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1.
The nicotinamide (pyridine-3-carboxamide, nia) complexes of silver(I), [Ag(nia)2(NO3)]·H2O (1), [Ag(nia)2(NO3)] (2), and {K[Ag(nia)2](NO3)2} n (3), were prepared and characterised by IR spectroscopy and TG/DTA thermal methods. The solid state structures of 2 and 3 were determined by single-crystal X-ray diffraction analysis. In both complexes two nicotinamide ligands are coordinated to silver(I) through the nitrogen atom of the pyridine ring in a near-linear fashion. In 2, additional coordination by two oxygen atoms of one nitrate group leads to the distorted tetrahedral coordination environment of silver(I). In 3, nitrate ions bridge potassium cations giving rise to a 2D coordination network which is further stabilised by cross-bridging of each two potassium atoms in [1 0 0] direction by complex cations, [Ag(nia)2]+. Despite different aggregation of 2 and 3 in the solid state, both complexes demonstrate quite similar thermal stability. The amide self-complementary hydrogen bonds appear to be the main driving force for establishing the crystal structures of both 2 and 3.  相似文献   

2.
The polarized (E‖a′, E‖b and E‖c) electronic absorption spectra of five natural chromium-containing clinopyroxenes with compositions close to chromdiopside, omphacite, ureyite-jadeite (12.8% Cr2O3), jadeite, and spodumene (hiddenite) were studied. The polarization dependence of the intensities of the Cr3+ bands in the clinopyroxene spectra cannot be explained by the selection rules for the point groups C 2 or C 2v but can be accounted for satisfactorily with the help of the higher order pseudosymmetry model, i.e. with selection rules for the point symmetry group C 3v. The trigonal axis of the pseudosymmetry crystal field forms an angle of 20.5° with the crystallographic direction c in the (010) plane. D q increases from diopside (1542 cm?1) through omphacite (1552 cm?1), jadeite (1574 cm?1) to spodumene (1592 cm?1). The parameter B which is a measure of covalency for Cr3+-O bonds at M1 sites in clinopyroxene depends on the Cr3+ concentration and the cations at M2 sites.  相似文献   

3.
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   

4.
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5.
Polarized electronic absorption spectra, E∥a(∥X), E∥b(∥Y) and E∥c(∥Z), in the energy range 3000–5000?cm–1 were obtained for the orthorhombic thenardite-type phase Cr2SiO4, unique in its Cr2+-allocation suggesting some metal-metal bonding in Cr2+Cr2+ pairs with Cr-Cr distance 2.75?Å along [001]. The spectra were scanned at 273 and 120?K on single crystal platelets ∥(100), containing optical Y and Z, and ∥(010), containing optical X and Z, with thicknesses 12.3 and 15.6?μm, respectively. Microscope-spectrometric techniques with a spatial resolution of 20?μm and 1?nm spectral resolution were used. The orientations were obtained by means of X-ray precession photographs. The xenomorphic, strongly pleochroic crystal fragments (X deeply greenish-blue, Y faint blue almost colourless, Z deeply purple almost opaque) were extracted from polycrystalline Cr2SiO4, synthesized at 35?kbar, above 1440?°C from high purity Cr2O3, Cr (10% excess) and SiO2 in chromium capsules. The Cr2SiO4-phase was identified by X-ray diffraction (XRD). Four strongly polarized bands, at about 13500 (I), 15700 (II), 18700 (III) and 19700 (IV) cm–1, in the absorption spectra of Cr2SiO4 single crystals show properties (temperature behaviour of linear and integral absorption coefficients, polarization behaviour, molar absorptivities) which are compatible with an assignment to localized spin-allowed transitions of Cr2+ in a distorted square planar coordination of point symmetry C2. The crystal field parameter of Cr2+ is estimated to be 10?Dq?10700?cm–1. A relatively intense, sharp band at 18400?cm–1 and three other minor features can, from their small half widths, be assigned to spin-forbidden dd-transitions of Cr2+. The intensity of such bands strongly decreases on decreasing temperature. The large half widths, near 5000?cm–1 of band III are indicative of some Cr-Cr interactions, i.e. δ-δ* transitions of Cr2 4+, whereas the latter alone would be in conflict with the strong polarization of bands I and II parallel [100]. Therefore, it is concluded that the spectra obtained can best be interpreted assuming both dd-transitions of localized d-electrons at Cr2+ as well as δ-δ* transitions of Cr2 4+ pairs with metal-metal interaction. To explain this, a dynamic exchange process 2 Crloc 2+?Cr2, cpl 4+ is suggested wherein the half life times of the ground states of both exchanging species are significantly longer than those of the respective optically excited states, such that the spectra show both dd- and δ-δ*-transitions.  相似文献   

