Hexagonal Layered Structure Research Articles

First-principles study of the stability of BN and C

First-principles total energy calculations are performed to study the phase stability of BN and C at low temperature. These are done using density functional theory (DFT) within the local density approximation (LDA) and the generalized gradient approximation (GGA). We find that for both materials LDA calculations predict dense cubic structures as the ground state structures, while the GGA calculations predict less dense hexagonal layered structures as the ground states. Our calculated results provide a stringent test for the validity of the functional used for the DFT calculations. Accurate experimental measurements of thermodynamic properties for BN and C at low temperature are needed to clarify the stability issues for these materials.

Plasma Treatments for the Low Temperature Crystallization of LiCoO2 Thin Films

As alternatives for the low temperature crystallization of thin films prepared by radio frequency (rf) reactive magnetron sputtering, two different plasma treatments, microwave and rf plasma treatment, were introduced for the first time. The films deposited at 350°C showed (003) preferred orientation corresponding to a hexagonal layered structure. From X-ray diffraction patterns, it was found that the microwave plasma treatment was effective for enhancing the crystallinity of the as-deposited films. However, the microwave plasma treatment did not enhance electrochemical properties at all, which was associated with the deterioration of surface conditions by the bombardment of energetic particles from the plasma. In the case of rf plasma treatment, the crystallinity improved effectively without severe surface degradation. At the same time, the films treated with rf plasma exhibited relatively good electrochemical properties in comparison with those of bulk or high temperature annealed thin films. © 2001 The Electrochemical Society. All rights reserved.

Microwave Dielectric Properties of BanLa4Ti3+nO12+3n Homologous Series

We have investigated the phase structure and microwave dielectric properties of the BanLa4Ti3+nO12+3n homologous series. In BaLa4Ti4O15 with n=1 composition, we obtained high Q characteristics at the dielectric constant εr=46, the temperature coefficient of the resonant frequency τf=-11 ppm/°C and Q·f=46000 GHz. In the BaO–La2O3–TiO2 system, BanLa4Ti3+nO12+3n compounds exist on the tie line BaTiO3–La4Ti3O12 near the tungstenbronze-type like Ba6-3xR8+2xTi18O54 (R=rare earth) solid solutions. There are three kinds of compounds: n=1, BaLa4Ti4O15; n=2, Ba2La4Ti5O18; and n=4, Ba4La4Ti7O24. These compounds have a layered hexagonal perovskite-like structure, which has a common substructure in the crystal structure. We have investigated the phase compositions and microwave dielectric properties of BanLa4Ti3+nO12+3n. With an increase of n, the dielectric constant, εr, increases and the quality factor, Q·f, decreases.

Raman spectroscopic determination of phase evolutions in LiAlxCo1−xO2 battery materials

Superior battery materials LiAlxCo1−xO2 (x = 0.0, 0.1, 0.3, 0.5, and 0.7) were synthesized using a solution-based route at various sintering temperatures (450–800 °C). In this communication, we report on the use of Raman spectroscopy to study effect of composition and sintering temperature on the resulting material. The phase evolutions in LiAlxCo1−xO2 compositions were studied using micro-Raman spectroscopy and a phase diagram is proposed based on the observations. For less Al content, the low-temperature phases of LiAlxCo1−xO2 showed Raman spectra corresponding to a monoclinic (space group C2/m) structure, while a low-temperature spinel (space group Fd3m) phase was observed for 50% or more Al in these compounds. All these compositions exhibited a layered hexagonal (space group R3m) structure when sintered above 700 °C. Raman spectra also revealed residual Co3O4 in the low-temperature forms of LiCoO2 and LiA10.01Co0.9O2.

Effect of LiOH concentration change on simultaneous preparation of LiCoO 2 film and powder by hydrothermal method

LiCoO 2 film and powder applicable to rechargeable lithium microbatteries have been directly prepared at the same time by hydrothermal treatment of a cobalt metal plate in a concentrated LiOH solution. The effect of LiOH concentration (0.1–6 M) on the structural change in film and powder has been examined using the XRD, micro-Raman and SEM analyses. It is found that all the powders with a layered hexagonal structure are precipitated by homogeneous nucleation regardless of LiOH concentration, whereas the dependence of film structure on the concentration is revealed by a selection of phase such as cobalt hydroxide species, hexagonal or cubic spinel phases formed by heterogeneous nucleation.

