Effect of bond polarity in Sb2Te3−x Se x single crystals

Sb₂Te_3?x Se _x (x=0·00?1·25) single crystals were prepared from 5N purity elements using a modified Bridgman method. Measurements of the reflectivity spectra in the plasma resonance frequency range, Hall constantR _H(B∥c) and electrical conductivityσ _⊥C were carried out on these samples at room temperature. With increasing selenium content a shift of the reflectivity minimum towards longer wavelengths was observed as well as an increase of the Hall constant and a decrease of the electrical conductivity — the incorporation of Se atoms into the Sb₂Te₃ crystal lattice results obviously in a decrease in the concentration of free carriers. This effect is accounted for by a change in the polarity of bonds in the Sb₂Te₃ crystal lattice, due to the formation of Se _Te ^x substitutional defects.     

Effect of Cd content on the structural characteristic and some physical properties of Zn1−x Cd x Se

The present work focuses on the structural, optical, and electrical properties of Zn_1?x Cd _x Se (0.1≤x≤0.25) compounds. The compounds were synthesized by solid state reaction. X-ray diffraction (XRD) patterns confirm that the samples have cubic single phase (zinc-blende) crystal structure with space group F-43m. The crystal structural parameters were refined by the Rietveld method using the FullProf program. It was found that the lattice parameters increase linearly with increasing the Cd content and obeys Vegard’s law. The refined values of the crystallite size and the bond lengths increase with increasing the Cd content. The energy band gap of the samples has been calculated and it was found that it decreased as Cd increased. The conductivity of the samples increases with increasing both of composition parameter x and temperature, and showing semiconducting behavior.     

Optical coefficients of Hg1 − x − y Cd x Eu y Se crystals

Optical properties of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals grown by the Bridgman method have been investigated based on the independent reflectance and transmittance measurements, which were performed on a Nicolet 6700 spectrometer at T = 300 K in the wavelength range 0.9 ≤ λ ≤ 26.6 μm. The values of refractive index n, absorption index k, and absorption coefficient α have been determined for the crystals studied. Based on the dependences α = f(hν), the presence of direct allowed interband optical transitions in the crystals is established and the band-gap values are determined. The influence of temperature on the transmittance and band gap are investigated in the range T = 114–300 K.     

Dielectric properties and phase transitions in crystals of TlInS x Se2 − x solid solutions

This paper reports on a study of the dielectric permittivity and electrical conductivity of single-crystal TlInS _x Se_{2 − x} solid solutions as functions of temperature and composition. It has been found that the dielectric permittivity ɛ and electrical conductivity σ decrease with increasing x and increase with growing temperature. The temperature dependences ɛ = f(T) and σ = f(T) for TlInS _x Se_{2 − x} crystals have been demonstrated to reveal anomalies in the form of maxima, which suggests that these crystals undergo phase transitions. The temperatures of the phase transitions increase with increasing x.     

Investigation of the effect of annealing on the magnetic properties of Sn1−xEuxTe single crystals

A study of the effect of annealing on the magnetic properties of single crystals Sn_1−xEu_xTe is reported. The width of the electron paramagnetic resonance line of the crystal is found to decrease upon annealing but its g-value of 1.991 is nearly unaffected. Magnetization results indicate that the pair exchange interaction is weakly antiferromagnetic with a value of −0.67 K for the non-annealed sample and −0.29 K after annealed sample. Susceptibility measurements performed as a function of temperature also indicate the presence of EuTe clusters in the as-grown Sn_1−xEu_xTe crystals. Therefore it was deduced that the Eu²⁺ ions tend to form clusters, particularly pairs, in the as-grown crystal and these clusters disappear after annealing, as the Eu²⁺ ions occupy isolated sites in the SnTe host lattice.     

Optical properties of ternary ZnS x Se1−x polycrystalline thin films

Ternary ZnS_xSe_1–x polycrystalline thin films were prepared by evaporation in vacuum of 10^–5 Torr. The molecular fractionx varied in the region ox1. The optical constants (the refractive indexn, the absorption indexk, and the absorption coefficient) were determined in the wavelength range 300–1600nm. A plot representing ₂=f(hv) shows that the ZnS_xSe_1–x polycrystalline thin films of different compositions have two direct transitions corresponding to the energy gapsE andE+. The variation in eitherE orE+ withx indicates that this system belongs to the amalgamation type. Such variation follows a quadratic equation. The bowing parameter was found to be 0.456 eV, roughly equal to the calculated value 0.60 eV using the empirical pseudopotential method based on the virtual-crystal approximation, in which the disorder effect has not been taken into account.     

