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1.
Metal K- and L3-, sulfur K- and arsenic K- and L3-edge X-ray absorption near-edge spectra of a series of metal disulfides, FeS2 (both pyrite and marcasite), CoS2, NiS2, and CuS2, and their isomorphs, FeAsS and CoAsS, are presented. The features in this region of these spectra are interpreted using band structure and molecular orbital calculations in terms of the transitions from the 1s or 2p3/2 state to unoccupied states. The 3d transition metal L3-edge spectra of these materials show dependence on the degree of multiplet splitting in the final state, and thus offer less information on the electronic ground state. There are substantial differences in the spectra of the isostructural materials, whereas the spectra of the isotopes pyrite and marcasite show several similarities, illustrating the dependence of near-edge region on electronic structure.  相似文献   

2.
The O2? ion is a useful concept in ionic solids. Its electron density and large polarizability is well described by the Watson sphere model. The large variation in the electronic structure of oxides is illustrated by discussing the ionic MgO, the partly ‘covalent’ Cu2O, and TiO and NbO which have defects with respect to the ideal NaCl structure. The variation in physical properties is shown for the oxides in the rutile structure ranging from the insulating TiO2, the metallic RuO2 to the ferromagnetic CrO2. Electron densities, total energies and densities of states are used to study these materials.  相似文献   

3.
A local density approximation (LDA) band structure method, the Linear Muffin-Tin Orbital Atomic Sphere Approximation (LMTO-ASA) method has been used to calculate the electronic structures of the pyrite-type disulphides (MS2, where M = Mn, Fe, Co, Ni, Cu, Zn). The total density of states has been calculated for 10 eV above and below the Fermi Level, along with the separate contributions from metal and sulphur and shows that the metal d band occurs above the sulphur p bands in MnS2, FeS2, CoS2 and NiS2, whereas in CuS2, the d band passes through the sulphur p band and in ZnS2, it lies below the sulphur p band. Substantial hybridization of the metal d states with the sulphur states occurs. FeS2 is calculated to be a semiconductor with a direct band gap of 0.64 eV in good agreement with experiment. The calculated local densities of states have been used in turn to calculate X-ray photoelectron spectra and Bremsstrahlung Isochromat spectra for this series of compounds, and these also show reasonable agreement with experimental data. A particular strength of the LMTO-ASA method is the ability to calculate and predict certain bulk properties of solids of interest in mineral physics. This has enabled the first reasonably accurate calculations of the total energy of the valence electrons of the system for pyrite (FeS2), given as — 345.885 rydbergs per unit cell, and the equilibrium unit cell volume which is within 3.3% of that determined experimentally. A theoretical pressure vs. volume curve for pyrite was also calculated along with values for the bulk modulus. However, our calculations predict a bulk modulus of 6.75 Mbar which is too high by a factor of 4.6 due to the simplifying assumption of a uniform scaling of interatomic distances on compression.  相似文献   

4.
This paper is concerned with the atomistic simulation of the surfaces of MgO, CaO, SrO, Li2O, c-ZrO2, SrTiO3, -Al2O3 and -Fe2O3. Details are reported of the structure and energy of pure, non-defective surfaces and the constant electrostatic potential in the bulk that results from surface relaxation and electronic polarisation. Cation impurity segregation is discussed in some detail with an emphasis on calculated heats of segregation, equilibrium surface coverages, the lattice structure of segregated surfaces and the energy of impure surfaces.Paper from Conference on Quantum Theory and Experiment, July 1986  相似文献   

5.
As is known, a variable parameter for partial ionicity is often involved in the techniques of modeling structure and properties of inorganic crystals by energy-minimization methods. In such a case, to make a calculated energy comparable to experimental estimate (atomization energy) there is a need to involve an intraatomic energy term, the so-called charge-transfer energy (Urusov and Eremin 1995). For this purpose a set of charge-transfer energies for valence states of many elements is compiled. The procedure based on aspiration of equalizing the electronic levels electronegativities is proposed to estimate the total contribution of the charge-transfer energy in the cohesive energy of a crystal. The other prescribed quantities are repulsive potential parameters which are related to the energetic propertis of valence shells of bonded atoms. The final potential function consists of an effective Coulomb attraction, repulsion and covalent Morse type term, properly weighted. The energy-minimization procedure is performed for various values of the ionicity degree parameter of the crystal under study. Thereafter the charge-transfer energy correction is introduced in order for the cohesive energy calculations to be made. The calculations of structure, energy and elastic properties were performed using the METAPOCS code with ionicity variation for halides (NaCl, CaF2), oxides (MgO, Al2O3, SiO2 quartz and SnO2), sulphide ZnS and silicate CaSnSiO5. The resulting cohesive energies usually agree with experiment much better than those calculated in pure ionic or covalent approximations. As to elastic properties of crystals under consideration, they are generally functions of the ionicity parameter too and in most instances the predicted properties at intermediate values of the ionicity degree are in a satisfactory accordance with experimental data.  相似文献   