Neutron and X-ray powder diffraction studies of the oxynitrides SrW(O,N)3, Ba3W2(O,N)8 and Ba3Mo2(O,N)8

Oxynitrides of composition SrW(O,N)3, Ba3W2(O,N)8 and Ba3Mo2(O,N)8 have been prepared by the ammonolysis of stoichiometric mixtures of oxides and carbonates. Combined Rietveld refinements of powder neutron and powder X-ray diffraction data have allowed the structure of each to be determined. SrW(O,N)3 was found to have a cubic Perovskite structure whereas Ba3W2(O,N)8 and Ba3Mo2(O,N)8 have a layered hexagonal structure. In the hexagonal structures the oxygen and nitrogen atoms are distributed almost randomly over two sites in the unit cell producing 4-fold co-ordination to tungsten and molybdenum, a distorted octahedral co-ordination to one barium site and a 10-fold co-ordination to the other.

Dicalcium nitride, Ca2N—a 2D "excess electron" compound; synthetic routes and crystal chemistry

The subnitride Ca2N has been prepared via several synthetic routes described for the first time, including reduction of Ca3N2 and reaction of Ca metal dissolved in liquid sodium with nitrogen gas. Products have been characterised by powder X-ray and powder neutron diffraction. Crystallite morphology and compound stoichiometry have been examined by SEM/EDAX. Ca2N crystallises with a hexagonal layered structure in space group Rtext-decoration:overline3m (a = 3.62872(3) A, c = 19.0921(4) A, V = 217.72(1) A3, Z = 3, c/a = 5.26). [NCa2]+ layers composed of compressed, edge-sharing NCa6 octahedra stack along the c-axis and are separated by large 'van der Waals' gaps. The material is metallic and paramagnetic at room temperature. The c-parameter, [NCa2]+ layer thickness and interlayer gap are larger than previously reported and there is no evidence of hydride or other anion intercalation between layers.

Synthesis and characterization of LiNi1−x−yCoxMnyO2 as the cathode materials of secondary lithium batteries

Abstract LiNi1−x−yCoxMnyO2 (0≤x≤0.5, 0≤y≤0.3) were prepared by heating Ni1−x−yCoxMny(OH)2 and LiNO3 in flowing oxygen for 10 h at 550°C, followed by another heating at 750°C. The XRD patterns of LiNi1−x−yCoxMnyO2 (0≤x≤0.5, 0≤y≤0.3) samples with different x and y values show a pure phase of layered hexagonal structure. The lattice parameters a, c and the unit cell volume are found to decrease with increasing x in LiNi1−xCoxO2. The partial substitution of Ni by Co and Mn as in LiNi1−xCoxO2 and LiNi1−yCo0.2MnyO2 has a positive effect on lithium stoichiometry. However, lithium deficiency is still found after a prolonged thermal treatments (24 h). Among the doped materials synthesized, LiNi0.8Co0.2O2 and LiNi0.7Co0.2Mn0.1O2 have shown the best characteristics in terms of initial capacity and cycle life.

Liquid−Liquid Phase Transition in Confined Water: A Monte Carlo Study

We report simulations of water confined between two parallel flat (hydrophobic) walls separated by 10.95 Å. Our results are consistent with the possibility of a liquid−liquid phase transition separating two phases of different densities. The low-density liquid phase displays tetrahedral “ice-like” local ordering, while the high-density liquid phase shows indications of a hexagonal layer structure similar to the quasi-2D ice forms recently found in simulations using the TIP4P potential.