6.
For ABO 4 type ternary oxides, high pressure phase transformations known up to the present are reviewed, and an attempt is made to explain and predict crystal structures of their high pressure phases. When ABO 4 type compounds are plotted based on the two variables, k=r A /r B and t=(r A +r B )/2r O, where r A , r B , and r O are the ionic radii of A and B cations and divalent oxygen, they can be classified into the major structure types. It is found empirically that a compound basically transforms to the structure type isostructural with a compound lying in a classified area with the same k and larger t values in the diagram.  相似文献   

7.
Two new protoamphibole-type amphiboles with space group type Pnmn, have been found in nature: protoferro-anthophyllite (Fe0.80Mn0.20)2 (Fe0.98Mg0.02)5 (Si4O11)2(OH)2, and protomangano-ferro-anthophyllite, (Mn0.70Fe0.30)2 (Fe0.82Mg0.18)5 (Si4O11)2(OH)2. Protoferro-anthophyllite (PFA) occurs in pegmatites at both Gifu Prefecture, Japan and at Cheyenne Mountain, El Paso County, Colorado, USA. Protomangano-ferro-anthophyllite, (PMFA) occurs in pegmatites at Fukushima Prefecture and in a Mn mine at Tochigi Prefecture, Japan. Structure determinations of the two amphiboles show that both are isostructural with the synthetic fluorian-amphibole, protoamphibole (= protofluorian-lithian-anthophyllite). A calculation of the procrystal electron density distributions, the bond paths and the bond critical point properties of PFA, PMFA, grunerite and protoamphibole indicates that the M4 cation in these amphiboles is 4-coordinated. A calculation of the electron density distributions at the Becke3LYP/6-311G(2d,p) level for model silicate tetrahedra for these amphiboles and anthophyllite reveals that the value of the electron density at the bond critical points, ρ(r c ), for the SiO(nbr) bonds is larger, on average (0.93 e/Å3), than that for the SiO(br) bonds (0.90 e/Å3). The observed SiO bond lengths decrease linearly with increasing ρ(r c ) while the magnitudes of the curvatures of ρ(r c ) both perpendicular and parallel to the bonds and the Laplacian of ρ(r c ) each increases. These trends are associated with an increase in the electronegativity of the Si cation, a possible increase in the covalent character of the SiO bond and a tendency for SiO(nbr) bonds to be involved in wider OSiO angles than SiO(br) bonds. It is possible, if not likely, that protoanthophyllite has often been misidentified as anthophyllite.  相似文献   

8.
We relate a single-crystal FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites. The structural refinements show that the oxygen site of the H2O molecule lies off the triad axis. The water molecule is almost symmetric and slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with Ow···O donor–acceptor distances >2.7 Å. The FTIR spectra show two main absorptions. The first at 3,602 cm?1 is polarized for E ⊥ c and is assigned to the ν3 mode. The second, at 3,531 cm?1, is also polarized for E ⊥ c and is assigned to ν1 mode. A weak component at 4,108 cm?1 could possibly indicate the presence of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1,300–1,500 cm?1 range are strongly polarized for E ⊥ c, and are assigned to the vibrations of the CO3 group.  相似文献   

9.
Metamorphic biotites examined by transmission electron microscopy contain planar defects on the (001) plane, superlattices, twins and a microstructure causing streaking of k≠3n rows. Analysis of the fringe contrast shows that the fault vectors associated with the planar defects are either R 1=±1/3 [010], R 2=±1/6 [310] or R 3=±1/6 [3 \(\bar 1\) 0]. Structural considerations indicate that a stacking fault R 1, R 2 or R 3 is most likely to exist in the octahedral layer rather than the potassium layer. The result of such a fault on a unit layer of mica is effectively to rotate it through ±120° about c* (equivalent to the common mica twin law). These stacking faults can provide the mechanism for producing the ±120° rotations associated with the common mica polytypes. Furthermore, many of the observed microstructures can be generated by these stacking faults.  相似文献   

10.
The cation distribution in the synthetic samples of olivine-type structure with composition (Fe x Mn1?x )2SiO4 was determined at room temperature and confirms previous Mössbauer results. At low temperature an antiferromagnetic ordering is observed. The magnetic structures can be described in the crystallographic cell (i.e. k=0). They are interpreted on the basis of the irreducible representations (modes) of the symmetry groups which are compatible with Pnma. The dominant modes observed for all compounds, including Fe2SiO4 and Mn2SiO4, only differ in their direction. The main direction of magnetization is dominated by the Fe2+ single-ion anisotropy. At 4.2K, for x=0.29, it is parallel to the c-axis, whereas for x=0.76 the direction is parallel to the b-axis. The anisotropy of the M1-sites dominates in the first case, whereas M2-anisotropy dominates in the second case. The influence of temperature is demonstrated for x=0.50 where c is the main direction at 4.2K, when it is b at 38K.  相似文献   

