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1.
EPR investigations of yellow-green titanites from four localities showed presence of V4+ and Fe3+. They were investigated at X-band and room temperature. For V4+ the principal values of the g matrix and 51V hyperfine splitting and the directions of their principal axes indicate presence of a VO2+ ion substituting for Ti. Due to a high zero-field splitting only approximate values of the fine structure parameters of Fe3+ could be determined. With directions of their principal axes very similar to those of V4+ this ion must also substitute for Ti. Unusually large linewidths for both ions with little variation for samples from different localities are ascribed to the reported domain structure of titanites and accumulation of impurities in the domain boundaries. While for Fe3+ a small variation of the fine structure parameters explains this broadening, for V4+ a distribution of g-factors equal to its total anisotropy must be responsible whereas the V=0 bond length is remarkably constant. Due to preferential incorporation of impurities in the grain boundaries a contribution of dipolar broadening cannot be excluded.  相似文献   

2.
Amelia albite annealed at > 1080 °C for 3200 hrs by Duba and Piwinskii (1974) shows very fine twin lamellae (～1 μm) after the albite law, suggesting that it once underwent transformation into monalbite. A fragment of this specimen was investigated at 27 °C, 300 °C, 550 °C, 800 °C and 930 °C using the high-temperature precession technique. As the temperature increases, the splitting angle of c *-axes (likewise c *-axes) of two twin individuals continues to decrease. The photographs taken at 930 °C show that these two splitting angles have converged to 0o, indicating completion of the transformation into monalbite. The transition point we observe supports the results of MacKenzie (1952) (920±20 °C) and Grundy et al. (1967) (930 °C) rather than those of Sueno et al. (1973) and Prewitt et al. (1974) (> 1080 °C); the discrepancy is most likely due to the differences in the degree of Al-Si disorder of the samples used in the experiments.  相似文献   

3.
The Mössbauer spectra of one chromite at 298 K and one chromite at 298, 200, 170, 140 and 90 K have been analyzed in this study. A Voigt-based quadrupole splitting distribution (QSD) method was used to analyze the spectra. The tetrahedral site Fe2+ and the octahedral site Fe3+ quadrupole splitting distributions (QSDs) were obtained from the Mössbauer spectra of chromites, and the multiple tetrahedral site Fe2+ Gaussian QSD components and the large widths σ Δ of the Gaussian QSD components of the tetrahedral site Fe2+ QSDs for chromites were attributed to next-nearest neighbor effects. In addition, temperature dependences of the isomer shift and the quadrupole splitting were presented and discussed. Comparisons between the Mössbauer parameters for thickness-corrected folded spectra and raw-folded spectra of chromites were made, and the results show that the two sets of the Mössbauer parameters and ratios of ferric to total iron as well as χ2 are very close to each other. This is because of the small absorber thickness of chromites in this study. Comparisons between the Mössbauer parameters of chromites obtained using the Voigt-based QSD method and a Lorentzian doublet method were also made. The results show that there are some differences between the two sets of the Mössbauer parameters and ratios of ferric to total iron, but not significant. However, much larger χ2 were obtained when the Lorentzian doublet method was used to fit the spectra of chromites. This indicates that the Voigt-based QSD method is more adequate to analyze the Mössbauer spectra of chromites from the point of view of statistics.  相似文献   

4.
The Mössbauer spectra of ilvaite CaFe 2 2+ Fe3+[Si2O7/O/OH] and their temperature dependence between 298 K and 455 K can be satisfactorily least-squares fitted by a superposition of the resonances for Fe2+(8d), Fe3+(8d) and Fe2+(4c). The relative areas under the three resonances are nearly equal and vary only weakly with temperature. No additional resonances or line broadenings have to be introduced, if we assume that the hyperfine interactions of Fe2+(8d) and Fe3+(8d) fluctuate between their values due to electron hopping between the iron ions at the 8d sites. Hopping can be assumed to occur homogeneously among nearly equivalent sites. The fluctuation rate is described by an Arrhenius law with a pre-exponent of about 9 × 108 s?1 and an activation energy of 0.11 eV indicating non-adiabatic hopping. In addition to the intersite hopping process, the strong decrease of the quadrupole splitting and the isomer shift of Fe2+(8d) between 298 K and 360 K suggests the occurrence of intrinsic charge delocalization from Fe2+(8d) which does not involve the neighbouring Fe3+(8d) ions.  相似文献   