Optical investigations of mixed-phase boron nitride thin films by infrared spectroscopic ellipsometry

We focus on the application of infrared spectroscopic ellipsometry (IRSE) to simultaneously determine phase and microstructure of mixed-phase thin films such as polymorphic-polycrystalline boron nitride (BN) thin films deposited by magnetron sputtering on (100) silicon. We discuss a recently presented microstructure-dependent model for infrared optical properties of mixed-phase thin films which contain anisotropic materials. In particular, the IRSE data are sensitive to the pure or mixed hexagonal (h) and cubic (c) BN thin film layer structure, phase composition and average grain c-axis distribution of the hexagonal phase.

Chemical Vapor Deposition of Hexagonal Gallium Selenide and Telluride Films from Cubane Precursors: Understanding the Envelope of Molecular Control

Gallium selenide (GaSe) thin films have been grown at 325−370 °C by atmospheric pressure metal−organic chemical vapor deposition (AP-MOCVD) using the single-source precursors [(R)Ga(μ3-Se)]4 (R = CMe3, CEtMe2, and CEt2Me). In contrast, the growth of gallium telluride from [(R)Ga(μ3-Te)]4 is accomplished at 285−310 °C by low-pressure metal−organic chemical vapor deposition (LP-MOCVD). Characterization of the films by Auger electron spectroscopy (AES), Rutherford backscattering (RBS), and wavelength-dispersive spectroscopy (WDS) microprobe analysis shows all the films to have Ga:E (E = Se, Te) compositions of 1:1 with a low degree of impurities (C < 2%; O < 0.1%). From X-ray diffraction (XRD) and transmission electron microscopy (TEM) the GaSe and GaTe films were found to be polycrystalline hexagonal layered GaE structures, with a preferred c-axis orientation independent of substrate. For GaSe this represents the thermodynamically stable phase, while hexagonal GaTe is a metastable phase that converts to the thermodynamic monoclinic form on annealing at 500 °C. Diffraction results yield appropriate lattice parameters of a = 4.1 Å (TEM) and c = 16.38 Å (XRD) for hexagonal GaTe. Crystallite and particle size measurements reveal that the as-deposited films consist of oriented ca. 10 nm crystalline particles. The formation of these hexagonal layered structures is proposed to occur as a consequence of the fragmentation of the cubane's Ga4E4 core during deposition, and the formation of trimeric “Ga3E3” building blocks.

Structural and electronic properties of Ce overlayers and low-dimensional Pt-Ce alloys on Pt{111}

The structural, thermal, chemisorptive, and electronic properties of Ce on Pt{111} are studied by photoemission, Auger spectroscopy, scanning tunnel microscope (STM), and low-energy electron diffraction (LEED). Stranski-Krastanov-like growth of low-density Ce layers is accompanied by substantial valence charge transfer from Ce to Pt: in line with this, the measured dipole moment and polarizability of adsorbed Ce at low coverages are $7.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}\mathrm{C}\mathrm{}\mathrm{m}$ and $\ensuremath{\sim}1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}29}{\mathrm{m}}^{3},$ respectively. Pt-Ce intermixing commences at $\ensuremath{\sim}400\mathrm{K}$ and with increasing temperature a sequence of five different ordered surface alloys evolves. The symmetry, periodicities, and rotational epitaxy observed by LEED are in good accord with the STM data which reveal the true complexity of the system. The various bimetallic surface phases are based on growth of crystalline ${\mathrm{Pt}}_{5}\mathrm{Ce},$ a hexagonal layer structure consisting of alternating layers of ${\mathrm{Pt}}_{2}\mathrm{Ce}$ and Kagom\'e nets of Pt atoms. This characteristic $\mathrm{ABAB}$ layered arrangement of the surface alloys is clearly imaged, and chemisorption data permit a distinction to be made between the more reactive ${\mathrm{Pt}}_{2}\mathrm{Ce}$ layer and the less reactive Pt Kagom\'e net. Either type of layer can appear at the surface as the terminating structure, thicker films exhibiting unit mesh parameters characteristic of the bulk alloy.