11.
The polarized far-infrared reflection spectra of single crystals of FeS2-marcasite are presented in the range from 40–700 cm?1. The spectra show 7 reststrahlen bands, as predicted by group theory. The oscillator parameters ?α ∞, ωα f, ?α f, γα f, and the transversal and longitudinal optical phonon frequencies ωTO and ωLO as well as effective ionic charges and oscillator strength weighted mean phonon frequencies were calculated. The anisotropic behaviour of these quantities is discussed in relation to the data for FeS2-pyrite. It is shown that the ionicity of marcasite is considerably smaller than that of pyrite, especially in the a and c direction. The directional dependence of the phonon frequencies is given and discussed with regard to the spectra of polycrystalline samples.  相似文献   

12.
In a group of minerals of reasonable complexity in which the structure topology is related but not identical, the general relation between structure topology and chemical composition is not known. This problem is of major significance. The structural hierarchy and stereochemistry are described for 27 titanium disilicate minerals that contain the TS (titanium-silicate) block, a central trioctahedral (O) sheet and two adjacent (H) sheets of [5]- and [6]-coordinated polyhedra and (Si2O7) groups and related delindeite. The TS block is characterized by a planar cell based on translation vectors, t 1 and t 2 , with t 1 ～ 5.5 and t 2 ～ 7 Å and t 1 ∧ t 2 close to 90°. The general formula of the TS block is A 2 P B 2 P M 2 H M 4 O (Si 2 O 7 ) 2 X 4 + n, where M 2 H and M 4 O = cations of the H and O sheets; MH = Ti (= Ti + Nb), Zr, Mn2+, Ca; MO = Ti, Zr, Mn2+, Ca, Na; A P and B P are cations at the peripheral (P) sites = Na, Ca, Ba; X = anions = O, OH, F; n = 0, 2, 4; the core part of the TS block is shown in bold and is invariant. Cations in each sheet of the TS block form a close-packed layer and the three layers are cubic close packed.There are three topologically distinct TS blocks, depending on the type of linkage of two H sheets and the central O sheet. The H sheets of one TS block attach to the O sheet in the same manner. All structures consist of a TS block and an I (intermediate) block that comprises atoms between two TS blocks. Usually, the I block consists of alkali and alkaline-earth cations, (H2O) groups and oxyanions (PO4)3?, (SO4)2? and (CO3)2?. These structures naturally fall into four groups, based on differences in topology and stereochemistry of the TS block. In Group I, Ti = 1 apfu Ti occurs in the O sheet, and (Si2O7) groups link to a Na polyhedron of the O sheet (linkage 1). In Group II, Ti = 2 apfu, Ti occurs in the H sheet, and (Si2O7) groups link to two M 2+ octahedra of the O sheet adjacent along t 2 (linkage 2). In Group III, Ti = 3 apfu, Ti occurs in the O and H sheets, and (Si2O7) groups link to the Ti octahedron of the O sheet (linkage 1). In Group IV, Ti = 4 apfu (the maximum possible content of Ti in the TS block), Ti occurs in the O and H sheets, and (Si2O7) groups link to two Ti octahedra of the O sheet adjacent along t 1 (linkage 3). The stability of the TS block is due to the ability of Ti (Nb) to have an extremely wide range in Ti (Nb)-anion bond lengths, 1.68–2.30 Å, which allows the chemical composition of the TS block to vary widely. In crystal structures so far known, only one type of TS block occurs in a structure. The TS block propagates close-packing of cations onto the I block. The general structural principles and the relation between structure topology and chemical composition are described for the TS-block minerals. These principles allow prediction of structural arrangements and possible chemical compositions, and testing whether or not all aspects of the structure and chemical formula of a mineral are correct. Here, I show how these principles work, and review recent results that show the effectiveness of these principles as a predictive technique.  相似文献   

