Room-temperature oxidation of La2−x Sr x CuO4

A series of oxidized La_2?x Sr _x CuO_4+y $(0.05 \preccurlyeq x \preccurlyeq 0.3)$ compounds has been prepared by treating the starting materials with a solution of Br₂ in NaOH at room temperature. The structural modifications due to the oxidation of the materials have been studied by X-ray diffraction. Some of the observed changes are: (i) a large increase in the long parameter of the unit cell for samples with Sr content $0.05 \preccurlyeq x \preccurlyeq 0.15$ and (ii) a slight decrease in a along the whole range ofx. Interesting features have been observed regarding the critical temperatures of these materials: transition temperatures are higher for those containing lower Sr amounts (some of them were even nonsuperconducting before the oxidation treatment) in contrast to materials with x?0.15, whoseT _c's do not change very much. The influence of both oxygen and strontium contents on the structural modifications and the superconducting properties of the oxidized materials will be discussed.     

Study of some physical properties of bulk CuGa x In1−x Se2

Structural electrical, and optical properties of polycrystalline bulk samples of CuGa _x In_1–x Se₂ (0.75x0) were studied as a function of composition. The homogeneity of the constituents is tested using microprobe analysis. The lattice parameters a and c vary linearly with the change of Ga composition but the c/a values depart from a linear variation. The constructed phase diagram was reported from X-ray and differential thermal analysis measurements. All investigated samples are p-type and the electrical conductivity , carrier concentration p, and Hall mobility _H decrease while the thermoelectric power Q increase as the Ga content (x) increases. The energy gap values obtained from diffused reflection measurements, exhibit bowing behavior expressed by the relationship E _g(x)=1.066+0.464x+0.144x ².     

Thermodynamic study of the magnetic ordering of Fe3−x Mn x Si alloys

The compounds Fe_3–x Mn _x Si exhibit a complex magnetic behavior for 0.6x1.75 involving ferromagnetic ordering followed by a reordering at lower temperature. Measurements of the thermal expansion and the specific heat C _p show only a weak anomaly at the Curie temperature, T _C, whereas a large, relatively sharp peak occurs at the magnetic reordering temperature, T _R. Thermal expansion measurements for the Fe_2.2Mn_0.8Si compound made about T _R are indicative of long-range ordering that can be suppressed in an applied field of 0.4T. Magnetization measurements show that low fields, 1–2 mT, strongly influence the bulk magnetization. Irreversible magnetization behavior, similar to that in spin-glass systems, is displayed.Paper presented at the Tenth International Thermal Expansion Symposium, June 6–7, 1989, Boulder, Colorado, U.S.A.     

Laser-Induced Crystallization of a Si1−x Ge x Microstructure

Laser-induced crystallization of SiGe was investigated using a crystallization technique of pulsed excimer KrF laser irradiation on a-SiGe films that were prepared by plasma-enhanced chemical vapor deposition on quartz. The crystallized SiGe sample was investigated by scanning electronic microscopy (SEM); the SiGe microcrystals are 0.5 m in size and embedded in the a-SiGe:H matrix. Strong photoluminescence with two peaks at 720 and 750 nm was observed at room temperature in the crystallized film, whereas the uncrystallized a-SiGe:H films emit do not emit light in the visible range. This indicates that laser-induced crystallization can be used to improve the luminescence efficiency for Si-based materials.     

Superconductivity of Nb1−x Mg x B2: Impact of Stretched c-Parameter

We have performed a systematic study on the occurrence of superconductivity in Nb_1?x Mg _x B₂ (0.0≤x≤0.40). X-ray diffraction and magnetization measurements are carried out to determine the changes in lattice parameters, superconducting transition temperature (T _c ) and critical field (H _c1). The substitution of Mg at Nb site results in considerable stretching of c-parameter with only a slight change in a parameter. Rietveld analysis on X-ray diffraction patterns gives a=3.11 Å and c=3.26 Å for pure NbB₂ while a=3.10 Å and c=3.32 Å for Nb_0.60Mg_0.40B₂. This increased c-parameter introduces superconductivity in niobium diboride. Magnetization measurements though indicate the absence of superconductivity in NbB₂, the same shows a clear diamagnetic signal at about 10 K for Nb_0.60Mg_0.40B₂ sample. The magnetization M(H) plots exhibit weak superconductivity like hysteresis loops. The stretching of c-parameter from around 3.26 to 3.32 i.e. by 0.06 cannot be explained solely by substitution of Nb by Mg in the lattice. It seems that some Nb deficiencies are introduced in the Nb_1?x Mg _x B₂ as Mg is not substituted completely at the vacant Nb sites. This could be seen from XRD results, where one can clearly notice the presence of small amount of MgO in Nb_1?x Mg _x B₂ samples.     