Pressure-driven valence change in ternary Eu pnictides

We observed a structural phase transition with extremely anisotropic changes of the lattice parameters as a function of pressure at 2.6 GPa in EuPdP, which crystallizes in the hexagonal layered structure type. On the basis of the results of pressure-dependent x-ray diffraction experiments on the isostructural series APdP and APdAs (A = Sr or a trivalent rare-earth element) we show that the phase transition in EuPdP is accompanied by a valence change of the Eu. Strong but continuous changes of the lattice parameters with increasing pressure, which are due to increase of the Eu valence, were observed in EuNiP, EuPtP and EuPdAs, too. An estimation of the average Eu valence in these compounds leads to preferred values of the order of .

Metal-Organic Layer Magnets with and without Metallic Conduction

A distinguishing feature of molecular-based magnetic materials is that the vast majority of examples are insulators, and the microscopic magnetic moments are therefore localised. Maximum connectivity between centres carrying the moments is therefore desirable to maximise ordering temperatures, so ambidentate ligands are most effective. We have been studying the bimetallic tris-oxalato series AMFeIII(C2O4)3 from this point of view and the lecture will give an overview of results. We concentrate on organic cations A stabilising a hexagonal layer structure since the details of magnetic ordering are highly sensitive to changes in the packing of A between the layers. In contrast when A = BEDT-TTF the organic layer is conducting and in one case superconductivity has been found.

Temperature induced valence instabilities in ternary Eu-pnictides: a comprehensive view

We report on measurements of the lattice constants, magnetic susceptibility, LIII X-ray absorption and Mossbauer-effect on EuNiP and EuPdP, which crystallize in the hexagonal layered Ni2In structure. In both compounds the Eu valence above 510 K is 2.33. With decreasing temperature they show one (EuPdP) or two (EuNiP) first order phase transitions with a valence increase of about 0.16. At the same time thec-axis shrinks while the a-axis even increases. From Mossbauer measurements we show that the nature of the valence mixing is static. In contrast, the valence mixing in isostructural EuPtP is static at low temperatures, too, but it becomes homogeneous valent above a first order phase transition at 235 K. The behaviour of these compounds (as well as that of EuPdAs) is explained in a new model of electrostatically charged layers. In this model we can explain the temperature dependence of the lattice constants, the static valence mixing and the occurrence of preferred valences of the order 2 6/n. Together with the compression shift model of Hirst we can also understand the mechanism of the phase transitions. A comparison with EuT2Si2 compounds in ThCr2Si2 structure shows that in EuTX compounds only the electronic structure of the transition elements is relevant for the occurrence of mixed valency.

EXAFS study of graphite intercalated with metal chlorides

Synchrotron EXAFS spectra of transition metal chlorides intercalated into a graphite matrix (1) Gr ∗CrCl 3, (2) Gr ∗FeCl 3, (3) Gr ∗CoCl 2, (4) Gr ∗NiCl 2, (5) Gr ∗CuCl 2 and (6) chemically reduced Gr ∗Cr, and X-ray powder data confirm the structure of hexagonal layers of the ‘guest’ chlorides stacked in the graphite matrix. The two nearest coordination spheres of the metal atoms in (1)–(5) corresponding to MCl (2.32–2.46 Å) and M … M distances (3.3–3.6 Å) agree well with the crystal structures of pure MCl n . In (6), zerovalent Cr atoms are probably η 6-coordinated to graphite hexagons; no CrCr contacts were observed.

Temperature and pressure driven valence change in ternary Eu-pnictides

We report on measurements of lattice constants as a function of pressure and temperature on EuPdAs and EuPtP (EuTX), which crystallize in the hexagonal layered Ni 2In structure. Below room temperature the valence changes dramatically ( Δν ≅ 0.16) at one (EuPdAs) or two (EuPtP) valence phase transitions accompanied by a giant collapse of the c-axis, while the a-axis increases slightly. The mean Eu-valence, which is temperature independent above the phase transitions, is ν ≅ 2.16. Pressure dependent measurements on EuPtP and LaPdAs show larger compressibilities in c- than in a-direction due to the anisotropic, layered structure of the compounds. A strong decrease of the volume (5%) from 0 to 1.5 GPa indicates (at least) one phase transition below 1.5 GPa. Furthermore we present a model of electrostatically charged layers that may explain the observed, unusual behaviour of the lattice constants phenomenologically.