5.
A single-crystal of topaz was studied by Raman spectroscopy to assign the internal modes of the high-frequency range and to compare with infrared data. All active modes exhibit an important Davydov splitting (150 cm?1) but we have found a small Bethe splitting (14.5 cm?1) consistent with a very regular SiO4 tetrahedron. Because of a high value of v 1 (～920 cm?1) the Raman active modes present a mixed v 1/v 3 character. Finally the substitution of OH for F splits an A g internal mode and lead to some proper modes at 3650 cm?1, 3639 cm?1 and 1165 cm?1.  相似文献   

6.
Despite a large number of studies of iron spin state in silicate perovskite at high pressure and high temperature, there is still disagreement regarding the type and P–T conditions of the transition, and whether Fe2+ or Fe3+ or both iron cations are involved. Recently, our group published results of a Mössbauer spectroscopy study of the iron behaviour in (Mg,Fe)(Si,Al)O3 perovskite at pressures up to 110 GPa (McCammon et al. 2008), where we suggested stabilization of the intermediate spin state for 8- to 12-fold coordinated ferrous iron ([8–12]Fe2+) in silicate perovskite above 30 GPa. In order to explore the behaviour in related systems, we performed a comparative Mössbauer spectroscopic study of silicate perovskite (Fe0.12Mg0.88SiO3) and majorite (with two compositions—Fe0.18Mg0.82SiO3 and Fe0.11Mg0.88SiO3) at pressures up to 81 GPa in the temperature range 296–800 K, which was mainly motivated by the fact that the oxygen environment of ferrous iron in majorite is quite similar to that in silicate perovskite. The [8–12]Fe2+ component, dominating the Mössbauer spectra of majorites, shows high quadrupole splitting (QS) values, about 3.6 mm s?1, in the entire studied P–T region (pressures to 58 GPa and 296–800 K). Decrease of the QS of this component with temperature at constant pressure can be described by the Huggins model with the energy splitting between low-energy e g levels of [8–12]Fe2+ equal to 1,500 (50) cm?1 for Fe0.18Mg0.82SiO3 and to 1,680 (70) cm?1 for Fe0.11Mg0.88SiO3. In contrast, for the silicate perovskite dominating Mössbauer component associated with [8–12]Fe2+ suggests the gradual change of the electronic properties. Namely, an additional spectral component with central shift close to that for high-spin [8–12]Fe2+ and QS about 3.7 mm s?1 appeared at ~35 (2) GPa, and the amount of the component increases with both pressure and temperature. The temperature dependence of QS of the component cannot be described in the framework of the Huggins model. Observed differences in the high-pressure high-temperature behaviour of [8–12]Fe2+ in the silicate perovskite and majorite phases provide additional arguments in favour of the gradual high-spin—intermediate-spin crossover in lower mantle perovskite, previously reported by McCammon et al. (2008) and Lin et al. (2008).  相似文献   

7.
Fe57 Mössbauer spectra were measured on compositions of the series Fe1?x/3Ta1+x/3O4, 0≤x≤1. The spectra are characterized by mixed valencies of Fe2+ and Fe3+ ions for 0<x<1. Starting from x=0 with rutile structure, a trirutile structure forms towards x=1. Quadrupole splitting QS of Fe3+ is QS(Fe3+)≈0.55 mm/s and isomer shift IS is IS(Fe3+)≈0.40 mm/s (referred to Fe); both quantities exhibit minor variations along the series. The Fe2+ subspectra for x>0.5 were fitted using one symmetrical doublet; however, for x<0.5 two symmetrical doublets were necessary to describe these patterns. QS(Fe2+)=2.0–3.2 mm/s and IS(Fe2+)=0.90–1.15 mm/s for all compositions. In the case x<0.5, marked temperature dependent QS values appear to exist. This feature may be related to short range order effects and possibly also in part to intervalence electron transfer betwee Fe2+ and Fe3+ ions.  相似文献   

