Determination of the E1 and E1+Δ1 energy bands by photoreflectance in AlxGa1−xAs in the region 0<x<0.55

The E₁ and E₁+Δ₁ energy bands of metal–organic chemical vapor deposition grown Al_xGa_1−xAs, with x in the range 0–0.55, have been determined using photoreflectance technique. The aluminum composition for each sample was determined using the energy of the room-temperature photoluminescence compensated peak value and a suitable fundamental band gap formula. The positions of the E₁ and E₁+Δ₁ peaks were determined from curve-fitting an appropriate theoretical model to our experimental data by a modified downhill simplex method. Using the results, we propose new E₁ and E₁+Δ₁ cubic expressions as functions of the aluminum composition, x, and compare them with the available reported expressions.     

Relaxation peaks associated with the oxygen-ion diffusion in La2−xBixMo2O9 oxide-ion conductors

The effects of bismuth doping on the oxygen-ion diffusion in oxide-ion conductors La_2−xBi_xMo₂O₉ (x=0.05, 0.1, and 0.15) have been studied by both internal friction and dielectric relaxation techniques. Two internal friction peaks of relaxation type (P₁ and P₂ peak) were observed at a measurement frequency of 4 Hz around 380 and 430 K, respectively. As for the dielectric measurement, a prominent dielectric relaxation peak (P_d) was found in all the Bi-doped samples around 700 K at a measurement frequency of 50 kHz, which actually consists of two sub-peaks (denoted as P_d1 and P_d2 peak). With increasing Bi-doping content, two peaks shift to higher temperature and decrease in height, while the activation energy of both peaks increases. The main reason was interpreted as the introduction of the lone-pair electrons of bismuth, which tends to block the diffusion of oxygen ion.     

Structural relaxation and crystallisation of bulk metallic glasses Zr41Ti14Cu12.5Ni10−xBe22.5Fex (x = 0 or 2) studied by mechanical spectroscopy

The measurements of the internal friction and dynamic shear modulus as well as differential scanning calorimetry have been performed in order to investigate the structural relaxation and crystallization of Zr₄₁Ti₁₄Cu_12.5Ni_10−xBe_22.5Fe_x (x=0 or 2) bulk metallic glasses. It is found that the glass transition is retarded and the thermal stability of supercooled liquid is increased by the Fe addition. The experimental results are well analyzed using a physical model, which can characterize atomic mobility and mechanical response of disordered condensed materials.     

Combinatorial pulsed laser deposition and thermoelectricity of (La1−xCax)VO3 composition-spread films

La_1−xCa_xVO₃ composition-spread film library was fabricated by combinatorial pulsed laser deposition and their thermoelectric properties were evaluated paralelly by the multi-channel probes of Seebeck coefficient and electric conductivity. Concurrent X-ray analysis verified the formation of solid soluted films in the full composition range (0x1) as judged from the linear variation of the lattice constants. The Seebeck coefficients of La_1−xCa_xVO₃ changed from a large negative value to almost zero with the increase of x, due presumably to the variation of valence in vanadium ions.The power factor in this library was as high as 0.6 μW/cm K², which was obtained at x=0, i.e. pure LaVO₃ grown at 800 °C.     

Electronic structure of the ladder-chain compound Sr14−xCaxCu24O41

The electronic structure of the ladder-chain compound Sr_14−xCa_xCu₂₄O₄₁ is studied by ab initio calculations within the local density approximation. The effects of Ca substitution and structure modulation on electronic structure are discussed. It is found that 0.05 holes per copper atom are on the ladder layers for fully substituted compound, Ca₁₄Cu₂₄O₄₁.     

Nucleation and evolution of Si1−xGex islands on Si(001)

In this study, we report a systematic investigation of the metastable morphologies of Si_1−xGe_x layers obtained by the interplay of kinetics and thermodynamics during growth on Si(001). We show that three main growth regimes can be distinguished as a function of the misfit and of the deposited thickness. They correspond to three equilibrium steady state morphologies that consist of (105)-facetted hut islands, huts and domes in co-existence, and a bimodal size distribution of domes, respectively. The shape transitions between these states are attributed to different levels of relaxation.     

