Atoms In Tetrahedral Sites Research Articles

Placement of H atoms in the crystal lattice of cubic titanium oxyhydride: Simulation and diffraction experiment

Possible models for the arrangement of hydrogen atoms in the sites of the cubic lattice of titanium oxyhydride TiOyHp with vacancies in the metallic and nonmetallic sublattices are considered for the first time. It has been established that interstitial H atoms in oxyhydrides occupy vacant octahedral sites 4(b) of the oxygen sublattice. No displacement of H atoms in tetrahedral sites 8(c) is observed.

Improved phase change properties in layered ScxIn2−xSe3 for multilevel information storage

Phase change random access memory stores information by utilizing large resistance differences between crystalline and amorphous states of phase change materials (PCM). It would be highly advantageous to realize multilevel devices with single PCM system in nanoscale to improve memory capacity and reduce power consumption. Experimental results verify the possibility of multilevel phase change in two dimensional In2Se3 thin layers with two stable single crystal phases (α and β). Here, we report, the phase change properties of layered In2Se3 can be greatly improved by substitutional doping with Sc atoms based on first-principle calculations together with semiclassical Boltzmann transport theory. The lattice thermal conductivity of both α- and β-ScxIn2−xSe3 are lower than that of the undoped models in the temperature range from 300 K to 700 K. Particularly, of β-Sc0.167In1.833Se3 at 300 K is 0.53 W m−1 K−1, which is reduced by 60% compared to β-In2Se3. The out-of-plane electrical conductivities also show a significant decrease after Sc doping. These results indicate better performance of Sc-doped In2Se3 PCM with minimized thermal crosstalk and lower RESET current. Moreover, we found the resistance difference between α and β phase can become 1–2 orders of magnitude larger in Sc0.167In1.833Se3, which is beneficial for the multilevel programming. The phase change process from α-In2Se3 to its β phase with different Sc doping sites are calculated by ab-initio molecular dynamics simulations. It is found that with the introduction of Sc atoms in tetrahedral sites, phase change speed can be significantly increased. Our studies shed light on the modification of phase change properties of layered In2Se3 with rare earth doping.

Quaternary Chalcogenides La3Sn0.5InS7 and La3Sn0.5InSe7

The quaternary chalcogenides La3Sn0.5InS7 and La3Sn0.5InSe7 were prepared by reactions of the elements at 1050 °C and 950 °C, respectively. They adopt noncentrosymmetric structures [hexagonal, space group P63, Z = 2; a = 10.2993(11) Å, c = 6.0921(6) Å for La3Sn0.5InS7; a = 10.6533(7) Å, c = 6.4245(4) Å for La3Sn0.5InSe7] in which the half‐occupancy of Sn atoms within octahedral sites classifies them as belonging to the La3Mn0.5SiS7‐type branch of the large family of quaternary rare‐earth chalcogenides RE3M1–xM′Ch7. The site distribution in La3Sn0.5InCh7, with higher‐valent Sn atoms occupying octahedral instead of tetrahedral sites, is reversed from the typical situation observed in other RE3M1–xM′Ch7 compounds. The ordered distribution of Sn atoms in octahedral sites and In atoms in tetrahedral sites was evaluated by bond valence sum analyses. Moreover, 119Sn solid‐state nuclear magnetic resonance (NMR) spectroscopy confirms the occupation of Sn4+ species exclusively within octahedral sites. An optical bandgap of 1.45 eV was found for La3Sn0.5InS7. Band structure calculations on an ordered superstructure model of La3Sn0.5InS7 reveal that avoidance of strongly Sn–S antibonding levels is an important driving force for the Sn deficiency.