6.
Two models have been used to explain the in-situ control of heavy metals: (1) solubility controls, where precipitation of a solid phase occurs under varying metal and ligand concentrations, and (2) surface chemical controls, where adsorption or exchange occurs at the solid/solution interface. Based on experiments presented in this paper, surface chemical controls can account for the removal of heavy metals, particularly Zn(II), from metal solutions which are undersaturated with respect to the hydroxide, oxide, or other controlling solid phases. Adsorption isotherms are presented for varying solution pHs, total metal ion concentrations, ionic strengths, and mineral substrates. The minerals chosen for illustration are SiO2, TiO2, FeOOH, Al2O3, MnOx and HgS which range widely in surface acidity, electrical double layer properties, specific surface areas, and surface functional groups.  相似文献   

7.
The three binary garnet solid solutions FeII3Al2Si3O12–XII3Al2Si3O12 (XII= MgII, MnII, CaII) have been investigated by 57Fe Mössbauer spectroscopy at 298 and 77 K and by electronic structure calculations in the local spin density approximation. The spectra yield isomer shifts and quadrupole splittings that are typical for FeII in the dodecahedral X-site of 222 point symmetry and are similar for each of the three binaries recorded. Conversely, electronic structure calculations based on the experimental crystal structure of the different end-member garnets exhibit pronounced variations in some of the electronic properties of FeII that are not reflected in the spectroscopic data. These results are interpreted as indicating that the different X–O bonds in garnet solid solutions retain to a large degree the intrinsic lengths that they possess in their respective end members, and that the Fe–O bond does not change greatly as a function of composition. This is evidence for the state of alternating bonds and not for the virtual crystal approximation in describing the X–O bond types or lengths in aluminosilicate garnet solid solutions. The observed degree and behavior of the FeII doublet asymmetry in the Mössbauer spectra for the three solid solution series do not indicate major variations in the anisotropic recoil-free fraction of FeII. Variations in doublet asymmetry are more likely a result of complex next-nearest X-site neighbor interactions and/or some degree of short-range cation ordering, though doublets representing different local X-site cation configurations cannot be resolved or fitted to the experimental spectra.  相似文献   

8.
The structural, electronic properties and stability of thorium orthosilicate ThSiO4 polymorphs: thorite and huttonite are investigated by means of the full-potential linearized augmented-plane-wave method with the generalized gradient approximation for the exchange-correlation potential (FLAPW-GGA). The forbidden gaps of thorite and huttonite are estimated at about 7.8 and 7.6 eV, respectively. It is found that Th5f states in ThSiO4 partially overlap with occupied O2p bands. The data obtained showed that thorite is more stable than huttonite; in turn both ThSiO4 polymorphs are unstable with respect to their constituent binary oxides (thorianite ThO2 and α-quartz SiO2) in agreement with the experiments. The theoretical shapes of X-ray emission (XES) (Si,O)Kα,β spectra for thorite, huttonite as well as for SiO2 and ThO2 are calculated and discussed. We show that the XES spectroscopy near the (Si,O)K edge may be very useful technique not only for detailed investigation of the bulk-electronic structure of Th silicates but also for the phase analysis of complex mineral samples containing these species.  相似文献   

