Phase transition in layered perovskite-like manganate Ca3Mn2O7 under high pressure

In situ high pressure energy dispersive X-ray diffraction measurements on the layered perovskite-like manganate Ca₃Mn₂O₇ powder under pressures were performed by using the diamond anvil cell with synchrotron radiation. The results show that the structure of layered perovskite-like manganate Ca₃Mn₂O₇ is unstable under pressure due to the easy compression of NaCl-type blocks. The structure of Ca₃Mn₂O₇ underwent two phase transitions under pressures in the range of 0–35 GPa. One was at about 1.3 GPa with the crystal structure changing from tetragonal to orthorhombic. The other was at about 9.5 GPa with the crystal structure changing from orthorhombic back to another tetragonal.     

Effects of Nb and Si on densities of valence electrons in bulk and defects of Fe3Al alloys

Positron lifetime measurements have been performed in binary Fe₃Al and Fe₃Al doping with Nb or Si alloys. The densities of valence electrons of the bulk and microdefects in all tested samples have been calculated by using the positron lifetime parameters. Density of valence electron is low in the bulk of Fe₃Al alloy. It indicates that, the 3d electrons in a Fe atom have strong-localized properties and tend to form covalent bonds with Al atoms, and the bonding nature in Fe₃Al is a mixture of metallic and covalent bonds. The density of valence electron is very low in the defects of Fe₃Al grain boundary, which makes the bonding cohesion in grain boundary quite weak. The addition of Si to Fe₃Al gives rise to the decrease of the densities of valence electrons in the bulk and the grain boundary thus the metallic bonding cohesion. This makes the alloy more brittle. The addition of Nb to Fe₃Al results in the decrease of the ordering energy of the alloy and increases the density of valence electron and the bonding cohesion of the grain boundary. However, since the radius of Nb atom is larger than that of Fe atom, when Nb atoms substitute for Fe atoms, they will distort the lattice and enlarge the volume of the lattice, which decreases the density of valence electron and the cohesion of metallic bond in the bulk of the alloy. Project supported by the National Natural Science Foundation of China (Grant No. 59561001) and the Foundation of Guangxi Education Committee.     

Microstructure of a-SiOx:H

A set of a-SiO_x:H (0.52 <x< 1.58) films are fabricated by plasma-enhanced-chemical-vapor-deposition (PECVD) method at the substrate temperature of 250°C. The microstructure and local bonding configurations of the films are investigated in detail using micro-Raman scattering, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). It is found that the films are structural inhomogeneous, with five phases of Si, Si₂O:H, SiO:H, Si₂O₃:H and SiO₂ that coexist. The phase of Si is composed of nonhydrogenated amorphous silicon (a-Si) clusters that are spatially isolated. The average size of the clusters decreases with the increasing oxygen concentration x in the films. The results indicate that the structure of the present films can be described by a multi-shell model, which suggests that a-Si cluster is surrounded in turn by the subshells of Si₂O:H, SiO:H, Si₂O₃:H, and SiO₂.     

Synchrotron radiation photoemission spectrum study on K3C60 film

K₃C₆₀, single crystal film was prepared on the cleaved (111) surface of C₆₀, single crystal. Synchrotron radiation angle-resolved photoemission spectra were measured at normal emission with sample temperature at × 150K. Up to four subpeaks of LUMO-derived band were observed. These sub-peaks exhibit distinct energy dispersions which resemble in general the theoretical ones calculated for K₃C₆₀ at low temperature with the so-called one-dimensional disordered structure. But there is large deviation of experimental sub-band intervals from the theoretical values. This result is meaningful for the studies of the physical properties of alkali-doped C₆₀ solids, e.g. the mechanism for superconductivity.     

Fibrous Filler-Matrix Interaction in Oxide Composites

The interaction between a fibrous alumina filler and a hydrated zirconia matrix during the fabrication of ceramic composites is studied. The influence of the crystal structure and porosity of the filler on the properties of the ZrO₂-Al₂O₃ composite material is investigated. It is found that, as a result of addition of alumina fibers of cubic and monoclinic structures (- and -phases), the ZrO₂ hydrogel interacts with fibers and solid ZrO₂ solutions of tetragonal structure are formed. These solutions exist over the range of 600 to 1200°C and, at annealing above 1200°C, due to crystallographic transitions the of - and -phases, they break down, namely Al₂O₃ transforms into -corundum and ZrO₂ into a monoclinic modification. However, the material does not fail and a stable porous ceramic composite is formed. To manufacture a composite with improved mechanical characteristics, fibrous alumina fillers with the structure of -corundum must be used.     