Oxygen defects in GaAs: A hybrid functional study

Using hybrid density functional calculations, we address the structural properties, formation energies, and charge transition levels of a variety of oxygen defects in GaAs. The set of considered defects comprises the bridging O atom in a As-O-Ga configuration, interstitial O atoms in tetrahedral sites, and O atoms substitutional to either Ga (O-Ga) or As atoms (O-As). In addition, we consider an As vacancy containing two O atoms, for which the most stable configurations are found through the use of molecular dynamics simulations, and defect complexes involving a O-As defect bound to either one or two As-Ga antisites, denoted As-Ga-O-As and (As-Ga)(2)-O-As, respectively. We find that the bridging O defect and the As-Ga-O-As and (As-Ga)(2)-O-As complexes are the most stable oxygen defects in GaAs. The actual occurrence of these defects is examined against two criteria. The first criterion concerns the stability against O dissociation and is evaluated via the calculation of dissociation energies. The second criterion involves the defect formation at thermodynamic equilibrium and is inferred from the comparison between the formation energy of the oxygen defect and that of its O-related dissociation product (bridging O defect). Both the As-Ga-O-As and (As-Ga)(2)-O-As complexes satisfy these criteria and are stable against O dissociation. Further analysis in cooled-down conditions leads us to dismiss the As-Ga-O-As defect due to the more favorable bonding of two rather than one As-Ga antisites. The conclusion that only the bridging O defect and the (As-Ga)(2)-O-As complex are expected to occur is in accord with experimental observations.

Formation of lithium clusters and their effects on conductivity in diamond: A density functional theory study

We have applied Density Functional Theory (DFT) within the Generalized Gradient Approximation to study the behavior of lithium impurities in the diamond lattice. It was found that, although isolated interstitial lithium atoms in tetrahedral sites have a donor state relatively close to the conduction band at 0.35 eV, their behavior is critically affected by adjacent interstitials. Lithium atoms that occupy adjacent interstitial tetrahedral sites cluster spontaneously and induce carbon–carbon bond breaking in their neighborhood. The corresponding charge distribution and electronic density of states analyses show that charge localization takes place at the carbon atoms around the lithium clusters and deeper mid-gap states are introduced in the diamond band gap. The formation energy was found to be 1.88 eV and 3.09 eV, for clusters of two and three lithium atoms, respectively, considered in this study. The calculations also indicate that Li clustering would not take place if Li ions instead of atoms were incorporated interstitially.

Hierarchical aluminosilicate macrochannels with structured mesoporous walls: Towards a single catalyst for multistep reactions

Two novel and simple synthesis pathways, one surfactant-assisted and one template-free, have recently been developed for the formation of aluminosilicate macrochannels with mesoporous walls. These thermally stable materials are made of straight tubular macrochannels separated by wormhole-like mesopores. This paper gives a comparative study of the materials obtained by both ways in order to determine the role played by the surfactant. High specific surface area materials with homogeneous tubular macrochannels are spontaneously formed without needing any templating procedure. The use of a non-ionic surfactant favors the incorporation of Al atoms in tetrahedral sites and also controls the homogeneity of the mesopores. The formation of the macrochannels is believed to result from the spontaneous and strong release of alcohol molecules due to high hydrolysis and condensation rates of the inorganic sources, which creates a hydrodynamic flow, thus channels of the solvent. The mesopores are generated by interparticular voids.

The Structure of Cu0.38NbSe2 An Orthorhombic Superstructure of a Hexagonal Basic Structure Occurring in Three Orientation Variants

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Darstellung und Kristallstruktur der Hochdruckphase AgAlS2-II / Preparation and Crystal Structure of the High-pressure Phase AgAlS2-II

AgAlS2-I with chalcopyrite structure transforms at 25 kbar and 300°C to the new highpressure phase AgAlS2-II. The crystal structure of AgAlS2-II is trigonal, space group P3ml, with a= 3,50 Å, c = 6,84 Å and Z = 1. The structure is based on a hexagonally close packed arrangement of sulfur atoms with aluminium atoms in octahedral sites and silver atoms in tetrahedral sites. The AgS4-tetrahedra are considerably distorted, giving a coordination number 3 + 1 for the silver atoms.