Occupation Of Tetrahedral Sites Research Articles

X-ray photoemission investigations of binary and ternary C60 fullerides of Na, K, Rb, and Cs.

Core-level photoemission studies of alkali fulleride ${\mathit{A}}_{\mathit{x}}$${\mathrm{C}}_{60}$ thin films indicate alkali bonding configurations that can be associated with octahedral and tetrahedral interstitial sites of the ${\mathrm{C}}_{60}$ lattice. For ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$, the K 2p core-level results suggest ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ nucleation for K concentration as low as x\ensuremath{\sim}0.1 with occupation of both octahedral and tetrahedral sites. In contrast, the Rb 3d and Cs 4d core-level spectra show filling of only octahedral sites until x\ensuremath{\sim}1, forming ${\mathit{A}}_{1}$${\mathrm{C}}_{60}$ phases. For intercalation beyond x\ensuremath{\sim}1, the fcc tetrahedral sites of Rb are occupied as ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ forms. Intercalation with K or Rb beyond x\ensuremath{\sim}3, or with Cs beyond x\ensuremath{\sim}1, results in a spectral feature that is consistent with formation of a body-centered phase with equivalent tetrahedral interstitial sites. Results for ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{C}}_{60}$ suggest that Na ions occupy off-center positions in the large octahedral sites. Studies of ternary fullerides ${\mathrm{K}}_{\mathit{x}}$${\mathit{A}}_{\mathit{y}}$${\mathrm{C}}_{60}$ (x+y3) demonstrate preferential filling of the octahedral sites by the larger ion.

Neutron diffraction study of the structure of chalcogen spinels Cu 1+yCr 2X 4 (X Se, Te)

The structure of the copper excess chromium spinels Cu 2Cr 2Se 4 and Cu 1.81Cr 2Te 4 was determined by neutron diffraction at 300 K and at 4 K. The structure refinement reveals a simultaneous occupation of tetrahedral as well as of pseudo-octahedral sites by the excess y copper ions in Cu 1+yCr 2X 4. The relatively high temperature factors are discussed in terms of a specific local disorder model within the spinel space group Fd3m. From these results and the change in the lattice parameter α o with y a model was derived that is able to describe the sequence of occupation of tetrahedral and pseudo-octahedral lattice sites in the course of copper intercalation.

Caracte´risation du domaine de solution solide de structure spinelle dans le systeme In 2S 3[sbnd]Cu 2S [sbnd]CuS

Abstract The study of the In 2S 3 Cu 2S CuS system has shown the existence of a solid solution range with In 2S 3 vacant spinel crystal structure. These solid solutions have been synthesized in the form of powders and single crystals by solid state and chemical vapor transport reactions. Two processes of this solid solution range formation have been proposed from X-ray powder diffraction studies. The cell parameters variations as a function of the composition have been discussed in terms of site occupancies and copper oxidation states. The crystal structure refinement carried out in the F4¯3m space group and quantitative chemical analysis lead to the formulas (In 3.71Cu I 2.72Cu II 1.08□ 0.49)[In 16S 32 and (In 3.70Cu I 0.98Cu II 1.96□ 1.36)[In 16S 32, which illustrates the occupancy of tetrahedral and octahedral sites by indium, mixed valence copper, and vacancies.

Faraday rotation of pure and substituted cobalt ferrite single crystals

Faraday rotation measurements have been performed on single crystals of CoFe 2O 4, CoFe 2− x Al x O 4 ( x = 0.27) and Co 1− y Cd y Fe 2O 4 ( y = 0.25) at 1150 nm wavelength as a function of temperature (5⩽ T⩽580 K). The magnetic field up to 2 T was applied along two crystallographic orientations [100] and [111]. Results show a very strong anisotropy of magnetic origin due to Co 2+ and/or tetrahedral site occupancy. Crystal structure determinations show that Al 3+ substitution in pure CoFe 2O 4 increases the tetrahedral Co 2+ amount, leading to a strong decrease of the absolute FR.