13.
The electron paramagnetic resonance (EPR) spectrum of Cr3+ in synthetic crystals of forsterite consists primarily of lines of Cr3+ “isolated” at the M1 and M2 positions in a “perfect” crystal environment without local charge compensation. In addition it shows two nonequivalent superhyperfine-split sextets with different intensities which are due to strong interaction of the Cr3+ electron spin S (S=3/2) with an adjacent nuclear spin I(I=5/2). Electron nuclear double resonance (ENDOR) experiments revealed that the sextets result from Cr3+ (M1) - Al3+ and Cr3+ (M2) - Al3+ pairs, Al being located at adjacent tetrahedral Si sites. The g, D, A, and A′ tensor components of the Cr3+ - Al3+ pairs have been determined at room temperature. The values of the pairs are distinct although they are not very different from the corresponding data of “isolated” Cr3+. From the intensities of the EPR spectra the relative concentration of the Cr3+ - Al3+ pairs with respect to “isolated” Cr3+ has been estimated.  相似文献   

14.
The mixed valence iron silicate ilvaite, CaFe 2 2+ Fe3+Si2O8(OH), displays electron delocalization associated with Fe2+→Fe3+ charge transfer as observed by Mössbauer spectroscopy. Previous studies report the observation of an ‘electron hopping phenomenon’ with resolution of discrete valence states below 320 K. Mössbauer spectra of a suite of naturally occurring ilvaites were recorded over a temperature range, 80 K to 575 K. Five quadrupole doublets were resolved by computer fitting and assigned to Fe2+(A), Fe2+(B), Fe3+(A), and Fe2+(A)→Fe3+(A)‖c and ⊥c. Contrary to prior work, doublets associated with electron delocalization are resolved at 80 K and preclude the use of a Verwey-type order-disorder model. We propose a thermal activation model and discuss its criteria from molecular orbital and mineralogical viewpoints.  相似文献   

15.
Polycrystalline material of a sulfate apatite with chemical composition Na6Ca4(SO4)6F2 or (Na2Ca4)Na4(SO4)6F2 has been synthesized by solid state reactions. Basic crystallographic data are as follows: hexagonal symmetry, a?=?9.3976(1) Å, c?=?6.8956(1) Å, V?=?527.39(1) Å3, Z?=?1, space group P63/m. For structural investigations the Rietveld method was employed. Thermal expansion has been studied between 25 and 600 °C. High temperature (HT) powder diffraction data as well as thermal analysis indicate that the apatite-type compound undergoes a reconstructive phase transition in the range between 610 and 630 °C. Single-crystals of the HT-polymorph were directly grown from the melt. Structural investigations based on single-crystal diffraction data of the quenched crystals performed at ?100 °C showed orthorhombic symmetry (space group Pna21) with a?=?12.7560(8) Å, b?=?8.6930(4) Å, c?=?9.8980(5) Å, V?=?1097.57(10) Å3 and Z?=?2. Unit cell parameters for a quenched polycrystalline sample of the HT-form obtained at ambient conditions from a LeBail-fit are as follows: a?=?12.7875(1) Å, b?=?8.7255(1) Å, c?=?9.9261(1) Å, V?=?1107.53(2) Å3. The lattice parameters of both modifications are related by the following approximate relationships: a HT?≈?2c RT, b HT?≈?-(½a RT?+?b RT), c HT?≈?a RT. The HT-modification is isotypic with the corresponding potassium compound K6Ca4(SO4)6F2. The pronounced disorder of the sulphate group even at low temperatures has been studied by maximum entropy calculations. Despite the first-order character of the transformation clusters of sulfate groups surrounding the fluorine anions can be identified in both polymorphs. Each of the three next neighbor SO4-tetrahedra within a cluster is in turn surrounded by 8–9 M-cations (M: Na,Ca) defining cage-like units. However, in the apatite structure the corresponding three tricapped trigonal prisms are symmetry equivalent. Furthermore, the central fluorine atom of each cluster is coordinated by three next M-neighbors (FM3-triangles), whereas in the HT-polymorph a four-fold coordination is observed (FM4-tetrahedra).  相似文献   

16.
The electron paramagnetic resonance (EPR) spectra of Fe3+ in a well cristallized kaolinite from Decazeville in France are well resolved. It is shown that in this sample there are mainly two slightly different spectra, well separated at low temperature and characterized at -150° C by the constants B 2 0 = 0.112 cm?1, B 2 2 = 0.0688 cm?1 for one and B 2 0 = 0.116 cm?1, B 2 2 = 0.0766 cm?1 for the second. These two spectra arise from Fe3+ substituted for Al3+ at the two octahedral positions in equal amounts. The temperature dependence of EPR spectra was studied and was explained by a modification of the octahedral sites.  相似文献   

