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1.
The electronic and magnetic structure of the octahydrophosphate vivianite, Fe3(PO4)2·8H2O, has been investigated by cluster molecular orbital calculations in local spin density approximation. Optical and Mössbauer spectra are well reproduced by the calculations, and the differences between the two iron sites can be correlated with differences in the geometrical structure of the first coordination sphere. The spin structure within the crystallographic ac plane is derived and explained on the basis of different superexchange pathways via edges of the phosphate tetrahedra. The calculations demonstrate that quite large clusters (up to 118 atoms) are necessary to arrive at reliable results. 相似文献
2.
Michael Grodzicki G. Redhammer M. Reissner W. Steiner G. Amthauer 《Physics and Chemistry of Minerals》2010,37(1):11-23
The electronic and magnetic structure of the chain silicate hedenbergite (CaFe2+Si2O6) has been investigated by a number of experimental methods (neutron diffraction, Mössbauer spectroscopy, low temperature magnetic measurements), as well as by electronic structure calculations for clusters of different size in the local spin density approximation. The calculated size-converged spectroscopic data (d-d excitation energies, hyperfine parameters) are in quantitative agreement with the respective experimental values. The calculated magnetic coupling constants are about +25 cm?1 and ?4 cm?1 for intra-chain and inter-chain coupling, respectively. The latter value shows that weak superexchange via edges of silicon tetrahedra is well reproduced by the calculations, and it is in qualitative agreement with an observed metamagnetic transition at 4.2 K in an external magnetic field with an onset around 4 T but saturation is not achieved in fields up to 14.5 T. The large ferromagnetic intra-chain coupling is attributed to a nearly degenerate ground state. The ratio between the two magnetic coupling constants agrees with earlier estimates on similar compounds. Finally, it is demonstrated how the detailed discussion of the various exchange pathways contributes to an improved understanding of the connection between magnetic properties and the geometrical structure. 相似文献
3.
Sven-Ulf Weber Michael Grodzicki Werner Lottermoser Günther J. Redhammer Gerold Tippelt Johann Ponahlo Georg Amthauer 《Physics and Chemistry of Minerals》2007,34(7):507-515
Natural alexandrite Al2BeO4:Cr from Malyshevo near Terem Tschanka, Sverdlovsk, Ural, Russia, has been characterized by 57Fe Mössbauer spectroscopy, electron microprobe, X-ray single-crystal diffractometry and by electronic structure calculations in order to determine oxidation state and location of iron. The sample contains 0.3 wt% of total iron oxide. The 57Fe Mössbauer spectrum can be resolved into three doublets. Two of them with hyperfine parameters typical for octahedrally coordinated high-spin Fe3+ and Fe2+, respectively, are assigned to iron substituting for Al in the octahedral M2-site. The third doublet is attributed to Fe3+ in hematite. Electronic structure calculations in the local spin density approximation are in reasonable agreement with experimental data provided that expansion and/or distortion of the coordination octahedra are presumed upon iron substitution. The calculated hyperfine parameters of Fe3+ are almost identical for the M1 and M2 positions, but the calculated ligand-field splitting is by far too large for high-spin Fe3+ on M1. 相似文献
4.
Danylo Zherebetskyy Georg Amthauer Michael Grodzicki 《Physics and Chemistry of Minerals》2010,37(7):455-464
The electronic and magnetic structure of the chain silicate orthoferrosilite Fe 2 2+ Si2O6 has been investigated by electronic structure calculations in the local spin density approximation. All calculations are based on experimentally determined geometrical data at room temperature. The calculated spin-allowed d–d excitation energies and hyperfine parameters are in quantitative agreement with the respective experimental data from optical absorption and Mössbauer spectroscopy. Inside one ribbon that is parallel to the crystallographic c axis and contains two non-equivalent M1 and M2 sites, all iron spins are ferromagnetically coupled with coupling constants of about +16 cm?1. Between these ribbons within the (b, c)-plane a weak ferromagnetic coupling of about +2 cm?1 is obtained. Neighboured (b, c)-planes are coupled antiferromagnetically via chains of Si B -tetrahedra but ferromagnetically via chains of Si A -tetrahedra. Such a theoretically determined "double-plane antiferromagnetic" spin structure is at variance with an experimentally derived magnetic structure. This discrepancy is attributed to differences between the geometry at room temperature and at temperatures below the Néel temperature currently not available. 相似文献
5.