Chemical variation in vesuvianite

Microprobeanalyses of 44 vesuvianite specimens of different color, morphology, and occurrence indicate the chemical variation of the species. Vesuvianites can be divided into four types, based on chemistry and color; most vesuvianites can be assigned to one of these types with confidence. Type 1 vesuvianites contain ≈ 2 atoms of Mg, 2.5 atoms of Mg, variable (divalent?) Fe, and <0.5 atoms of Ti. In color they are yellow, yellow-brown, or yellow-green. Type 3 vesuvianites contain divalent Fe, 0.5 to 1.5 atoms of Ti and close to 18 atoms of Si (full occupancy of tetrahedral sites). They are yellow, brown, red-brown, or black. Type 4 vesuvianites are manganoan or cuprian; the studied samples are from Pajsberg, Sweden, Franklin, New Jersey, and Telemark, Norway. In color they are red-brown, purple, blue (“cyprine”), or green. Iron and much of the Mn is apparently trivalent.

Hydrogen site distribution in the alloy system Nb 100 − xV xH y studied by neutron vibrational spectroscopy

Nb 100 − x V x H y is a random b.c.c. alloy with a hydrogen sublattice disordered with respect to site energies. We have determined the energetically most favourable hydrogen sites by means of neutron vibrational spectroscopy, which is a powerful tool for this purpose when the translational symmetry is strongly reduced. In Nb 95V 5H y , Nb 90V 10H y and Nb 50V 50H y both octahedral and tetrahedral site occupation was observed. On the basis of spectroscopic and statistical arguments we assign the low temperature vibrational peaks in the niobium-rich NbVH alloys at low hydrogen content to Nb 4V 2 octahedral and NbV 3 tetrahedral sites.

Electron spectroscopic studies of Mo- and Pt-Modified γ-Al 20 3 model catalysts

Model catalysts based on γ-Al 20 3, prepared by oxidizing Al foils and thin films, are chemically modified by Mo and Pt. Electron spectroscopies reveal the chemical states of Mo and Pt depending on the treatment in oxidizing, reducing, and sulfidizing atmospheres. We find Mo to exist in the oxidation states 4+, 5+, and 6+, whereas Pt appears as Pt 0 and Pt 2+. For Pt-modified films agglomerates of a mixed Pt-AI oxide grow beside large areas which are free of Pt. The surface morphology of the Mo-modified films is found to depend on the Mo concentration. For the reduced and sulfided state a phase transition due to concentration-dependent occupation of tetrahedral and octahedral aluminum defect sites by Mo is followed, whereas at low Mo concentrations a homogeneous distribution can be deduced.

Hydrogen site distribution in the alloy system Nb100−xVxHy studied by neutron vibrational spectroscopy

Nb100 − xVxHy is a random b.c.c. alloy with a hydrogen sublattice disordered with respect to site energies. We have determined the energetically most favourable hydrogen sites by means of neutron vibrational spectroscopy, which is a powerful tool for this purpose when the translational symmetry is strongly reduced. In Nb95V5Hy, Nb90V10Hy and Nb50V50Hy both octahedral and tetrahedral site occupation was observed. On the basis of spectroscopic and statistical arguments we assign the low temperature vibrational peaks in the niobium-rich NbVH alloys at low hydrogen content to Nb4V2 octahedral and NbV3 tetrahedral sites.

ChemInform Abstract: Neutron Diffraction Studies of Li2CrCl4.

AbstractAs shown by neutron diffraction studies at 350 °C, the cubic HT polymorph of Li2CrCl4, prepared by melting together the anhydrous binary chlorides, crystallizes in the space group Fd3m with Z = 8. Its inverse spinel structure shows a large Frenkel disorder of the tetrahedrally coordinated lithium ions (incomplete occupation of tetrahedral 8a sites (24%) and very strong population of the interstitial octahedral 16c sites (38%)).

Variation in sericite compositions from fracture zones within the Coso Hot Springs geothermal system