Heating effect of BaTiO3 in microwave field and microstructure of BaTiO3

Microwave equipment at 2 450 MHz was employed to prepare BaTiO₃. The heating effect of the system in the microwave field, which was influenced by several factors including dielectric properties of synthesis system and thermal insulate structures, was discussed in detail. The heating rates of the synthesis system were mainly determined by BaCO₃ and TiO₂ at low temperature and by TiO₂ and BaTiO₃ at high temperature. The results show that the heating effects in microwave field are greatly different from those in conventional furnace. The reaction of BaCO₃ and TiO₂ only lasts for 3 min at 1 100°C, and the fine, narrow-distributed and well-crystallized powders were prepared. Project supported by the National Natural Science Foundation of China.     

Electronic structure of ZnO and its defects

The electronic structure of ZnO and its native point defects has been calculated using full potential linear Muffin-tin orbital ( FP-LMTO) method for the first time. The results show that Zn3d electrons play an important role in the bonding of ZnO. Vacant Zn (V_zn) and interstitial O (Oi) produce the shallow acceptor levels at 0.3 eV and 0.4 eV above the top of the valence band (VB), while interstitial Zn (Zni) produces a shallow donor level at 0.5 eV bellow the bottom of the conduction band (CB). However, Vacant O (Vo) produces a deep donor level at 1.3 eV below the bottom of CB. On the basis of these results, we confirm that Zni is the main factor to induce the native n-type conductivity in ZnO     

Infrared absorption spectra of some lanthanide acetylacetonate complexes

The infrared absorption spectra of 12 lanthanide acetylacetonate complexes were measured in the region 400–2000 cm^?1 and discussed. Assignments of the bands especially those due to metal-oxygen (M—O), C=O and C=C stretching vibration is given. It is found that the band at 530 ± 5 cm^?1 is due mainly to Ln—O stretching vibration. In the carbonyl region, it is confirmed that the band at lower frequency is a C=C stretching vibration while that at higher frequency is due to C=O stretching vibration. Integral intensities for the M—O, C=O and C=C stretching vibrations were calculated in KB₁ and CHCl₃ solutions. The variation of the intensities of the M—O bands witn complexed cation were discussed in terms of crystal field stabilization energy (CFSE).     

Preparation and third-order nonlinear optical properties of self-organized complex oxide Ce: BaTiO3 quantum dots

Pulsed laser ablation epitaxial Strancky-Krastanow growth of self-organized complex oxide Ce:BaTiO₃ quantum dots on MgO(lOO) substrates is demonstrated. Atomic force microscopy and X-ray diffraction are used to observe the structure and the growth process of the self-organized complex oxide quantum dots. The average dimension, height, and the density of self-organized complex oxide Ce:BaTiO₃ quantum dots are given. The nonlinear refractive indexes of the wetting layer and the selforganized ordered quantum dots are determined by the single beam Z-scan method. The mechanisms of the nonlinear effect enhancement for these low-dimensional structure are discussed.     

Magnetovolume and chemical bonding effects of Ni atom in γ’-(Fe1- x Ni x )4N compounds

By X-ray diffraction and high pressure Mossbauer spectroscopy, the structure and the hyperfine parameters of Ni substituted γ’-Fe₄N were investigated. The results of X-ray diffraction indicate that single phase γ’-(Fe₁-_xNi_x)₄N compounds can be prepared in the composition range of 0⩽x⩽0.6, and with the increase of Ni content the lattice parameter is fit for the relationshipa ₀(x)=3.790 5-0.021 57x-0.031 67x ². By high pressure Mossbauer spectra, effects of magnetovolume and chemical bonding of Ni atom on hyperfine magnetic field and isomer shift of iron were distinguished for the first time, and their composition dependences for different lattice sites were studied simultaneously. It is found that the magnetovolume and chemical bonding have different influences on the properties of γ’-(Fe₁-_xNi_x)₄N, and the latter one plays a key role in the property changes of γ’-(Fe₁-_xNi_x)₄N. Project supported by the National Natural Science Foundation of China, the Natural Science Fund of Gansu Province and the Postdoctoral Fund of China.     