Phase relations, lattice thermal expansion in Ce1−xEuxO2−x/2 and Ce1−xSmxO2−x/2 systems and stabilization of cubic RE2O3 (RE: Eu, Sm)

The phase relations in CeO₂–Eu₂O₃ and CeO₂–Sm₂O₃ systems have been established under slow-cooled conditions from 1400 °C. The two-phase relations differ as the CeO₂–Eu₂O₃ system showed only two monophasic phase fields, namely F-type cubic and C-type cubic, whereas CeO₂–Sm₂O₃ system showed three phase fields namely F-type cubic, C-type cubic and a biphasic field comprising of C-type cubic and monoclinic phase. An interesting observation of this investigation is the stabilization of C-type rare-earth oxide after Ce⁴⁺ substitution, which is attributed to decrease in average cationic size on Ce⁴⁺ substitution at RE³⁺ site. The lattice thermal expansion behavior of F-type solid solution and C-type solid solution in CeO₂–Eu₂O₃ system was investigated by high-temperature XRD.     

Bond-lengths of the atomic nearest-neighbor and next-nearest-neighbor in A1−xBxC1−yDy III–V solid solutions

On the basis of the FDUC model and the hypothesis of the constant covalent radii, the expressions of the atomic nearest-neighbor and the next-nearest-neighbor bond-lengths were derived for A_1−xB_xC_1−yD_y III–V quaternary solid solutions. This set of bond-length expressions predicts the averaged bond-lengths and bond angles at any concentration (x, y) for the III–V pseudobinary and quaternary solid solutions, which are only dependent on the lattice parameters and the concentrations of the pure end compounds. When x=0, 1 or y=0, 1, A_1−xB_xC_1−yD_y III–V quaternary solid solutions degenerate into the relative pseudobinary solid solutions, in which the nearest-neighbor and the next-nearest-neighbor bond-lengths agree well with the experimental results. Further discussion and comparison with other theoretical models are also given in this paper.     

Mechanical characterization of Ni1−xZnxFe2O4 prepared by non-conventional methods

The mechanical properties like hardness, H_v and compressive strength, σ of Ni_1−xZn_xFe₂O₄ (x = 0.2, 0.3, 0.4 and 0.5) prepared by the non-conventional flash combustion and citrate-gel decomposition techniques are studied and reported. It is observed that there is an increase in hardness with zinc content as well as sintering temperature. The hardness in the order of 2.0–3.63 GPa and compressive strength in the order of 150–240 MPa are obtained for Ni–Zn ferrites prepared by these non-conventional techniques. The influence of density, porosity and microstructure on hardness and compressive strength of Ni–Zn ferrites with respect to sintering temperature was studied.     

Spin-dependent tunnelling in La0.7Sr0.3MnO3/SrTiO3 superlattices

La_0.7Sr_0.3MnO₃ is predicted to show half-metallic behaviour at low temperature, which gives rise to a metallic character for one spin direction and an insulating character for the other. This 100% polarisation of the conduction band should enhance the spin dependent tunnelling in manganite-based tunnel junctions. La_0.7Sr_0.3MnO₃/SrTiO₃ epitaxial superlattices were grown on LaAlO₃(001) substrates by metal–organic chemical vapour deposition (MOCVD). These multilayers consist of 15 epitaxial bilayers of La_0.7Sr_0.3MnO₃ and SrTiO₃. The junctions were patterned using UV lithography and Ar ion milling to carry out transport measurements in the current perpendicular-to-plane geometry (CPP). A temperature-independent non-linear I–V curve, which is characteristic of a tunnelling conduction mechanism, was observed below 50 K. At higher temperatures, the I–V curves are found to become linear and temperature-dependent. Up to 30 K, a constant tunnel magnetoresistance (TMR) (3%) is measured. The switching field is consistent with the film coercive field (a few 10s of mT). At higher temperatures, the TMR decreases rapidly. This temperature dependence is compared to the expected behaviour of a spin tunnel junction with half-metallic electrodes, with thermal activation or the loss of spin polarisation taken into account.     