Two types of white micas are found in drillhole samples within the geothermal system at Coso Hot Springs. Low-permeability zones of the crystalline basement contain coarse-grained relict muscovite, whereas rock alteration near fracture zones at temperatures > 150°C is characterized by abundant finegrained sericite in association with secondary calcite and quartz and unaltered relict microcline. In this hydrothermal sericite there is an increase in interlayer K, octahedral Mg + Fetotal and tetrahedral Al with increasing temperature between ~215° to >250°C. Thermodynamic activity of the Al2Si4O10(OH)2 component of serielles, calculated using site-mixing approximations of Helgeson and Aagaard (1985), decreases with increasing temperature between ~215° and >250°C. As a consequence of the observed variations in the interlayer, octahedral and tetrahedral site occupancies, the activity of KAl2(AlSi3O10(OH)2 is essentially constant in the Coso seriates over this temperature range. The calculated equilibrium distribution of aqueous species in the hydrothermal solutions produced from well 16−8, together with cation-activity phase diagrams that account for variations in sericite composition, requires a pH of ~6.7–6.8 at temperatures between 236° and 250°C. Comparison of predicted and observed phase relations with fluid compositions indicate that seriates are in local equilibrium with the geothermal reservoir fluid, whereas relict metamorphic muscovites are metastable. Although the compositions of hydrothermal sericites are a complex function of temperature, pressure and geothermal fluid composition, compositional relations observed at Coso together with published compositions of hydrothermal dioctahedral layer-silicates from the Salton Sea geothermal system demonstrates that elemental compositions of interlayer K and tetrahedral Al increase systematically with increasing temperature despite the dramatic differences in fluid compositions between these two geothermal systems. This suggests that the observed variations in interlayer and tetrahedral site occupancy is largely dependent on the enthalpy of hydrolysis reactions representing equilibrium between sericite solid solutions and the geothermal reservoir fluids.

Lattice distortion in deuterium implanted niobium

Abstract Using the channeling method, implanted deuterium in niobium has been found to occupy a displaced tetrahedral site, in contrast to the normal tetrahedral site occupancy of dissolved deuterium. The comparison of experimental angular scans with computer simulations indicates that deuterium is displaced by about 0.02 nm towards the octahedral site. Moreover, the deuterium lattice site occupancy is found to depend upon the relaxation direction of the host lattice. The results are interpreted by taking into account the uniaxial distortion of the lattice parameter (up to 2%) within the implanted layer, as measured by channeling experiments.

A single-crystal neutron diffraction study of the distribution and thermal motion of silver ions in alpha- and beta- Ag 3SI

Single-crystal neutron diffraction data have been used to examine the distribution of mobile Ag + ions in anion-ordered β-Ag 3SI (space group Pm3m) and anion-disordered α-Ag 3SI (space group Im3m) at 8 temperatures in the overall range 23° to 475°C. The Ag density in the β-phase is confined to a cluster of four tetrahedral sites grouped at the centers of the cell faces, closely spaced as a result of the different distances between Ag + and the two species of anion. The individual sites are not resolved in Fourier syntheses because of large thermal displacements within a flat potential well, but the refinements permit rejection of occupancy of the octahedral site. The Ag + distribution in α-Ag 3SI displays a 2-coordinated site on the body diagonal of the cell and density delocalized in <100 > bands which display fine structure not present in α-AgI or β-Ag 2S. This feature may be qualitatively interpreted as an average over positional disorder of the equilibrium position within the tetrahedral interstice, the location depending on the short-range configuration of anions about the site. Models employing partial Ag + occupancy of the 2-coordinated position plus either octahedral or tetrahedral site occupancy involve the same number of structural parameters and refined to comparable levels. Tetrahedral site occupancy is judged to be somewhat more likely on the basis of agreement for the higher temperature data sets and physical criteria.

Ordering of aluminum in tetrahedral sites in mixed-layer 2:1 phyllosilicates by solid-state high-resolution NMR

Al and Si MAS NMR results are presented for the mixed-layer 2:1 phyllosilicate rectorite. Separate Si spectra, obtained by partially relaxed inversion-recovery experiments, for the two types of tetrahedral sheets present are consistent with one sheet being smectite-like, i.e. low aluminum occupancy of tetrahedral sites, and the other sheet being mica-like, i.e. high aluminum occupancy of tetrahedral sites. Comparison of the relative areas of the Si(0-3Al) resonances from the mica-like sheets with those predicted by computer modeling indicates a high degree of ordering of substituted aluminum. The observed areas indicate that minimization of both Al-O-Al (Loewenstein's rule) and Al-O-Si-O-Al (Dempsey's rule) interactions is important. For the tetrahedral sheets Si/Al ratios derived from Al MAS spectra indicate a much higher aluminum occupancy than determined by XRF or Si NMR.