8.
Mössbauer spectra (MS) of anapaite (Ca2 Fe2+(PO4)2?·?4H2O) and of a sample after being immersed in a 4% H2O2 solution at room temperature (RT) over 12 days (hereafter an4ox) were collected at temperatures in the range 4.2 to 420?K and 11 to 300?K respectively. All MS consist of symmetrical doublets, hence magnetic ordering was not observed. The temperature dependencies of the Fe2+ centre shifts of anapaite and an4ox were analysed with the Debye model for the lattice vibrations. The characteristic Mössbauer temperatures were found as 370?K?±?25?K and 340?K?±?25?K, and the intrinsic isomer shifts as 1.427?±?0.005?mm/s and 1.418?±?0.005?mm/s respectively. From the external-field (60?kOe) MS recorded at 4.2 and 189?K for the non-treated sample, the principal component V zz of the electric field gradient (EFG) is determined to be positive and the asymmetry parameter η?≈?0.2 and 0.4 respectively. The temperature variations of the quadrupole splittings, ΔE Q(T), cannot be interpreted on the basis of the thermal population of the 5 D electronic levels resulting from the tetragonal compression of the O6 co-ordination. The low-temperature linear behaviour of ΔE Q(T) is attributed to a strong orbit-lattice coupling. A field of 60 kOe applied to anapaite at 4.2?K produces magnetic hyperfine splitting with effective hyperfine fields of ?136, ?254 and ?171?kOe along the principal axes Ox, Oy and Oz of the EFG tensor respectively. Additional oxidation treatments in solutions with various H2O2 concentrations up to 20% and subsequent Mössbauer experiments at room temperature, have revealed that the anapaite structure is not sensitive to oxidation since eventually only a small amount of Fe2+ (～6.5%) is converted into Fe3+.  相似文献   

9.
It is shown that under ambient atmospheric conditions heating causes the crystal structure of βFeOOH (synthetic akaganeite) to degenerate gradually into a quasi amorphous intermediate state, before the final phase transformation to αFe2O3 (hematite) takes place. Using X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Mössbauer Spectroscopy, this amorphization process is monitored and the structural, morphological and magnetic features of the intermediate phase as a function of the isochronal heating temperature are discussed: the crystallites develop macropores on their surface, the adsorption capacity raises up to 10 percent of the initial mass, a third type of Fe3+ coordination, having an extremely large quadrupole splitting, is created and the Néel temperature, after an initial decrease, exhibits a sharp increase at higher heating temperatures. The magnetic behaviour of this intermediate phase at low temperatures and in high external fields suggests this antiferromagnet undergoes magnetic phase transitions (metamagnetism and spin flop) at unusually low critical fields.  相似文献   

10.
The Mössbauer spectra of several blue beryls have been obtained in the temperature range of 4.2–500 K. A common feature observed in all room-temperature spectra is the presence of an asymmetric Fe2+ doublet (ΔE Q ?～?2.7?mm?s?1, δ?～?1.1?mm?s?1), with a very broad low-velocity peak. This asymmetry seems to be related to a relaxation process involving ferrous ions and water molecules in the structural channels, as suggested by Price et?al. (1976). Surprisingly, the spectrum at 500?K also shows a broad, but symmetrical, doublet, with a clear splitting of the lines indicating the presence of at least two Fe2+ components. The room-temperature spectrum obtained after the 500?K run shows the same features as prior to the heating. At 4.2?K the spectrum of a deep blue beryl was well fitted with four symmetrical doublets, one of which could be related to Fe2+ in the structural channels. Ferrous ion was also found to occupy the octahedral and tetrahedral sites, whereas ferric ion is most probably located in the octahedral site. A meaningful fit of the room-temperature spectrum, as well as an explanation for the temperature dependence of the Mössbauer spectra, are discussed. Finally, it is believed that the color in beryl will be dictated by the relative proportions of Fe3+ in the octahedral sites and of Fe2+ in the channels.  相似文献   

11.
Troilite close to FeS, with 0.17 weight percent Cr as main impurity, was obtained from the Agpalilik meteorite. Powder Mössbauer spectroscopy was made in the temperature range 77–645 K. The full Hamiltonian was applied in the fittings. Assuming the asymmetry parameter η to be constant on passing from the high-temperature NiAs-type structure to the medium-temperature MnP-type structure yields a quadrupole splitting (dq=0.5e2 qQ(1+(η2)/3)1/2) value of ?0.25(2) mm/s for these phases. In low-temperature troilite |dq|=0.85 mm/s at room temperature. The combinations of (η, θ, φ) in troilite giving identical spectra range from (0, 49°, -) to (1, 45°, 50°) for negative V zz or from (0.3, 57°, 78°) to (1, 58°, 54°) for positive V zz . Assuming a negative V zz and B‖c gives a θ value in agreement with the shortest Fe-S join being the V zz orientation. The magnetic spin flip of 90° is proposed to occur in the MnP-phase only. The MnP phase-troilite transition occurs at lower temperatures and is more sluggish than in pure FeS.  相似文献   