Vapor Growth of Pb1 – x In x Te Crystals

Vapor growth of In-doped PbTe crystals by the sublimation–condensation and vapor–liquid–solid (VLS) processes is examined. Well-faceted Pb_{1 – x} In _x Te crystals with x = 0.04–0.06 are prepared by the sublimation method. The effects of the charge composition on the facial development and growth rate in the range 0 x 0.02 are discussed. The growth process at x 0.02 is shown to follow the VLS mechanism. Bulk Pb_{1 – x} In _x Te crystals with x 0.05 are grown by a vertical VLS process. The crystal composition is shown to depend significantly on the rate of ampule translation through the temperature field of the furnace and the separation between the evaporation and condensation zones. The longitudinal indium profiles in the crystals are correlated with growth kinetics.     

Dielectric properties of the Sr1−x La x Sn1−x Co x O3 system

The possibility of the formation of a solid solution in the Sr_1–x La _x Sn_1–x Co _x O₃ system has been explored. Single-phase solid solution forms in the compositions for x0.10. All single-phase solid solution compositions have a cubic structure similar to SrSnO₃. The dielectric behaviour of these solid solution compositions has been studied as a function of temperature and frequency. The frequency dependence of dielectric constant and dielectric loss in these materials indicates that space charge polarization contributes significantly to their observed dielectric parameters. Microstructural studies show the presence of well-faceted grains. The average grain size in these samples is small.     

Crystallization behaviour of amorphous Sc100−x Fe x alloys near x=25 composition

A detailed study of the process of crystallization in amorphous Fe₂₅Sc₇₅, Fe₂₈ Sc₇₂ and Fe₂₅Sc₄₅Zr₃₀ alloys has been carried out using differential scanning calorimetry, X-ray diffraction and Mössbauer spectroscopic techniques. The complex multi-stage transformation process in Fe₂₅Sc₇₅ and Fe₂₈Sc₇₂ is understood in terms of polymorphous formation of an intermediate metastable crystalline phase of orthorhombic structure (a = 0.521 nm, b = 0.648 nm and c = 1.212 nm) and of likely Sc₃Fe stoichiometry, before final eutectic crystallization to stable Fe₂Sc and -Sc.     

Thermodynamic properties of Ca1 − x Er x F2 + x and Ca1 − x Yb x F2 + x heterovalent solid solutions

The heat capacity of single crystals of the Ca_{1 ? x} Er _x F_{2 + x} (x = 0.05, 0.10) and Ca_0.95Yb_0.05F_2.05 fluorite solid solutions was determined by adiabatic calorimetry in the temperature range 55–300 K. The results were used to obtain temperature dependences of the Debye characteristic temperature, entropy, and enthalpy for the solid solutions.     

Structural and optical properties of Bi x Se1−x films

Bi _x Se_1–x thin films have been studied because of their structural and optical properties with a view to judging their suitability as the recording medium in phase-change type optical recording. Amorphous films deposited at room temperature were crystallized by thermal annealing. X-ray diffraction analysis and surface morphological studies are reported. A maximum reflectivity difference of 25% at =830 nm was obtained upon amorphous-to-crystalline transition. The optical constants calculated by the Newton-Raphson method using the experimental transmittance, reflectance and thickness data are reported.     

Properties of Nd1 – x Gd x CoO3 Solid Solutions

NdCoO₃, GdCoO₃, and Nd_{1 – x} Gd _x CoO₃ solid solutions with x= 0.1–0.9 are prepared by solid-state reactions, their lattice parameters are determined, and their electrical conductivity and thermal expansion are measured between 300 and 1110 K in air. All of the solid solutions are found to have an orthorhombically distorted perovskite structure and to exhibit anomalies in conductivity and thermal expansion, due to a semiconductor–metal transition.     

Magnetoresistance of Sr1−x K x BiO3: a second-family of bismuth-oxide-based superconductors

The superconducting Sr _1–x K _x BiO ₃ samples with x = 0.45 – 0.6 were synthesized by the high-pressure-high-temperature technique in a belt type apparatus (2 GPa, 700 °C, 1 h, Pt capsules) with stoichiometric mixtures of Sr ₂ Bi ₂ O ₅ , Bi ₂ O ₃ and KO ₂ as described earlier. ¹ The X-ray diffraction results appear as a single perovskite-like phase. The superconductivity occurs at T _c 12 K in the A. C. susceptibility measurement. The onset (zero resistivity) temperature of superconductivity in resistivity measurement of the investigated sample was . The transition temperature region was a little bit broad and a shoulder was present about 11.3 K indicating probably the existence of crystallites of different K content. The particularly interesting point is that the resistance begins to reappear at T < 6 K at zero magnetic field. As the external magnetic field is applied, the reentrant resistance disappears and superconductivity is recovered until the applied magnetic field becomes higher than 0.65 Tesla. The superconductivity for T < 6 K is destroyed for the higher magnetic field. The T_c onset decreases as the magnetic field increases like in the BCS type superconductors. The transition region becomes broader under the magnetic field, which indicate a kind of vortex transition as in the case of high T _c cuprates.     