Photoelectrochemical properties of sol–gel-derived Ti1−xVxO2 solid solution film photoelectrodes

Ti_1−xV_xO₂ solid solution film photoelectrodes were prepared by the dip-coating sol–gel method. X-ray diffraction and X-ray photoelectron spectroscopy were employed to ensure the formation of the solid solution and their composition. Obvious photoresponses were observed in the visible region for the solid solution film electrodes with x0.05 and the red shift of the photoresponse was enhanced with increasing x. Moreover, the solid solution film electrodes were found to be photoelectrochemically stable. However, the onset potential of photocurrent shifted positively with increasing x. Band model of the solid solution was suggested to explain the effects of the vanadium incorporation on the photoelectrochemical properties.     

Optical transitions near the band edge in bulk CuInxGa1−xSe2 from ellipsometric measurements

From the analysis of the variation of optical absorption coefficient with incident photon energy between 0.8 and 2.6 eV, obtained from ellipsometric data, the energy E_G of the fundamental absorption edge and E_G′ of the forbidden direct transition for CuIn_xGa_1−xSe₂ alloys are estimated. The change in E_G and the spin-orbit splitting Δ_SO=E_G′−E_G with the composition x can be represented by parabolic expression of the form E_G(x)=E_G(0)+ax+bx² and Δ_SO(x)=Δ_SO(0)+a′x+b′x², respectively. b and b′ are called “bowing parameters”. Theoretical fit gives a=0.875 eV, b=0.198 eV, a′=0.341 eV and b′=−0.431 eV. The positive sign of b and negative sign of b′ are in agreement with the theoretical prediction of Wei and Zunger [Phys. Rev. B 39 (1989) 6279].     

Magnetism in Si1 − xMnx diluted magnetic semiconductor thin films

We have studied the electrical and magnetic properties of p-type semiconductor thin films of Si_1 − xMn_x/Si (x = 0.036 and 0.05) grown by molecular beam epitaxy. Experimental results reveal that the resistivity of the samples decreases gradually with increasing measurement temperature, which can be described well by Mott's variable-range-hopping model. All the samples exhibit the ferromagnetic ordering above room temperature. Among these samples, Si_0.95Mn_0.05 has a higher hole density and magnetization. This indicates an enhancement of hole-mediated ferromagnetic exchange interactions when the Mn-doping concentration is increased.     

Combinatorial synthesis and rapid characterization of Mo1−xSnx (0x1) thin films

Thin films of Mo_1−xSn_x, continuously and linearly mapped for 0<x<1, have been prepared by d.c. magnetron sputter deposition under various growth conditions. X-ray diffraction results indicate that as x in high-pressure deposited Mo_1−xSn_x increases from 0 to approximately 0.45, the bcc lattice expands and no new phases are formed. At low deposition pressures, Mo₃Sn, a β-tungsten structured phase, is formed along with the bcc Mo–Sn solid solution for 0.1<x<0.3. The variation of the lattice parameter for this intermetallic phase also indicates that solid solutions, possibly of the form Mo_3+ySn, are being formed. These materials are of special interest as anode candidates in lithium-ion batteries.     

Study of optical constants in CdxSe1−x

Thin films of Cd_xSe_1−x (0<x<1) have been deposited by the vacuum evaporation technique onto ultra clean substrates maintained at 300°C. The optical properties (specially transmission spectra) of these films were studied. The wavelength dependence of refractive index and extinction coefficient of the films along with the variation of these constants with composition was studied.     