Laser molecular-beam epitaxy and second-order optical nonlinearity of BaTiO3/SrTiO3 superlattices

A series of c-axis oriented BaTiO₃/SrTiO₃ superlattices with the atomic-scale precision were epitaxially grown on single-crystal SrTiO₃(l00) substrates using laser molecular-beam epitaxy (LMBE). A periodic modulation of the intensity of reflection high-energy electron diffraction (RHEED) in BaTiO₃ and SrTiO₃ layers was observed and attributed to the lattice-misfit-induced periodic variation of the terrace density in film surface. The relationship between the second-order nonlinear optical susceptibilities and the superlattice structure was systematically studied. The experimental and theoretical fitting results indicate that the second-order nonlinear optical susceptibilities of BaTiO₃/SrTiO₃ superlattices were greatly enhanced with the maximum value being more than one order of magnitude larger than that of bulk BaTiO₃ crystal. The mechanism of the enhancement of the second-order optical nonlinearity was discussed by taking into account the stress-induced lattice distortion and polarization enhancement.     

Crystal structure of zirconia by Rietveld refinement

The crystal structures and phase transformation of zirconia ceramics have been investigated by means of X-ray powder diffraction and Rietveld powder diffraction profile fitting technique. A structural transition from monoclinic to tetragonal occurs when Y₂O₃ and₂ are doped into zirconia. The space group of the tetragonal structure is P4₂/nmc,Z=2. The lattice parameters are α=0.362 6(5)nm,c=0.522 6(3)nm for CeO₂ doped zirconia and α=0.360 2(8)nm,=0.517 9(1)nm for Y₂O₃ doped zirconia, respectively. In each unit cell, there are two kinds of equivalent positions, i.e. 2b and 4d, which are occupied by Zr⁴⁺, M(M=Y³⁺, Ce⁴⁺) cations and O^2- anions, respectively. The crystallographic correlation among the cubic, tetragonal and monoclinic structures of ZrO₂ is discussed.     

Microstructure of a-SiOx:H

A set of a-SiO_x:H (0.52 <x< 1.58) films are fabricated by plasma-enhanced-chemical-vapor-deposition (PECVD) method at the substrate temperature of 250°C. The microstructure and local bonding configurations of the films are investigated in detail using micro-Raman scattering, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). It is found that the films are structural inhomogeneous, with five phases of Si, Si₂O:H, SiO:H, Si₂O₃:H and SiO₂ that coexist. The phase of Si is composed of nonhydrogenated amorphous silicon (a-Si) clusters that are spatially isolated. The average size of the clusters decreases with the increasing oxygen concentration x in the films. The results indicate that the structure of the present films can be described by a multi-shell model, which suggests that a-Si cluster is surrounded in turn by the subshells of Si₂O:H, SiO:H, Si₂O₃:H, and SiO₂.     

Liquid structure of undercooled Cu70Ni30 alloy

Experiments of X-ray diffraction for liquid Cu₇₀Ni₃₀ alloy above and below its liquidus (l 230°C) have been carried out. By the analysis of experimental results, it is discovered that difference between structures of liquid and undercooled Cu₇₀Ni₃₀ alloy is their cluster sizes. The correlation radius of cluster is 1.125 nm and the atom number of cluster is 403 at l 250—l 400°C, and they are 1.3 nm and 704 respectively at the undercooled liquid state (1 200°C). The structure of liquid Cu₇₀Ni₃₀ alloy is fcc short order and its solid structure, fcc, is kept from liquid fcc short order.     

Electron paramagnetic resonance of Mn2+ in NaF single crystals

The electron paramagnetic resonance of Mn²⁺ in NaF single crystals is investigated at different temperatures from 573° K to 93° K. Four different spectra designated as I, II, III₂ and III₄ are observed. Spectrum I consists of a single broad resonance corresponding to precipitated Mn²⁺ ions. Spectrum II is isotropic and centred nearg = 2.00. This spectrum corresponds to substitutional Mn²⁺ ions with remote charge compensating sites and therefore with local cubic symmetry. Spectrum III₂ withg = 2.014 and spectrum III₄ withg = 1.995 are also due to substitutional Mn²⁺ ions but subjected to tetragonal crystalline fields and are the same as those reported by earlier workers. Superhyperfine structure has been observed in spectra II, III₂ and III₄. The analysis of that structure in spectra II and III₂ has been carried out for the first time and the constants A_s, and A_σ are given. The spectra are analysed by the usual spin-Hamiltonian method.     