12.
Magnetization, susceptibility and Mössbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of theT 2g triplet of 1,120K. Predominant exchange interactions in the iron-rich samples are ferromagnetic withJ=1.2 K, as for other trioctahedral ferrous minerals. A peak in the susceptibility of thuringite occurs atT m=5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the Mössbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4 Å?1). A factor favouring this random ferromagnetic ground state over the planar antiferromagnetic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illustrate how long range ferromagnetic order may be destroyed by vortices forming around the Fe3+-Fe3+ pairs.  相似文献   

13.
57Fe Mössbauer spectra are presented for synthetic cation-deficient Fe2TiO4 and FeCr2O4 spinel particles (<1μm) at various temperatures. The spectra of ferrimagnetic cation-deficient Fe2TiO4 show characteristic features due to relaxation because of superparamagnetism and spin relaxation in the temperature range 5–294 K. At 5 K and 78 K, a superposition of at least two sextets is observed which appear to arise from Fe3+ onA-sites (Fe A 3+ andB-sites (Fe B 3+ ) of the spinal lattice with magnetic hyperfine fields at 5 K ofB hf ((Fe B 3+ )≈47.5 T andB hf (Fe B 3+ )≈51.0 T, respectively. Cation-deficient FeCr2O4 particles reveal at 78 K a fieldB hf (Fe3+)≈46.9 T and exhibit relaxation spectra as a consequence of superparamagnetism in the temperature range 80 K - ～300 K. At 294 K, quadrupole splitting Δ(Fe A 3+ )=0.92 mm/s and isomer shift δ(Fe A 3+ )=0.29 mm/s (relative to metallic Fe) are measured. For both compounds the magnetic hyperfine fieldsB hf are discussed in terms of supertransferred hyperfine fields involving vacancies and in the case of cation-deficient Fe2TiO4 also diamagnetic Ti4+ neighbours of the Fe ions.  相似文献   

14.
We have measured in situ Mössbauer transmission spectra of iron silicate spinel (γ-Fe2SiO4) in a diamond anvil cell at room temperature and pressures up to 16 GPa. The observed spectra show a doublet characteristic of the paramagnetic state. The isomer shift and quadrupole splitting at atmospheric pressure are 1.10 and 2.63 mm/s, respectively, which are smaller than those of fayalite (α-Fe2SiO4). Both the isomer shift and quadrupole splitting decrease linearly with pressure with slope of ?0.003(1) and ?0.020(1) mm/sec · GPa, respectively. This simple linear trend suggests that no electronic or polymorphic transitions occur under 16 GPa except for those due to the small and continuous changes of volume and local symmetry under pressure. On the basis of a crystalline field calculation, the negative pressure derivative of the quadrupole splitting is associated with a trend towards an ideal cubic symmetry of the oxygen sublattice.  相似文献   

15.
Room temperature X-irradiation of some natural beryls produced several new absorption lines in the electron paramagnetic resonance (EPR) spectrum, a known series of optical absorption lines in the 500–700 nm range, and a shift of the absorption edge to lower energies. Several of the new EPR lines and part of the irradiation-induced shift of the absorption edge disappeared after a few days at room temperature, and were not examined in detail. However, three of the paramagnetic centres responsible for the new EPR lines were stable at room temperature and two of these have previously been identified as atomic hydrogen and the methyl radical, CH3. These species were stable to ～150 and ～450°C respectively. The third stable species, hitherto unreported, showed a single-line EPR spectrum of axial symmetry, with g∥=2.0051 and g⊥=2.0152. This spectrum was found to be intensity-correlated with the series of optical bands in the 500–700 nm range, after thermal bleaching at 175°C. The EPR and optical spectra are therefore assigned to the same species. It is argued that this species is the CO 3 ? molecular ion, located in the widest part of the structural channel and aligned with the plane of the molecule perpendicular to the c axis. The EPR spectrum is consistent with a 2 A′2 ground state of a CO 3 ? molecule with trigonal symmetry, and this requires that the optical transition has a 2 A′2 → 2 E′ character. Most of the features in the optical spectrum can be assigned to coupling of a totally symmetric mode of frequency ～1020 cm?1 onto a zero-phonon line at 14,490 cm?1 and a second weaker line at 16,020 cm?1. However, both of these two fundamental lines are structured, and the two components show strong temperature-dependent derivative-shaped magnetic circular dichroism (MCD). Furthermore, the overall sign of the MCD for the line at 16,020 cm?1 is opposite to that at 14,490 cm?1. The separation (～120 cm?1) of the two components of the 14,490 cm?1 line is much larger than that expected from spin-orbit interaction, and the origin of this splitting is not yet understood.  相似文献   