Analysis of P-1, I-1 and C-1 plagioclase structures

The paper reports the refinement of five plagioclase structures from different geological settings. Compositions and space groups of the samples are An0 (C-1), An66 (I-1), An91 (I-1), An94 (I-1), and An94 (P-1). Emphasis is on calcic plagioclases with highly significant results for 'a', 'b', 'c', and 'd' type reflections. The structural contribution to 'b', 'c', and 'd' type superstructure reflections is investigated with Fourier analysis. The resolution of the X-ray determination of the Al, Si tetrahedral site occupancies is explored. The Na, Ca positions are refined as split sites and found to be differently occupied by Na and Ca atoms, respectively. Individual tetrahedral bond lengths are shown to be highly correlated to their structural environment. Average tetrahedral bond lengths indicate that those four tetrahedra which represent an "albite-like" structural unit in a four-ring chain extending along z in I-1 anorthite maintain their identity also at intermediate compositions while the other four non-equivalent tetrahedra gradually change their Al content from an "anorthite-like" to an "albite-like" ordering pattern.

The positive muon as a tracer for the study of dynamic correlation effects in metal hydrogen systems

In order to investigate whether the positive muon (μ+) can be used as a tracer in hydrogen diffusion studies, we have made what we believe to be the first systematic study of muon–hydrogen correlations in a metal hydride (NbHx) over a large range of hydrogen concentrations. The following observations were made from transverse field μSR experiments: (i) Loading with hydrogen has a strong impact on muon mobility in Nb, and shifts the motional narrowing region from 60 to above 170 K; (ii) in β-NbHx, the inverse muon correlation times τ−1μ are of the same order as the hydrogen jump rates τ−1H. However, the activation energies are distinctly different (EHa≂230 meV, Eμa≂140–150 meV). Moreover, the concentration dependence of τ−1μ exhibits strong negative deviations from a (1−c) behavior; (iii) the H/D isotope effects as measured by μ+ diffusion in β-NbH(D)x is significantly nonclassical. While the activation energy for τ−1μ is slightly larger in the deuteride, the ratio of jump rates is significantly larger than √2; (iv) at low temperature the μ+ mobility exhibits an irregular concentration dependence which can be associated with phase transitions to subhydride phases. They are characterized by an ordering of the vacancies on the H sublattice which apparently suppresses long-range muon motion; (v) single crystal experiments support tetrahedral site occupancy as expected for a muon on a substitutional site of the H sublattice. The data are interpreted with the aid of Monte Carlo calculations, where the diffusion of tracer particles in a concentrated lattice gas was simulated. There is clear evidence that repulsive muon–hydrogen interactions are an important feature of the dynamic muon–hydrogen correlations.

Proton NMR studies of hydrogen diffusion and electronic structure in crystalline and amorphous titanium-copper hydrides

The proton line shapes, spin-lattice and rotating-frame relaxation times, and Knight shifts have been measured in crystalline TiCu${\mathrm{H}}_{0.94}$, ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{1.9}$, and ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{2.63}$, and in amorphous $a$-TiCu${\mathrm{H}}_{1.4}$. The second moments of the rigid-lattice line shapes indicate that protons occupy ${\mathrm{Ti}}_{4}$ interstitial sites in crystalline TiCu${\mathrm{H}}_{0.94}$ and ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{1.9}$ while both ${\mathrm{Ti}}_{4}$ and ${\mathrm{Ti}}_{4}$${\mathrm{Cu}}_{2}$ sites are occupied in crystalline ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{2.63}$. Although a detailed determination of hydrogen site occupancy in amorphous $a$-TiCu${\mathrm{H}}_{1.4}$ was not possible, the proton second moment implies octahedral site occupancy in addition to tetrahedral site occupancy. The host-metal structure and hydrogen site occupancies strongly influence both the hydrogen-diffusion behavior and electronic properties. The proton hyperfine interactions appear to be dominated by the transferred core-polarization terms from the Ti $d$ states near the Fermi level as previously found in $\ensuremath{\gamma}$-phase $\mathrm{Ti}{\mathrm{H}}_{x}$ and other Ti-based ternary hydrides. The density of electron states at the Fermi level as monitored at the proton sites is apparently reduced in amorphous $a$-TiCu${\mathrm{H}}_{1.4}$ when compared with densities for the crystalline samples. It is suggested that this reduction may reflect a smearing of the energy bands with the elimination of long-range order in the amorphous phase. The greatly enhanced hydrogen diffusion (relative to crystalline TiCu${\mathrm{H}}_{0.94}$ and $\ensuremath{\gamma}\ensuremath{-}\mathrm{Ti}{\mathrm{H}}_{x}$) has been confirmed for amorphous $a$-TiCu${\mathrm{H}}_{1.3\ifmmode\pm\else\textpm\fi{}0.1}$. The diffusion behavior in crystalline ${\mathrm{Ti}}_{2}\mathrm{Cu}{\mathrm{H}}_{x}$ supports the view that H-atom diffusion jumps through octahedral sites have lower activation energies than the direct tetrahedral-to-tetrahedral jump paths.