In situ growth of layered, spinel, and rock-salt LiCoO2 by laser ablation deposition

A study is reported of the in situ growth of three distinct phases of LiCoO2 by laser ablation deposition on heated substrates in an oxygen background. Films were characterized by x-ray diffraction from which crystal structure, crystal orientation, lattice constants, and phase information were obtained. Electron microscopy was used to investigate crystal grain size and overall film morphology. For deposition under 2000 mTorr O2, substrate temperatures of 22–250 °C resulted in a rock-salt structure, 300–450 °C produced a modified spinel structure (low-temperature LiCoO2), and 680 °C gave a hexagonal layered structure (high-temperature LiCoO2). Growth at 500 and 550 °C produced mixed-phase spinel and layered LiCoO2; however, at 550 °C, reducing the O2 pressure to 100 mTorr resulted in single-phase high-temperature LiCoO2. In the case of the rock-salt phase, modified film stoichiometries of composition Li0.5Co1.5O2 and Li0.75Co1.25O2 were also produced by using ablation targets of the same composition and growing at 150 °C under 200 mTorr O2. Average crystal grain size for all films was found to increase with growth temperature. Ordering of the cations in rock-salt films by heating was unsuccessful.

Electronic structure and crystal chemistry of niobium oxide phases.

Results of local-density linear muffin-tin orbital (LMTO) electronic-structure calculations for both known and recently obtained niobium oxide phases with different crystal lattices and different coordination of Nb atoms are presented. Perovskite-like compounds with one- or two-dimensional Nb-O condensed clusters, as well as hexagonal layered structures with trigonal-prismatic coordination of Nb atoms, are used. For ${\mathrm{SrNbO}}_{3}$, ${\mathrm{BaNb}}_{5}$${\mathrm{O}}_{8}$, ${\mathrm{BaNb}}_{4}$${\mathrm{O}}_{6}$, and ${\mathrm{Sr}}_{2}$${\mathrm{Nb}}_{5}$${\mathrm{O}}_{9}$, the Fermi level is located at the shoulder of the Nb d conduction band with a low density of oxygen states. For hexagonal ${\mathit{M}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ (M=Li,Na) compounds, oxygen contributions definitely appear at ${\mathit{E}}_{\mathit{F}}$, but remain small compared with those of the high-${\mathit{T}}_{\mathit{c}}$ superconducting cuprates. Pair-interaction energies and superexchange-interaction parameters were calculated using the LMTO Green-function method. Typical for high-${\mathit{T}}_{\mathit{c}}$ superconductors, antiferromagnetic coupling between metal atoms in the Nb-O planes is shown to exist, but not the ferromagnetic exchange between metal d and oxygen p states. The calculations show that perovskite niobium oxide systems are not good candidates in the search for new high-${\mathit{T}}_{\mathit{c}}$ superconducting materials, but that hexagonal layered niobium oxide phases of the type considered in this paper might be of interest in studies of low ${\mathit{T}}_{\mathit{c}}$ superconducting oxides.

Photo-induced phase inhibition during growth of boron nitride thin films

Boron nitride exists in three polymorphic forms which are thermally stable to decomposition, chemically inert and possess a wide band gap. Deposition of thin films containing various BN phases has been achieved using gas source molecular beam epitaxy in tandem with a 500 W mercury arc lamp. Thermal evaporation of elemental boron from an effusion cell and the decomposition of molecular nitrogen in an electron cyclotron resonance plasma provided the reactive species for film growth at 400 and 500 °C on single-crystal substrates of Si (100). The films contained both amorphous and hexagonal (layered structure) BN without the use of the arc lamp. Films growth with the arc lamp showed a dramatic decrease in the concentration of the hexagonal phase and evidence of an increase in the concentration of the cubic BN.