. No dependence was found between L and the pulsar’s kinematic age t k = |z|/〈v z〉, where |z| and 〈v z〉 = 300 km/s are the pulsars’ height above the plane of the Galaxy and mean velocity. A dependence of L on the rate of rotational energy losses ? was found for both groups of pulsars. It is shown that L ∝ ? 1/3 for the entire sample. The pulsar luminosity function is constructed, and the total number and birth rate of pulsars in the Galaxy are calculated.  相似文献   

19.
A computer model for cubic sodalite structures, general formula M 8(T 12O24)X 2 where M, X and T are the cavity cation and anion and framework cation respectively, has been devised. It has been used to determine the effect of changing cavity cation and anion radii on the cell edge, tilt angle of the tetrahedra and T-O-T angle for the following sodalite frameworks: (Al6Si6O24)6?, (Be6Si6O24)12?, (Al12O24)12?, and (B12O24)12?. After fixing the T-O distance(s), the cavity cation-framework oxygen distance and taking a value of 1.4 Å for the radius of oxygen the model was used to calculate atomic coordinates and interatomic distances and angles for selected aluminosilicate-sodalites. The structure calculated for Na8(Al6Si6O24)Cl2 agrees closely with that determined for natural sodalite (Löns and Schulz, 1967). The model is also applied to the estimation of the effective radii of the tetrahedrally-coordinated cavity anions which can be accommodated in natural and synthetic sodalites: OH? 1.48–1.51, Cl? 1.78, Br? 1.93, I? 2.14–2.17, SO 4 2? 2.37–2.57, MoO 4 2? 2.70 and WO 4 2? 2.79 Å.  相似文献   

20.
This study was conducted in six plots along an elevation gradient in the Qinghai spruce (Picea crassifolia Kom.) forest ecosystem of the Qilian Mountains, northwest China. Soil CO2 efflux over bare soil (R s) and moss covered soil (R s+m) were investigated from June to September in 2010 and 2011 by means of an automated soil CO2 flux system (LI-8100). The results showed that R s ranged from 1.51 to 3.96 (mean 2.64 ± 0.72) μmol m?2 s?1 for 2010, and from 1.41 to 4.09 (mean 2.55 ± 0.70) μmol m?2 s?1 for 2011. The daily change trend of R s resembled that of air temperature (T a), and there was a hysteresis between R s and soil temperature (T s). The seasonal variations of R s at lowlands (i.e., Plot 1, Plot 2 and Plot 3) were driven by soil moisture and temperature (T a and T s), while that at highlands (i.e., Plot 4, Plot 5 and Plot 6) were obviously affected by temperature. There were higher values at Plot 2 and Plot 6, which were caused by the interaction between soil moisture and temperature. In addition, soil CO2 efflux over moss covered soil (R s+m) was 8.83 % less than that over bare soil (R s), indicating that moss was another factor affecting R s. It was concluded that R s had temporal and spatial variations and was mainly controlled by temperature and soil moisture; the main determinants differed at different elevations; moss could reduce R s.  相似文献   

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