9.
2p (L 2,3) X-ray absorption spectra are presented for a range of minerals to demonstrate the usefulness of L-edge spectroscopy as a symmetry- and valenceselective probe. 2p XAS provides a sensitive fingerprint of the electronic states of 3 d transition metals and can be applied to phases containing mixtures of such elements. Calculated spectra for 3d n → 2p 5 3d n+1 transitions provide a basis for the interpretation of the measured spectra. Thus, in principle, multiple valence states of a particular 3 d metal can be precisely characterized from a single L-edge spectrum. Examples of vanadium L-edge spectra are presented for a range of minerals; these complex spectra hold information concerning the presence of vanadium in multiple valence states. The Cu L-edge spectrum of sulvanite (Cu3 VS4) indicates the presence of both Cu+ and Cu2+; the V L-edge spectrum of the same sample shows that both V2+ and V5+ are present. Spectral simulations representing mixtures of Fe d 5 and Fe d 6 states are used to quantify Fe3+/∑Fe in a spinel, a glass, and an amphibole, all of which contain Fe as a major component. To illustrate the sensitivity of 2p XAS in a dilute system, the Fe L-edge spectrum of amethyst (α-SiO2: Fe) has been recorded; this spectrum shows that ～68% of the Fe in amethyst is Fe2+, and ～32% is Fe3+. Although previous studies on amethyst using other spectroscopic methods cite evidence for Fe4+, there is no indication in the L-edge spectrum for Fe4+ in amethyst. Comparison of theoretical and experimental spectra not only allows the valence states of 3 d ions to be recognised, but also provides site-symmetry information and crystal field parameters for each ion site.  相似文献   

10.
We prepare TiO2 film by ALD and study their electronic properties by soft X-ray absorption spectroscopy (XAS) and photoelectron spectroscopy. We focus on XAS and X-ray linear dichroism to indentify band onset and learn about local distortion of the Ti–O octahedral atomic and about defects which cause Ti-based electronic states within the band gap.  相似文献   

11.
We studied the photoluminescence properties of a sample of SiO2-clathrate Melanophlogite, a crystalline microporous material which is found in nature as a rare mineral. Upon β irradiation, the material displays an intense light emission under near-UV illumination. We studied in detail this optical activity by steady-state and time-resolved photoluminescence measurements as a function of temperature. The spectroscopic properties we find can be ascribed to a population of quasi-free molecules trapped within each of the two different types of cage available in the structure of this clathrate, although the spectroscopic properties of the guest molecules are affected by their interactions with the host matrix. Based on the available data, we attribute the observed photoluminescence to trapped S2 molecules, emitting from their excited 3Σ u ? or 3Π u electronic states, depending on the cage they are trapped in and on temperature. Our results have an impact on the fundamental understanding of host–guest interactions characteristic of microporous systems such as clathrates. Indeed, the data highlight that even a relatively weak coupling between quasi-free S2 molecules and the two types of cages provided by the Melanophlogite host has a surprisingly complex influence on the optical properties of the guest.  相似文献   

12.
The structure and electronic properties of trigonal and orthorhombic boric oxide (B2O3) are studied using periodic ab initio Hartree-Fock method. The optimised structural parameters for two B2O3 polymorphs are in good agreement with experimental data. The analyses of their electronic structures provide insights into the chemical nature of the B–O bond and the way in which it changes with the coordination number around boron and oxygen. Our quantum-chemical study suggests that the orthorhombic form is more ionic than the trigonal form and that the coordination number of boron around oxygen plays a more dominant role than that of oxygen around boron in B2O3 crystals.  相似文献   

13.
The Vea catchment, mainly underlain by crystalline basement rocks, is located in Northern Ghana. Hydrogeochemical studies were carried out in this area with the objective of identifying the geochemical processes influencing water quality and suitability of surface and groundwater for agricultural and domestic uses. Sixty-one groundwater and four surface water samples were collected from boreholes, dams and rivers and analysed for Ca2+, Mg2+, Na+, K+, HCO3 ?, Cl?, and SO4 2?, Fetot, PO4 3?, Mntot, NH4 +, NO3 ?, NO2 ?. In addition, pH, total dissolved solids, electrical conductivity, total hardness, turbidity, colour, salinity and dissolved oxygen were analysed. Chloro-alkaline indices 1 and 2, and characterization of weathering processes suggest that the chemistry of groundwater is dominated by the interaction between water and rocks. Cation exchange and silicate weathering are the dominant processes controlling the chemical composition of the groundwater in the area studied. Mineral saturation indices indicate the presence of at least three groups of groundwater in the Vea catchment with respect to residence time. The meteoric genesis index suggests that 86% of the water samples belong to the shallow meteoric water percolation type. The findings further suggest that the groundwater and surface water in the basin studied are mainly Ca–Mg–HCO3 water type, regardless of the geology. Compared to the water quality guidelines of WHO, the study results on sodium absorption ratio, sodium percentage, magnesium hazard, permeability index and residual sodium carbonate indicate that groundwater and surface water in the Vea catchment are generally suitable for drinking and irrigation purposes.  相似文献   