Phase Formation in the Course of Oxidation of In x Se1 − x Alloys (0.47 ≤ x ≤ 0.53)

We have determined the characteristic temperature ranges of heating of In _x Se_{1 – x} (0.50 x 0.52) alloys in air. We have established that the annealing of InSe powder in selenium vapors leads to the stabilization of the rhombohedral structure.     

Piezoelectric Properties of TlIn1−x Nd x Se2 Crystals

The piezoelectric properties of TlIn_1–x Nd _x Se₂ crystals (0 x 0.05) have been investigated. It has been found that these crystals possess high coefficients of tensosensitivity which change on partial substitution of the indium atoms by neodymiun atoms and strongly depend on the intensity of the spectral composition of optical illumination.     

Thermal Diffusivity of La1−x Sr x MnO3 (x < 0.3)

Perovskite manganites are interesting because of their colossal magnetoresistance. In this work high resolution thermal diffusivity measurements of La_1–x Sr _x MnO₃ (0 x 0.3) single crystals in the temperature range from 250 to 400 K are presented. A photopyroelectric device in the standard back configuration has been used. The thermal diffusivity through second-order magnetic phase transitions, as well as through first- and second-order structural phase transitions has been measured. The critical parameters of the sample with x = 0.3 at the ferromagnetic-to-paramagnetic transition have been obtained, and are close to the values predicted by the Ising model.     

The dielectic response of K x Al x Ti8−x O16 and K x Mg x/2 Ti8−x/2 O16

Materials of the hollandite structure with the general formulae K_x Al_x Ti_8–x O₁₆ and K_x Mg_x/2 Ti_8–x/2 O₁₆ have been synthesized in the composition range 1.6x2.0 and their dielectric properties have been measured in the temperature range 77 to 800 K and the frequency range 10^–3 to 10⁶ Hz. The observed response shows a whole range of features characteristic for both charge carrier and dipolar polarization processes and these are seen as being associated with the one-dimensional transport in channels in the hollandite structure. At low temperatures the dominant response is the universal dielectric relation in which the loss follows the law x() ^n–1, with the exponent n<1 and equal specifically to approximately 0.7. This is followed at 120 to 180 K by a distinct loss peak superimposed on the above law, and finally at higher temperatures by a region of strong dispersion which is associated with strongly interacting many-body processes between charged carriers restricted by defects to move in limited regions of the channels.     

Effect of the Crystallographic Orientation of GaAs Substrates on the Composition of Ga x In1 – x P Layers

The effect of the crystallographic orientation of GaAs substrates on the composition of Ga _x In_{1 – x} P solid solutions grown by liquid-phase heteroepitaxy is studied. It is shown that GaAs-lattice-matched Ga _x In_{1 – x} P solid solutions can be grown, under identical conditions, on (100), (111), and (111) GaAs substrates using different melt compositions. The results are interpreted in terms of the contribution from the energy of the crystal–melt interface to the total energy of mixing of the solid solution.     

Synthesis of Cr x Ti1−2x Nb x O2O20≤x≤0.5 rutile solid solutions from alkoxides

In this study, Cr _x Ti_1–2x Nb _x O₂ (0 x 0.5) rutile solid solutions have been synthesized from gels built from hydrolysis-condensation of Cr (III) acetylacetonate, NbCl₅ and Ti (IV) isopropoxide mixture (polymeric gel). Characterization of these solid solutions was carried out by X-ray diffraction, ultraviolet-visible and infrared spectroscopy, differential thermal and thermogravimetric analysis and CIELAB (Commission Internationale del'Eclairage L^*a^*b^*) parameter measurements. The results obtained by the polymeric gel method were compared with those obtained by traditional ceramic synthesis. This comparison reveals some differences with regard to synthesis temperatures and reaction mechanisms. The formation of Cr _x Ti_1–2x Nb _x O₂ (0 x 0.5) rutile solid solutions by the ceramic method requires temperatures of about 1200°C and soaking times of several days. These solid solutions are synthesized at 1000°C in 24 h by the polymeric gel method. In ceramic synthesis, the CrNbO₄ compound with rutile structure appears as an intermediate compound in the formation of rutile solid solutions. In polymeric gel synthesis, however, the CrNbO₄ rutile compound was not detected in the samples.     

Bond Angle Study of Self-compensating Y1−x Ca x Ba2−x La x Cu3O z System

Single-phase samples of a self-compensating Y_1?x Ca _x Ba_2?x La _x Cu₃O _z system were synthesized through a solid-state reaction method with x<0.4. The structure of all samples were characterized by X-ray diffraction and refined by the Rietveld method. Superconducting properties have been investigated by the DC magnetization measurement. The critical temperature (T _c ) decreases evidently with the increment of x although the carrier concentration remains constant in the samples for different doping level. Careful study of the chemical bonds in the crystalline lattice demonstrates that the T _c is closely correlated to four pairs of bond angles in the unit cell. The analysis indicates that crystalline structure is one of the important factors to high-T _c superconductivity, and its influence is independent of the carrier concentration.