Reduction In Lattice Parameter Research Articles

Carbon and germanium distributions in Si 1− x− yGe xC y layers epitaxially grown on Si(001) by RTCVD

Silicon germanium carbon with up to 1.5% of substitutional carbon can be grown epitaxially on Si(001) by rapid thermal chemical vapor deposition (RTCVD) using methylsilane as a precursor. The germanium and carbon atomic distributions are studied for a C-rich Si l− x− y Ge x C y heterostructure using high-resolution transmission electron microscopy (HREM), high-resolution X-ray diffraction and Raman spectrometry. HREM images show the formation of regularly spaced carbon-rich tilted sublattices and local germanium fluctuations. 2D reciprocal space mapping and Raman spectroscopy confirm this thin tilted sublayers formation. A new model for perpendicular lattice parameter reduction is proposed. The structure remains highly distorted and a statistical description of these strain fluctuations is exposed.

PHOTOACOUSTIC SPECTRA STUDIES OF NANOMETER SrTiO3

Photoacoustic spectroscopy has been used to study nanoscaled SrTiO3 powder with different annealing temperature. The photoacoustic spectra show that the absorption edge of nanoscaled SrTiO3 powder has red shift with the rise of annealing temperature,which demonstrates that the energy gap becomes narrow. The decrease of energy gap is related to the growth of grains and the reduction of lattice parameter. In addition,the absorption band appeared around 700nm is induced by the transition of the captured electrons at the additional energy level of oxigen defects. The variation of the band is strongly dependent upon the sizes of the grains.

The effect of Ag diffusion on properties of BiPbSrCaCuO thin films

Superconducting BiPbSrCaCuO films have been evaporated on Ag/MgO and MgO substrates (with and without Ag buffer layer) by the electron-beam technique. The influence of annealing temperature on the crystalline structure and superconducting properties of BiPbSrCaCuO films was investigated by X-ray diffraction, X-ray fluorescence spectroscopy, scanning electron microscopy, critical temperature, critical current density and room temperature resistivity measurements. It was shown that the annealing of both types of films at 835°C resulted in formation of the mixed Bi-2223 and Bi-2212 phases with a high degree of preferential orientation with the c-axis perpendicular to the substrates. Annealing of films on Ag/MgO substrates is additionally accompanied by Ag diffusion from the buffer layer into BiPbSrCaCuO films. The higher rate of crystallization of the Bi-2223 and Bi-2212 phases, the higher degree of c-axis orientation, the higher dense surface morphology, the reduced lattice parameter c (by 0.6–0.8%) the reduced room temperature resistivity (2–3 times), the significantly enhanced critical temperature ( T c=106 K at R=0) and the critical current density ( J c = 1.1 × 10 4 A/cm 2 at 77 K) were observed for the Ag-doped films (on Ag/MgO substrates), in comparison with those for the undoped films (on MgO substrates). The temperature dependence of the Ag diffusion coefficient in Bi-2212 films in the range 650–800°C was described by the relation D=2.2×10 −5 exp(−1.2/ kT).

Pressure effect on yttrium doped La0.60Y0.07Ca0.33MnO3 compound

We report the effect of hydrostatic pressure up to 8 kbar on magnetoelastic, magnetic, and transport properties in the temperature range 10–300 K. An increase of Curie temperature Tc with pressure ΔTc/Δp=2.6 K/kbar was determined from ac susceptibility measurements. The anomalies in resistivity and magnetovolume effects are significantly suppressed by pressure but the temperatures of these anomalies increased with pressure with the same slope as Tc. These results indicate that impurity state of yttrium rather than lattice parameter reduction is responsible for the large increase in the magnetoresistance observed in the doped compound.

Microstructural Effects on Electrocatalytic Oxygen Reduction Activity of Nano‐Grained Thin‐film Platinum in Acid Media

The oxygen reduction activity of thin‐film platinum deposits on carbon and its relationship to catalyst microstructure has been investigated using rotating‐disk‐electrode techniques. The thin‐film form of platinum is a viable tool for catalyst study as it can provide intrinsic activity data on finely divided carbon‐supported platinum. Sputtered polycrystalline thin‐film platinum deposits on carbon (loading from 6 to 200 μg/cm2) have been characterized, and their oxygen reduction activity at 25°C in 1M sulfuric acid determined. Catalyst characterization consisted of electrochemical surface‐area measurement and establishing grain size, morphology, and lattice parameter using transmission electron microscopy. Thin‐film platinum exhibits a reduced lattice parameter at very small grain size. Mass activity and specific activity at 0.9 V vs. reversible hydrogen electrode both decrease with increasing grain size (and decreasing specific surface area). Peak mass activity for oxygen reduction was 25 A/g of Pt, and peak specific activity was 0.070 mA/cm2 of Pt surface. The activity trends are attributed to the changes in electronic and geometric properties.