14.
The catalytic properties of spores of a marine Bacillus known to oxidize divalent manganese were used to perform laboratory Mn(II) oxidation experiments at environmental conditions of pH and Mn(II) concentration. We found that at pH 7.8 the initial kinetics of Mn(II) oxidation facilitated by the spores was four orders of magnitude greater than that which would be expected for abiotic autocatalysis on a colloidal MnO2 surface. The rate progressively decreased as the spores became coated with manganese oxide, eventually becoming very near that predicted for abiotic surface catalysis. Transmission electron microscopic observations and oxidation state measurements of solids precipitated at pH 7.5 and [Mn(II)] < 50 nM indicated that the initial oxidation product was hausmannite (Mn3O4 or MnOx where x = 1.33) which aged to more highly oxidized MnO2 (x = 1.9) in the time scale of weeks. By utilizing spores to catalyze the oxidation rate, we were able to maintain our experimental system within the seawater range of pH and Mn(II) where highly oxidized manganese oxide precipitates are thermodynamically stable. In doing so we obtained, for the first time, laboratory precipitates with oxidation states similar to that found in marine particulate material. These results suggest that the concentration of manganese in seawater and the oxidation state of marine manganese oxides are controlled by the rapid precipitation of Mn3O4, which can be microbially mediated, followed by the disproportionation to MnO2.  相似文献   

15.
Polarized S K- and L-edge, Mo L3- and L2-edge x-ray absorption near-edge structure (XANES) of natural molybdenite (2H-MoS2) have been measured with synchrotron radiation. These results are qualitatively interpreted using the energy band model of molybdenite and provide important information on the unoccupied states of molybdenite. The valence band (VB) maximum of molybdenite is characterized by fully occupied Mo 4dz 2, and the conduction band (CB) minimum of molybdenite is characterized by unoccupied Mo 4d states. The unoccupied Mo 4d band is split into two sub-bands, designated as t 2g – /t 2g + and e g – /e g + sets. Although the relative energy of these two sets are difficult to be evaluated, probably the former has the lower energy than the latter, both two sets have the combination wave functions of the other unoccupied Mo 4d components, rather than the simple 4dx 2 — y2 and 4dxy states. The unoccupied Mo 4d sub-bands contain significant DOS of both S 3 p- and 3 s-like states, indicating strong hybridization with S 3s and 3 p states. In the lower energy sub-band, the DOS of the S pz- and px,y-like states are very similar. However, in the higher energy sub-band, the DOS of the S 3 px,y-like state is lower than that of the S 3pz state. Polarized S K-edge XANES also reveal the features of antibonding S pz- and px,y-like states in molybdenite. The feature assigned to the S 3 pz-like states is stronger and sharper, and shifts to lower energy by about 2 eV relative to that for the S 3 px,y-like states.  相似文献   

16.
We present a systematic density-functional study of phase relations in three 4d-transition-metal sesquioxides: Y2O3, Rh2O3, and In2O3. Y2O3 and In2O3 undergo pressure-induced transitions to phases with larger cation coordination number (from 6 to 7) at low pressures. However, this does not occur in Rh2O3 at least up to ~300 GPa. This cannot be explained by usual arguments based on ionic-radii ratios often used successfully to explain phase relations in simple-metal and rare-earth sesquioxides and sesquisulfides. Inspection of their electronic structures shows that, in Rh2O3, the electronic occupancy of 4d orbitals, 4d 6, plays a fundamental role in the extraordinary stability of the Rh2O3(II)-type phase with respect to coordination increase. We point out that d-orbital occupancy is a fundamental factor in explaining phase relations in transition-metal sesquioxides and sesquisulfides.  相似文献   