The Origin of Sillimanite in Glen Clova, Angus 总被引：2，自引：3，他引：2
At the sillimanite isograd in Glen Clova, sillimanite appearsto have formed within biotite, rather than in kyanite. Biotiteis thought to have been a nucleating agent, the trigonally arrangedoxygen octahedra and tetrahedra in the alternate mica layersacting as nuclei for the growth of the octahedral Al–Oand the tetrahedral (Al, Si)–O chains that constitutethe sillimanite structure. Nucleation seems to have been dominantlyepitaxial; no permanent breakdown of biotite was involved, andit is suggested that Al and Si for sillimanite growth was mainlyderived from the solution of unstable kyanite. 相似文献
6.
J. D. C. McConnell A. De Vita S. D. Kenny V. Heine 《Physics and Chemistry of Minerals》1997,25(1):15-23
Ab initio total energy calculations based on a new optimised oxygen pseudopotential has been used to determine the enthalpy
of disorder for the exchange of Al and Si in tetrahedral coordination in simple derivative aluminosilicate structures based
on the high temperature tridymite structure. The problem has been studied as a function of defect interaction, and defect
concentration, and the results indicate that the energy for Al/Al neighbouring tetrahedra can be assigned primarily to two
effects, the first, a coulombic effect, associated with the disturbed charge distribution, and the second associated with
the strain related to misfit due to the very different dimensions of the Si and Al containing tetrahedra. In practice each
of these effects contributes approximately 0.2 eV per Al−Al neighbour to the overal disorder enthalpy. These simple results
were obtained after a careful study of possible chemical interaction between adjacent Al/Si containing tetrahedra which showed
that chemical interaction was effectively absent. Since individual Al/Si tetrahedra proved to be discrete entities that are
individually heavily screened by the shared oxygens it follows that coulombic and strain effects in disorder effectively account
for the whole of the disorder enthalpy. The complete set of results have been used to establish new criteria for the structure
and disorder enthalpies of the feldspar group of minerals and their long period derivatives.
Received: October 7, 1996 / Revised, accepted: March 17, 1997 相似文献
7.
G. Longworth M. G. Townsend R. Provencher H. Kodama 《Physics and Chemistry of Minerals》1987,15(1):71-77
Mössbauer spectra of biotites (1) and (2) with relative iron concentrations ～1:1·6 and of their oxidation products are recorded at 4 K in zero field and in applied fields up to 5 T. Magnetic susceptibility data are also reported. The results show that Fe III spins are in a ferromagnetic configuration in the c-plane in both biotites. Partial oxidation of biotite (1) leads to a canted ferromagnetic structure, while complete oxidation of biotite (2) yields an antiferromagnetic spin configuration. Nearest-neighbour antiferromagnetic Fe III-O-Fe III, and ferromagnetic Fe III-O-Fe II and Fe II-O-Fe II superexchange can account for the results. For biotites with higher concentrations of iron, Fe II and Fe III seem to be distributed randomly in the triangular lattice. From susceptibility results in biotites dilute in iron, an estimate of the ratio of nearest-neighbour and next-nearest neighbour magnetic interaction in the triangular lattice is derived. 相似文献
8.
S.-U. Weber M. Grodzicki C. A. Geiger W. Lottermoser G. Tippelt G. J. Redhammer M. Bernroider G. Amthauer 《Physics and Chemistry of Minerals》2007,34(1):1-9
Three natural lawsonites from Syke Rock, Mendocino Co., Reed Ranch, Marin Co., and Blake Gardens, Sonoma Co., all from the Coast Range Region in California, were studied by 57Fe Mössbauer spectroscopy, electron microprobe analysis, and X-ray powder diffraction. The samples contain about 0.6, 1.0, and 1.4 wt% of total iron oxide, respectively. 57Fe Mössbauer spectra are consistent with the assumption that high-spin Fe3+ substitutes for Al in the octahedrally coordinated site. The Mössbauer spectrum of lawsonite from Syke Rock exhibits a second doublet with 57Fe hyperfine parameters typical for octahedrally coordinated high-spin Fe2+. Electronic structure calculations in the local spin density approximation yield quadrupole splittings for Fe3+ in quantitative agreement with experiment indicating, however, that substitution of Al by Fe3+ must be accompanied by local distortion around the octahedral site. Model calculations also reproduce the room temperature hyperfine parameters of ferrous high-spin iron assuming the substitution of Ca by Fe2+. However, it cannot be excluded that Fe2+ may occupy a more asymmetric site within the microstructural cavity occupied by Ca and a H2O molecule. 相似文献
9.