16.
A natural sample of ankerite has been characterized by chemical analysis, X-ray diffraction and differential thermal analysis. The composition was found to be (Ca1.11Mg0.50Fe0.33Mn0.09) [Co3]1.99.57Fe Mössbauer effect measurements were performed at temperatures between 4.2 and 400 K. At low temperatures (T < 25 K) relaxation effects are clearly dominant. The temperature dependence of the center shift is remarkably well reproduced by a model based on the Debije approximation of the lattice vibrations. In contrast, the temperature dependence of the quadrupole splitting cannot be described by any reasonable crystal field model. It is argued that an orbit-lattice coupling might explain the observed quadrupole splittings. A spectrum recorded in an applied field of 6 T reveals a positive electric field gradient from which an orbital doublet ground state is concluded. Highly anisotropic field reductions are derived but cannot be quantitatively explained due to a lack of knowledge concerning the magnetic structure of ankerite. The line widths decrease significantly with increasing temperature which is only partly due to the decreasing Mössbauer fraction.  相似文献   

17.
Cerussite (PbCO3) has been investigated by high-pressure and high-temperature Raman spectroscopy up to pressures of 17.2 GPa and temperatures of 723 K. Two pressure induced phase transitions were observed at about 8.0(2) and 16.0(2) GPa, respectively. The post-aragonite transition (PbCO3-II) at 8.0(2) GPa is accompanied by softening of the v 2-out-of-plane mode of the CO 3 2? group and disappearance of the B1g (v 4-in-plane band of the CO 3 2? group) mode. Stronger shifts of the carbonate group modes after the phase transition suggest that the new structure is more compressible. The formation of a second high-pressure polymorph begins at about 10 GPa. It is accompanied by the occurrence of three new bands at different pressures and splitting of the v 1-symmetric C–O stretching mode of the CO 3 2? group. The transitions are reversible on pressure release. A semi-quantitative phase diagram for PbCO3 as a function of pressure and temperature is proposed.  相似文献   

18.
Hyperfine parameters of 57Fe in anthophyllites (Mg2+, Fe2+)7 Si8O22(OH, F)2 mainly depend on the amount of Al present in the structure. The quadrupole splitting of the doublet due to Fe2+ in M1, M2 and M3 decreases systematically with the Al content, whereas that of the doublet due to Fe2+ in M4 and the half-width of the combined M1, M2, M3 doublet increases. Structurally these variations suggest that, with the incorporation of Al (miscibility towards gedrite), the distortion of the M4 polyhedron decreases, whereas the M1, M2 and M3 polyhedra become more distorted and dissimilar.  相似文献   

19.
Using the superposition model in conjunction with our crystal field analysis package recently developed for 3d ions doped at arbitrary low symmetry sites in crystals, the energy levels and statevectors have been predicted within the whole 3d 3 configuration of Cr3+ at the four possible triclinic sites in kyanite (Al2O3∶SiO2). The values of the ground state zero-field splitting for each of the four Al sites are evaluated. The splittings of the lower excited state 2 E as well as the admixture of 4 T 2 state into 2 E have also been determined. The predicted results are compared with the available experimental data on the four possible, but so far not uniquely identified, substitutional Cr3+ sites in kyanite thus enabling correlation of the spectroscopic properties and substitutional sites.  相似文献   

20.
γ LiAlO2 doped with Fe3+ in the tetrahedral site has been examined by extended X-ray absorption fine structure (EXAFS) analysis, and Mössbauer and optical spectroscopy. The isomer shift (IS) is ?0.026 mm/s (Fe-Pd); the quadrupole splitting (QS) is 0.62 mm/s. Anisotropic optical absorption is prominent at ～391, 452, and 463 nm. The K-edge absorption spectrum shows a prominent absorption near 7,113 ev typical of tetrahedrally coordinated Fe3+.  相似文献   

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