Formation and behavior of BGaVAs complex defects in gallium arsenide grown by liquid-encapsulated Czochralski method

The effect of wafer annealing on lattice parameter of liquid-encapsulated Czochralski (LEC) grown undoped semi-insulating GaAs crystals is investigated, showing that wafer annealing increases the lattice parameter from an anomalous state below that predicted by Vegard’s law. The sheet-carrier concentration of a silicon-implanted layer formed on undoped semi-insulating GaAs increases with the increase in lattice parameter caused by the wafer annealing. The reduction of BGaVAs complex defects is a reasonable explanation for the increase in lattice parameter and sheet-carrier concentration. In LEC-grown silicon-doped GaAs crystals, no anomalous reduction of lattice parameter is observed in as-grown crystals. It is speculated that the silicon introduced into the crystal from the melt fills the VAs vacancies in the GaAs crystal before BGaVAs forms. BGaVAs complex defects in undoped GaAs crystal must be formed during the post-growth cooling process.

X-ray investigations of La 2CuO 4 and Pr 2CuO 4 under high pressure

X-ray investigations of Pr 2CuO 4 and La 2CuO 4 under pressure have been carried out at 300 K and 30 K. The lattice parameters decrease monotonically for both structures, thus preserving the structure of the materials. For La 2CuO 4 the lattice parameter reduction is isotropic with increasing pressure, but the lattice parameters of Pr 2CuO 4 are found to behave anisotropic, as the c-axis decreases faster than the a-axis. La 2CuO 4 has a bulk modulus of 1850 kbar at 300 K, whereas Pr 2CuO 4 has a bulk modulus of 1600 kbar at 300 K and 1650 kbar at 30 K, due to the faster decreasing c-axis lattice parameter. In the a and b direction both materials have a similar compressibility. Assigning this behaviour to the common copper-oxide layers of both structures gives the possibility to generalize these results. For several high temperature superconductors we estimated the linear compressibilities in the a, b direction being in the range of 1.5 to 2.0×10 -4 kbar -1. In the c-direction the compressibility apparently depends on the degeneracy of the oxygen octahedra and the number of intercalated layers orstructural elements.

Anomalous reduction of lattice parameter by residual impurity boron in undoped Czochralski-grown GaAs

An anomalous reduction of lattice parameter is observed for undoped semi-insulating liquid encapsulated Czochralski-grown GaAs by precision lattice parameter measurements with the use of the bond method. The anomalous reduction is though to be due to the incorporation of residual boron from the B2O3 encapsulant. The reduction rate is about 2 times larger than the predicted rate for the boron concentration derived from Vegard’s law. Possible defect models for this phenomenon are discussed.

Structural analysis of amorphous (Fe1-x Mn x )78B22 alloys by X-ray diffraction and electrical resistance

The structure of amorphous (Fe1−xMnx) alloys prepared by a single roller technique has been investigated in terms of X-ray diffraction and electrical resistance. The lattice parameter of the crystalline precipitates, which wereα-Fe and b c t (FeMn)3B, was determined under different heat treatments. On heating up to 440° C where a mixture of amorphous and crystalline phases exists and up to 550° C corresponding to the completion of crystallization, the lattice parameter of theα-Fe phase rises to that of pureα-Fe with increasing manganese concentration. In samples annealed at 660° C for 5 h, the opposite behaviour is observed. These results can be explained on the basis of the position of the boron atom occupying theα-Fe lattice, the pressure effect exerted by the environment, and the enhancement of the chemical short-range ordering between manganese and boron atoms with manganese concentration. In the b c t phase, which shows a reduction in lattice parameter with manganese concentration independent of heat treatment, the effect of redistribution of the atoms in the unit cell should be also taken into account.

The rare-earth arsenides: Non-stoichiometry in the rocksalt phases

The rare-earth arsenides with the rocksalt structure are non-stoichiometric compounds. Both anion and cation lattice sites are incompletely occupied. A range of arsenic solubility is observed in all cases, and in samarium arsenide at 700 °C, it extends from Sm 0.98As 0.81 to Sm 0.98As 0.98. Over part of the solubility range at the arsenic-rich end, variations in arsenic content can occur with no detectable change in lattice parameter. Further decrease in arsenic content can occur at 700 °C in all but Y, Ho, Er and Yb arsenides, and is accompanied by a reduction in the lattice parameter. A range of metal solubility exists at high temperatures for all phases and results in considerable reduction of lattice parameter at the lower metal concentrations. There is no evidence for vacancy ordering.

Fast Neutron Effects in Tetragonal Barium Titanate

Tetragonal BaTiO3 is transformed into cubic BaTiO3 as a result of irradiation with fast neutrons at a temperature of approximately 100°C. Sufficient atomic displacements are produced by bombardment with an integrated fast neutron flux of 1.8×1020 n/cm2 so that tetragonal single crystals (c0=4.0349 A and a0=3.9923 A) expand anisotropically to form perovskite-type cubic single crystals with a0=4.0824 A. The irradiated material remains cubic to the lowest temperature of measurement, 78°K, showing none of the low temperature phase transitions of unirradiated BaTiO3, and after annealing at 1000°C the crystal remains cubic but with a reduced lattice parameter. X-ray, thermal, and optical methods were employed in studying these effects.