10.
Eric Dowty 《Physics and Chemistry of Minerals》1987,14(2):122-138
Normal coordinate calculations, producing synthetic infrared and Raman spectra, were carried out on melilites, pyroxenes, silica polymorphs and feldspars. Atomic motions are complex in the high-frequency Raman modes of melilites and aluminous pyroxenes. The symmetric T-Onb stretching vibrations of Si and Al tetrahedra with different numbers of bridging oxygens are separate from each other, but may combine individually with oscillation of bridging oxygens between Si and Al tetrahedra. The latter type of vibration tends to dominate as Al/Si increases. The frequencies of these vibrational components and the degree of such intermixing depend on T-O force constants, which vary greatly depending on local bonding configurations; individual bands in the high-frequency Raman cannot in general be assigned to single structural entities or fixed combinations thereof. Calculations confirm that in some Al-Si glasses such as jadeite and spodumene, i.e. those in which all Al can be tetrahedral without non-bridging oxygens, Al-O-Al linkages or linkage of more than two tetrahedra by a single oxygen, aluminum is predominantly in tetrahedral coordination. Other Al-Si glasses which are richer in aluminum or which have non-bridging oxygens may contain Al tetrahedral triclusters, non-tetrahedral Al, or both. On the basis of distinctive 450–750 cm?1 infrared bands, both silica and feldspar glasses resemble tridymite and related stuffed derivatives, not other crystalline silica polymorphs or feldspars. Either these glasses have a structure like that of tridymite on a local scale, or the disorder of the glasses causes drastic modification to the vibrations in question. 相似文献
11.
Silicon K-edge x-ray absorption near-edge structure (XANES) spectra of a selection of silicate and aluminosilicate minerals have been measured using synchrotron radiation (SR). The spectra are qualitatively interpreted based on MO calculation of the tetrahedral SiO 4 4? cluster. The Si K-edge generally shifts to higher energy with increased polymerization of silicates by about 1.3 eV, but with considerable overlap for silicates of different polymerization types. The substitution of Al for Si shifts the Si K-edge to lower energy. The chemical shift of Si K-edge is also sensitive to cations in more distant atom shells; for example, the Si K-edge shifts to lower energy with the substitution of Al for Mg in octahedral sites. The shifts of the Si K-edge show weak correlation with average Si-O bond distance (dSi-O), Si-O bond valence (sSi-O) and distortion of SiO4 tetrahedra, due to the crystal structure complexity of silicate minerals and multiple factors effecting the x-ray absorption processes. 相似文献
12.
Werner Lottermoser Günther J. Redhammer Sven-Ulf Weber Fred Jochen Litterst Gerold Tippelt Stephen Dlugosz Hermann Bank Georg Amthauer Michael Grodzicki 《Physics and Chemistry of Minerals》2011,38(10):787-799
This work reports on the evaluation of the electric field gradient (EFG) in natural chrysoberyl Al2BeO4 and sinhalite MgAlBO4 using two different procedures: (1) experimental, with single crystal Mössbauer spectroscopy (SCMBS) on the three principal sections of each sample and (2) a “fully quantitative” method with cluster molecular orbital calculations based on the density functional theory. Whereas the experimental and theoretical results for the EFG tensor are in quantitative agreement, the calculated isomer shifts and optical d–d-transitions exhibit systematic deviations from the measured values. These deviations indicate that the substitution of Al and Mg with iron should be accompanied by considerable local expansion of the coordination octahedra. 相似文献
13.
14.