17.
The orthopyroxene crystal structure can be viewed as the stacking of alternating tetrahedral and octahedral layers parallel to the (100) plane. Easy glide occurs in the (100) plane at the level of the octahedral layer to prevent breakage of the strong Si-O bonds. Dislocations with c and b Burgers vectors have been activated in (100) by room temperature indentation in an orthoenstatite gem quality single crystal. Investigations in transmission electron microscopy show that the b dislocations (b?9 Å) are not dissociated while the c's (c=5.24 Å) are dissociated into four partials. This result is interpreted by considering the oxygen sublattice as a distorted FCC one. The four c partials are thus Shockley partials bounding three stacking faults. For the two outer ones, synchroshear of the cations is necessary to keep unchanged their sixfold coordination; the oxygen sublattice is locally transformed into a HCP lattice. This accounts for the observed low splitting (?100 Å) of these faults as compared to the median one (?500 Å) which does not affect the oxygen sublattice and does not require cation synchroshear. In a Fe rich orthopyroxene (eulite), semi coherent exsolution lamellae have been studied. Either only c edge dislocations or both b and c edge dislocations occur in the phase boundaries depending upon the thickness of the lamellae. Only the c dislocations are dissociated. From the observed spacing between these mismatch dislocations a crude estimate of the exsolution temperature is proposed T ex ? 700° C.  相似文献   

18.
Single crystals of hexagonal and monoclinic pyrrhotite, Fe1?xS, have been experimentally deformed by uniaxial compression at 300 MPa confining pressure, and at a strain rate of 1 × 10?5 s?1 in the temperature range from 200° C to 400° C. Very high anisotropy characterizes the mechanical behaviour of the crystal structure. During compression parallel to thec-axis, when no slip system may be activated, the maximum strength is observed. One or two degrees of non-parallelism between [c] and σ1 results in slip on the basal plane, illustrating the very low resistance of the lattice against shear in this plane. At σ1 Λ(0001)=45°, i.e. when maximum resolved shear stress is attained on the basal plane, the strength reaches a minimum. Thecritical resolved shear stress (CRSS) increases from less than 4.7 MPa at 400° C to 52 MPa at 200° C. A new slip system, \((10\overline 1 0)\parallel \left\langle {1\overline 2 10} \right\rangle \) prism slip, is described. It is activated only at high angles (>70°) between σ1 and [c]. The CRSS of the prism slip ranges from 7 MPa (400° C) to 115 MPa (200° C). Twinning on \((10\overline 1 2)[(10\overline 1 2):(1\overline 2 10)]\) , earlier reported by several authors, has been produced only at the highest temperature either as secondary feature during pressure release (compression ‖[c]) or in heterogeneously strained areas (compression ⊥[c]). As twinning and prism slip attain their maximum values of the Schmidt factor under nearly equal stress conditions it is postulated that the former of the two deformation modes has the higher shear resistance.  相似文献   

19.
The borate polyhedra BO 3 3? , B(OH)3, BO 4 5? , and B(OH) 4 ? are studied using the ab initio and multiple scattering Xα quantum mechanical methods. The ab initio self-consistent-field (SCF) molecular orbital (MO) method, at the minimum basis set level, predicts equilibrium B-O distances within 0.04 Å of their average values in solids so long as the polyhedron charge is small. Orbital energies from double zeta basis set ab initio calculations and analogies with isoelectric compounds are used to assign the X-ray spectra of BO 3 3? and to predict the valence region spectra of BO 4 5? . Contour maps of the difference between molecular and superimposed free atom electron densities show charge buildup along the B-O bond which is only slightly smaller than that observed in CO 3 2? .  相似文献   

20.
Two natural clinopyroxene single crystals were investigated, an aegirine-augite (AEG) and a magnesian hedenbergite (HED). Both samples were carefully characterized by electron microprobe, X-ray diffraction, and Mössbauer spectroscopy. Magnetic susceptibility measurements of powdered samples reveal low temperature antiferromagnetic coupling and Curie-Weiss behaviour with T N =7.5(5)?K, Θ P =?19(1)?K for AEG, and T N =31(1)?K, Θ P =+21(1)?K for HED, respectively. Low temperature Mössbauer spectra exhibit relaxation phenomena. Magnetic susceptibility measurements of the single crystals show the direction of the magnetic moments to be lying within the a/c plane for both samples: 50(±2)° from a and 57(±2)° from c in AEG, and 45(±2)° from a and 60(±2)° from c in HED, respectively. The antiferromagnetic interchain interaction competes with the ferromagnetic intrachain interaction in both pyroxenes. In the magnesian hedenbergite a field induced magnetic transition is found. Its dependence on temperature, magnetic field and crystallographic direction is investigated and described.  相似文献   

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