Investigation of transition metal oxides by ultrasonic microscopy

Ultrasonic microscopy is now well established in non-destructive testing and has become a versatile method for investigating solid materials. It may serve both as imaging technique for large scale inspection of sample homogeneity and also as measuring device for a local determination of elastic properties. Our investigations of single crystalline vanadium dioxide nicely demonstrate this capability of flexible use and are exemplary for the beneficial effect of ultrasonic microscopy on materials research, in general. Here we focus on three different subjects: the detection of odd phases in as-grown single crystals, the inspection of structural transformations accompanying the metal–insulator transition and the investigation of elastic properties performed on small crystallites.     

Sol–gel preparation of boron-containing cordierite Mg2(Al4−xBx)Si5O18 and its crystallization

Five cordierite-based powders were investigated regarding their thermal and crystallization behaviors. The powders were obtained from amorphous gels having nominal compositions of 2Mg : xAl : (4 − x)B : 5Si where x = 4 down to 0. Thermal gravimetry analysis of the dry gels showed some absorbed water and decomposition of organic ligands in addition to network condensation. Gradual substitution of B for Al in the dried gel powders showed a new band in their infrared spectra corresponding to triangular BO₃, whereas the bands corresponding to Al vanished. This also showed a noticeable effect on the crystallization trends, type and stability of cordierite. Cordierite crystallized in samples of B/Al ratio up to 1 while protoenstatite predominated in samples of higher B/Al ratios. In addition, some silica minerals, with little amorphous phase, were formed. Incorporation of boron and increase in temperature enhanced the transformation of γ cordierite to its form.     

Magnetoimpedance effect at various temperatures for manganite La0.7Ca0.3MnO3−δ

In the present work, the AC magnetoimpedance effect in La_0.7Ca_0.3MnO_3−δ at various temperatures are investigated. The peak of the metal–insulator transition occurs in the temperature dependence of impedance. Negative magnetoimpedance effect in the La_0.7Ca_0.3MnO_3−δ is obtained at frequencies f≤10 MHz. In the magnetoimpedance effect of manganites, the magnetic field not only decreases the permeability μ_t, but also reduces the resistivity ρ by aligning the local spins and varying the transfer integral t_ij. The AC magnetoimpedance participated by the DC colossal magnetoresistance (CMR) in manganites, should be connected with the combined effects of double exchange interaction, electron–phonon coupling and skin effect.     

Synthesis, characteristics and oxygen-sensitive properties of SrMgxTi 1 − xO3 nanocrystals

SrMg_xTi_{1 - x}O₃ nanocrystals (x = 0.1–0.6) were synthesized by the stearic acid gel method. Powder samples were characterized by X-ray diffraction and X-ray photoelectron (XP) spectroscopy. The results showed that the lattice parameter a and the O 1s XP spectrum changed not only with the Mg content x but also with the grain size d of the samples. The conductivity of a thick film specimen fabricated on an aluminium oxide wafer was investigated in a nitrogen—oxygen atmosphere.     

La(1−x)SrxCoO3 for thin film thermocouple applications

In this study, the LSCO (lanthanum strontium cobalt oxide) family has been investigated for thin film thermocouple applications. Thin films of La_(1−x)Sr_xCoO₃ (x=0.3,0.5,0.7) were prepared on sapphire substrates by pulsed laser deposition. The films were annealed at different temperatures in air and characterized for phase, composition and microstructure to determine their thermal stability. From the phase and composition analyses, it is clear that as the Sr content in LSCO increases, the thermal stability decreases. Among the three compositions studied, x=0.3 had the best phase and chemical stability, and microstructural properties. It was observed that La_0.7Sr_0.3CoO₃ possesses excellent phase, composition and microstructural stability up to 1273 K. Above 1273 K, however, LSCO decomposes resulting in the loss of cobalt and formation of individual oxide phases. Electrical resistivity and Seebeck coefficients were measured in situ as a function of temperature in air up to 1023 K. The electrical and Seebeck coefficient properties were found to be stable for all the three compositions up to 1023 K and studies indicated that electrical conduction occurs through a small polaron hopping mechanism. In conclusion, LSCO possessed good thermal stability in air up to 1273 K and exhibits excellent potential in thin film thermocouple applications.