Electric field gradient, anisotropic shift, tetrahedral site occupation, and atomic motion in Li xTiS 2

Electric field gradient, anisotropic shift, tetrahedral site occupation, and atomic motion in Li xTiS 2

Electric field gradient, anisotropic shift, tetrahedral site occupation, and atomic motion in Li xTiS 2

New measurements of NMR lineshape parameters and relaxation rates have been made to characterize a carefully prepared set of samples of Li x TiS 2, with x spanning the composition range from one-tenth to one. Electric quadrupole coupling constants are in general agreement with previous measurements, but display differences at the high end of the composition range and an unpredicted upturn as x tends toward zero. An anisotropic shift of the line has been discovered and its possible origins are discussed. An unexpected discontinuity in the linewidth-composition relation may be due to lithium occupation of tetrahedral sites at the level of a few percent. Lithium hopping rates derived from continuous wave NMR and pulsed NMR measurements are found to agree quite well with one another.

A comparison of the luminescence of LiAl 5O 8: Fe and LiGa 5O 8: Fe-- II. Fe 3+ in octahedral sites

The dominant flourescence of Fe 3+ in ordered LiAl 5O 8 and LiGa 5O 8 is due to tetrahedral site occupancy. We have observed a very weak flourescence in LiGa 5O 8 due to Fe 3+ in octahedral sites. At low temperatures, the flourescence shows a zero phonon line with vibronic sidebands whose presence is evident even at room temperature. On the other hand. Fe 3+ in octahedral sites of LiAl 5O 8 (in the disordered phase) exhibits a broad band flourescence at room temperature that does not narrow appreciably upon cooling. We attribute this effect to inhomogeneous site broadening.

Electronic structure of metal hydrides. V.x-dependent properties ofLaHx (1.9&lt;~x&lt;~2.9)andNdHx (2.01&lt;~x&lt;~2.27)

The composition-dependent electronic structure of fcc $\mathrm{La}{\mathrm{H}}_{x}$, $1.9l~xl~2.9$, has been studied using photoelectron spectroscopy with synchrotron radiation ($10l~\mathrm{hv}l~50 \mathrm{eV}$). Complementary optical reflectance measurements ($0.16l~\mathrm{hv}l~4 \mathrm{eV}$) have been performed for fcc $\mathrm{La}{\mathrm{H}}_{x}$, $1.9l~xl~2.9$, and $\mathrm{Nd}{\mathrm{H}}_{x}$, $2.01l~xl~2.27$. For $\mathrm{La}{\mathrm{H}}_{x}$, $x\ensuremath{\simeq}2$, the occupied $d$ bands are \ensuremath{\sim}1.5 eV wide and the hydrogen-induced bonding band is centered \ensuremath{\sim}5 eV below the Fermi level, ${E}_{F}$ (full width \ensuremath{\sim}6 eV). Hydrogen occupation of both octahedral and tetrahedral sites is revealed for $xl~2$, analogous to what has been observed for other metal dihydrides. With increasing hydrogen concentration, emission from the $d$ bands near ${E}_{F}$ decreases and the bonding band shifts to higher binding energy; the optical spectra show a red shift of interband absorption features, increased octahedral site occupation, and increased screening of a low-energy plasmon. For $\mathrm{La}{\mathrm{H}}_{x}$ samples at the upper end of the composition range, $x\ensuremath{\sim}2.9$, the photoemission spectra show very weak valence-band emission, and the optical spectra suggest semiconducting behavior. The binding energies of the ${5p}_{\frac{1}{2},\frac{3}{2}}$ cores, ${E}_{B}$, measured relative to ${E}_{F}$ are shown to increase with $x$ (the total shift is \ensuremath{\sim}0.8 eV for La \ensuremath{\rightarrow} La${\mathrm{H}}_{2}$ and 0.9 eV for La${\mathrm{H}}_{2}$ \ensuremath{\rightarrow} La${\mathrm{H}}_{3}$). Our results are compared to band calculations by Gupta and Burger and by Misemer and Harmon and to results of NMR, specific heat, and resistivity studies.