Mark J. Nilges Yuanming Pan Rudolf I. Mashkovtsev 《Physics and Chemistry of Minerals》2009,36(2):61-73
The X- and W-band single-crystal electron paramagnetic resonance spectra of an electron-irradiated natural quartz permit quantitative
analysis of a 29Si hyperfine structure (A ~12.6 MHz) and an 27Al hyperfine structure (A ≤ 0.8 MHz) for a previously reported hole-like center. The 29Si hyperfine structure arises from interaction with two equivalent Si atoms and is characterized by the direction of the unique
A axis close to a Si–O bond direction. The 27Al hyperfine structure, confirmed by pulsed electron nuclear double resonance and electron spin echo envelope modulation spectra,
is characterized by the unique A axis approximately along a twofold symmetry axis. These 29Si and 27Al hyperfine data, together with published theoretical results on peroxy radicals in SiO2 as well as our own density functional theory (DFT) calculations on model peroxy centers, suggest this hole-like center to
have the unpaired spin on a pair of oxygen atoms linked to two symmetrically equivalent Si atoms and a substitutional Al3+ ion across the c-axis channel, a first peroxy radical in quartz. The nuclear quadrupole matrix P also suggests that the Al3+ ion corresponds closely to the diamagnetic precursor to the [AlO4]0 center.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
15.
W. Lottermoser K. Forcher G. Amthauer H. Fuess 《Physics and Chemistry of Minerals》1995,22(4):259-267
Mössbauer measurements on synthetic iron orthosilicate Fe2SiO4 (fayalite) were carried out in the antiferromagnetic spin state below T
N
65 K. The Mössbauer parameters isomer shift , inner magnetic field H(0), angle between H(0) and the z-component of the electric field gradient (efg), quadrupole splitting QS and asymmetry parameter were determined as a function of temperature. These parameters could be attributed to the two crystallographic sites M1 and M2.The smaller isomer shift on M1 with respect to M2 displays the more covalent character of the Fe-O bond on M1, which is supported by previous neutron diffraction experiments. H(0) shows a Brillouin-type behaviour with different fields on the two crystallographic sites (stronger on M1) and a small discontinuity at T = 23 K which corresponds with previous magnetic measurements. The quadrupole splitting is equal on both sites within error bars, in agreement with previous theoretical results and in contradiction to previous Mössbauer refinements published elsewhere. 相似文献
16.
S. Lauterbach C. A. McCammon P. van Aken F. Langenhorst F. Seifert 《Contributions to Mineralogy and Petrology》2000,138(1):17-26
(Mg,Fe)(Si,Al)O3 perovskite samples with varying Fe and Al concentration were synthesised at high pressure and temperature at varying conditions of oxygen fugacity using a multianvil press, and were characterised using ex?situ X-ray diffraction, electron microprobe, Mössbauer spectroscopy and analytical transmission electron microscopy. The Fe3+/ΣFe ratio was determined from Mössbauer spectra recorded at 293 and 80?K, and shows a nearly linear dependence of Fe3+/ΣFe with Al composition of (Mg,Fe)(Si,Al)O3 perovskite. The Fe3+/ΣFe values were obtained for selected samples of (Mg,Fe)(Si,Al)O3 perovskite using electron energy-loss near-edge structure (ELNES) spectroscopy, and are in excellent agreement with Mössbauer data, demonstrating that Fe3+/ΣFe can be determined with a spatial resolution on the order of nm. Oxygen concentrations were determined by combining bulk chemical data with Fe3+/ΣFe data determined by Mössbauer spectroscopy, and show a significant concentration of oxygen vacancies in (Mg,Fe)(Si,Al)O3 perovskite. 相似文献
17.