17.
The effect of the group IA and VIIA ions, as well as Mg2+, and the molecules H2O, CO2, H3O+ and OH? on the energy of the Si-O bond in a H6Si2O7 cluster has been calculated using semiempirical molecular orbital calculations (CNDO/2). Three types of elementary processes, i.e. substitution, addition, and polymerization reactions have been used to interpret data on the dynamic viscosity, surface tension and surface charge, hydrolytic weakening, diffusivity, conductivity, freezing point depression, and degree of polymerization of silicates in melts, glasses, and aqueous solutions. As a test of our calculational procedure, observed X-ray emission spectra of binary alkali silicate glasses were compared with calculated electronic spectra. The well known bondlength variations between the bridging bond [Si-O(br)] and the non-bridging bond [Si-O(nbr)] in alkali silicates are shown to be due to the propagation of oscillating bond-energy patterns through the silica framework. A kinetic interpretation of some results of our calculations is given in terms of the Bell-Evans-Polanyi reaction principle.  相似文献   

18.
A model has been obtained from a strict expression of crystal energy by introducing S and valence approximations. The Mulliken approximation has been used for double-centered exchange and three-center integrals. The calculations have been carried out for Slater type atomic orbitals, varying the screening parameters. The bond energy has been represented as a sum of the energy for forming partial differential electronic densities, E A, the interatomic interaction energy E IA, and the correlation energy of atomic states in the crystal, E S . The computed bond energies for four oxides of alkaline-earth metals and six oxides of transition (iron group) metals are in good agreement with experimental data.  相似文献   

19.
Insight into the complexation of organic molecules on hematite surfaces was obtained from molecular-level studies of a simple probe molecule (methanol) with the R-cut surface of hematite. The R-cut crystal orientation of hematite, designated in this paper as α-Fe2O3(012), has two stable surface structures under ultrahigh vacuum (UHV) conditions based on low-energy electron diffraction (LEED) measurements. These are a (1×1) structure consisting of a bulk terminated arrangement of undercoordinated Fe3+ and O2− surface sites and a (2×1) reconstructed structure with unknown atomic structure. Whereas the (1×1) surface is essentially free of Fe2+, the (2×1) surface possesses a high surface concentration of Fe2+ sites based on electronic structure measurements using electron energy loss spectroscopy (EELS). Methanol adsorbs dissociatively on the (1×1) surface by coordination of the molecule’s oxygen atom at a Fe3+ site followed by transfer of the alcohol proton to a bridging O2− surface site, resulting in terminal OCH3 and bridging OH groups. Most of the dissociated methanol molecules recombine during heating and desorb in vacuum as methanol at 365 and 415 K for the (1×1) and (2×1) surfaces, respectively. However, a significant amount of the terminal OCH3 and bridging OH groups interchange as the surface is heated above room temperature (RT), resulting in bridging OCH3 and terminal OH groups. The bridging OCH3 groups are retained on the surface to higher temperature than the terminal OCH3 groups, but eventually decompose at about 550 K via a disproportionation reaction that forms gaseous CH3OH and H2CO. As a result of the disproportionation reaction, some surface Fe3+ sites are reduced to Fe2+ sites. The exchange process competes more successfully with recombinative desorption of methanol (from reaction of terminal OCH3 and bridging OH groups) on the (2×1) surface, despite the fact that this surface is already partially reduced, because terminal OCH3 groups are more stable on this surface than on the (1×1) surface. Based on these molecular-level findings, extensive exchange terminal organic ligands and bridging OH groups may play a significant role in stabilizing organics on hematite mineral surfaces. Such exchange processes may also play a role in destabilizing hematite surfaces toward reductive dissolution.  相似文献   

20.
Several additional pieces of evidence are offered in support of the recent reinterpretation (Tossell et al. 1981a, b) of the electronic and geometric structure of lollingite, FeAs2, and related minerals. This evidence consists of: (1) the experimental electron affinities of diatomic molecules such as P2 and S2, (2) an analysis of the structure and spectra of TiP2, (3) quantum mechanical calculations on FeAs6, CoAs6 and As4 polyhedra which allow prediction of the photoelectron spectra of FeAs2 and CoAs3, (4) an analysis of bimetal cluster and band calculations suggesting metallic behavior for a hypothetical Fe4+As 4 4? species and (5) qualitative molecular orbital (MO) interpretations of the stability of As 4 4? polyhedra, the interaction between adjacent As 2 2? anions in FeAs2, and the variations in <M-S-S in pyrites and in R(As-As) in diarsenides. It is suggested that previous theories of sulfide and arsenide electronic structure have put too much emphasis upon metal-metal interaction and too little upon anion-anion interaction.  相似文献   

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