Interface interaction and inter-osmosis effect of Fe x (SiO2)1-x nanocomposite materials on magnetic properties

Fe _x (SiO₂)_1-x nanocomposites prepared by using mechanical alloying method were reported. The microstructure character and magnetic properties of Fe _x (SiO₂)_1-x nanocomposite samples with different Fe content and different ball milling time were studied by using X-ray diffraction (XRD), transmission electron microscopy (TEM), Mössbauer spectroscopy, and Faraday magnetic balance in a wide temperature range. The results indicate that the rnicrostructure and magnetic properties are closely related to ball milling time and Fe content. When Fe content is less than 20 wt%, the sample after 80-h ball milling has very complex microstructure. Small α-Fe grains and Fe cluster are implanted in SiO₂ matrix. And there are not only isolated α-Fe granular and Fe cluster, but also nanometer scaled sandwich network-like structure. Fe _x (SiO₂)_1-x nanocomposite samples display a rich variety of physical and chemical properties as a result of their unique nanostructure, strong interface interaction and inter-osmosis effect in Fe-SiO₂ boundaries, and the grain size effect.     

Resonance spectrum for one-dimensional layered media

We consider the “weighted” operator P_k=????_x a(x)? _x on the real line with a step-like coefficient which appears when propagation of waves through a finite slab of a periodic medium is studied. The medium is transparent at certain resonant frequencies which are related to the complex resonance spectrum of P_k. If the coefficient is periodic on a finite interval (locally periodic) with k identical cells, then the resonance spectrum of P_k has band structure. In the article, we study a transition to semi-infinite medium by taking the limit k→?∞?. The bands of resonances in the complex lower half plane are localized below the band spectrum of the corresponding periodic problem (k=∞) with k???1 or k resonances in each band. We prove that as k→?∞?, the resonance spectrum converges to the real axis.     

Phonon spectroscopy and transport properties of Y1-x Nd x Sr2Cu2.7Mo0.3O7-δ

The Raman and infrared spectra, resistivity as well as thermoelectric power of Y_1-xNd_xSr₂Cu_2.7Mo_0.3 O₇ δ(.x =0, 0.1. 0.2. 0.5, 0.8 and 1.0) are studied carefully. It was found that the structure characteristics of RSr₂Cu_2.7Mo_0.3O₇ δ cuprates are different from those of RBa₂Cu₃0₇ δ. The variations of the microstructure in Y_1-x Nd_xSr₂Cu_2.7Mo_0.3O₇ δ with Nd-doping affect the carrier distribution. so as a result the superconductivity changes. Furthermore, it is pointed out that the microstructure variations with the large rare earth ionic substitution cause the widely existing rare earth ionic size effect in HTSC. Project supported by the National Natural Science Foundation of China and the National Center for R & D Superconductivity.     

X-ray spectroscopic study of chemical bonding in MnSe2 and CoSe2

X-ray absorption study of two intermetallic compounds MnSe₂ and CoSe₂ has been carried out using a Cauchois type bent crystal spectrograph. The metal K absorption edges in both the compounds are found to shift toward the high energy side with respect to the discontinuities in the pure metals. On the other hand, the Se K absorption edge in both these compounds is found to shift toward the lower energies. Emission study of the compound MnSe₂ has shown that the Mn Kβ ₅ band in this compound is shifted toward the high energy side with respect to that in the pure metal. From the magnetic data and the results obtained in this work it is possible to obtain the chemical bonding pictures in these compounds. For MnSe₂ the bondings sp ³ d ² for the metal atom andsp ³ for the metalloid atom have been suggested. For CoSe₂ the bondings appear to bed ² sp ³ for the metal atom andsp ³ for the metalloid atom. These bondings are compatible with the pyrite type structure of these compounds. It is possible to explain the electrical behaviour of the compounds on the basis of these bonding pictures.     

Synthesis and characterization of C3N4 hard films

C₃N₄ films have been synthesized on both Si and Ft substrates by microwave plasma chemical vapor deposition (MPCVD) method. X-ray spectra were calculated for single phase α-C₃N₄ and β-C₃N₄ respectively. The experimental X-ray spectra of films deposited on both Si and Pt substrates showed all the strong peaks of α-C₃N₄ and β-C₃N₄ so the films are mixtures of α-C₃N₄ and β-C₃N₄. The N/C atomic ratio is in the range of 1.0–2.0. X-ray photoelectron spectroscopy (XPS) analysis indicated that the binding energy of Is and N ls are 286.2 eV and 399.5 eV respectively, corresponding to polarized C-N bond. Fourier transform infrared absorption (FT-IR) and Raman spectra support the existence of C-N covalent bond in the films. Nano-indentation hardness tests showed that the bulk modulus of a film deposited on Pt is up to 349 GPa