Ryosuke Sinmyo Elena Bykova Catherine McCammon Ilya Kupenko Vasily Potapkin Leonid Dubrovinsky 《Physics and Chemistry of Minerals》2014,41(6):409-417
Magnesium silicate perovskite is the predominant phase in the Earth’s lower mantle, and it is well known that incorporation of iron has a strong effect on its crystal structure and physical properties. To constrain the crystal chemistry of (Mg, Fe)SiO3 perovskite more accurately, we synthesized single crystals of Mg0.946(17)Fe0.056(12)Si0.997(16)O3 perovskite at 26 GPa and 2,073 K using a multianvil press and investigated its crystal structure, oxidation state and iron-site occupancy using single-crystal X-ray diffraction and energy-domain Synchrotron Mössbauer Source spectroscopy. Single-crystal refinements indicate that all iron (Fe2+ and Fe3+) substitutes on the A-site only, where \( {\text{Fe}}^{ 3+ } /\Upsigma {\text{Fe}}\sim 20\,\% \) based on Mössbauer spectroscopy. Charge balance likely occurs through a small number of cation vacancies on either the A- or the B-site. The octahedral tilt angle (Φ) calculated for our sample from the refined atomic coordinates is 20.3°, which is 2° higher than the value calculated from the unit-cell parameters (a = 4.7877 Å, b = 4.9480 Å, c = 6.915 Å) which assumes undistorted octahedra. A compilation of all available single-crystal data (atomic coordinates) for (Mg, Fe)(Si, Al)O3 perovskite from the literature shows a smooth increase of Φ with composition that is independent of the nature of cation substitution (e.g., \( {\text{Mg}}^{ 2+ } - {\text{Fe}}^{ 2+ } \) or \( {\text{Mg}}^{ 2+ } {\text{Si}}^{ 4+ } - {\text{Fe}}^{ 3+ } {\text{Al}}^{ 3+ } \) substitution mechanism), contrary to previous observations based on unit-cell parameter calculations. 相似文献
18.
19.
Sven-Ulf Weber Michael Grodzicki Werner Lottermoser Günther J. Redhammer Dan Topa Gerold Tippelt Georg Amthauer 《Physics and Chemistry of Minerals》2009,36(5):259-269
Natural sinhalites, MgAlBO4, from the Ratnapura District, Sri Lanka, and from Bodnar Quarry near Hamburg, Sussex Co., New Jersey, USA, have been characterized by 57Fe Mössbauer spectroscopy, electron microprobe, X-ray single-crystal diffractometry and by electronic structure calculations in order to determine the oxidation state and site occupancy of iron in the sinhalite structure. The samples contain about 3.35 and 1.46 wt% of total iron oxide, respectively. The structure refinement is successful and reproduces the total iron content provided that the substitution of Mg2+ by Fe2+ on the M2 position only is assumed. The 57Fe Mössbauer spectra at 77, 293, 573 and 773 K can be resolved into two doublets with hyperfine parameters common for octahedrally coordinated high-spin Fe2+. There is no evidence for iron in the tetrahedral site. Electronic structure calculations in local spin density approximation yield hyperfine parameters for Fe2+ on the M2-site at 0, 293, 573 and 773 K in quantitative agreement with experiments. Calculated spectroscopic properties for Fe2+ on the M1-site are at variance with the experimental data and, thus, indicate that substitution of Al3+ by Fe2+, if occurring at all, must be accompanied by considerable local expansion and distortion of the M1-octahedron. 相似文献
20.
J. M. D. Coey O. Ballet A. Moukarika J. L. Soubeyroux 《Physics and Chemistry of Minerals》1981,7(3):141-148
Three iron-rich 1:1 clay minerals, greenalite [Si2]{Fe 3 2+ }O5(OH)4, berthiérine [Si, Al]2{Fe2, Mg, Fe3+, Al}3 O5(OH)4 and cronstedtite [Si, Fe3+]2{Fe2+, Fe3+}3O5(OH)4 have been studied by Mössbauer spectroscopy, magnetization measurements and neutron diffraction to determine their magneticproperties. The predominant magnetic coupling is ferromagnetic for pairs of ferrous ions in the octahedral sheet, but antiferromagnetic for ferric pairs. The crystal field at Fe2+ sites in greenalite and berthiérine is effectively trigonal with an orbital singlet l z=0 as ground state. These mainly ferrous minerals order magnetically at 17K and 9K respectively. The magnetic structure of greenalite consists of ferromagnetic octahedral sheets, with the moments lying in the plane, coupled antiferromagnetically by much weaker interplane interactions. The ratio of intraplane to interplane coupling is of order 50, so the silicate has a two-dimensional aspect, both structurally and magnetically. Although the overall magnetic order is established as antiferromagnetic by neutron diffraction, the magnetization curves resemble those of a ferromagnet because of the very weak interplane coupling. Cronstedtite orders antiferromagnetically around 10K. Moments within the planes are antiferromagnetically coupled. The magnetism has no particular two-dimensional character because exchange paths between the layers are provided by the ferric cations present in the tetrahedral sheets. 相似文献