Mössbauer studies of Fe x Mn1 − x S single crystals

Single crystals of iron manganese sulfides Fe _x Mn_{1 ? x} S (0.25 ≤ x ≤ 0.29) are experimentally investigated using Mössbauer spectroscopy and x-ray diffraction. The Mössbauer spectra measured at 300 K exhibit a single broadened line characteristic of paramagnets. The isomer shift of this line is equal to 0.92–0.94 mm/s, which is typical of Fe²⁺ ions in the octahedral position. The quadrupole splitting (0.18–0.21 mm/s) suggests a distortion of the coordination polyhedron of iron ions in the Fe _x Mn_{1 ? x} S compounds.     

Effect of Sr content on structure and electrical properties of La1−x Sr x MnO3 from ITSOFC cathode view point

The samples belonging to La_1−x Sr _x MnO₃ series, prepared by combustion route without using fuel, exhibit crystal structural phase transition with the change in Sr content. Less than 40 mol% Sr is partially substituted in the crystal structure of LaMnO₃. The structural phase transition from rhombohedral to cubic, cubic to tetragonal, and tetragonal to orthorhombic takes place on 40, 60, and 80 mol% addition of Sr. The highest electrical conductivity in La_0.4Sr_0.6MnO₃ is understood to be due to the maximum concentration of polaron. The polarons are formed due to the conversion of Mn³⁺ to Mn⁴⁺ so as to achieve electroneutrality after substitution of Sr²⁺ for La³⁺.     

Heat capacity and thermal conductivity of Fe x Mn1 − x S single crystals

This paper reports on the results of investigations of the thermal properties and thermal conductivity of single crystals of homogeneous solid solutions Fe _x Mn_{1 ? x} S with a cubic NaCl structure, which have been prepared by the cation substitution for divalent manganese ions in manganese monosulfide. It has been revealed that the heat capacity and thermal conductivity exhibit anomalies in the range of the magnetic transition. The cation substitution is accompanied by an increase in the phase transition temperature.     

Specific heat of Na1−x V2O5 single crystals

Temperature dependences of the specific heat C and the magnetic susceptibility χ of Na_1?x V₂O₅ single crystals (x=0, 0.01, 0.02, 0.03, and 0.04) are studied. In NaV₂O₅, the transition to the spin-gap state (T _c =34 K) is accompanied by a sharp decrease in χ, while C exhibits a λ-shaped anomaly. At low temperatures, the specific heat of NaV₂O₅ is approximated by the sum of phonon ～T ³ and magnon ～exp(?Δ/T) contributions, which makes it possible to estimate the Debye temperature Θ_D=336 K and the gap in the magnetic excitation spectrum Δ=112 K. With the departure from stoichiometry, the anomalies observed in the behavior of χ and C are spread and shifted to lower temperatures. The low-temperature specific heat of nonstoichiometric samples is determined by the sum of phonon and magnon components and the contribution due to the presence of defects. The values of magnetic entropy characterizing the phase transitions in Na_1?x V₂O₅ are calculated.     

Birefringence of CuGa(S1−x Sex)2 single crystals

The magnitude and dispersion of birefringence of single crystals of CuGa(S_1?x Se_x)₂ solid solutions is studied in the spectral region of 0.5–2.5 μ at T=300 K. The effect of the substitution of selenium for sulfur on special features of birefringence dispersion is analyzed within the framework of the single-oscillator model.     

Thermoelectric properties of n-Bi2Te3 − x − y Se x S y solid solutions under high pressure

The thermoelectric properties of n-Bi₂Te_{3 ? x ? y} Se _x S _y solid solutions with atomic substitutions in the tellurium sublattice (x = 0.27, 0.3, y = 0, and x = y = 0.09) have been studied under a pressure to 8 GPa. It has been found that the Seebeck coefficient and the resistance decrease with increasing P, and power factor χ increases in all compositions and becomes maximal at pressures of 2–4 GPa. It has been shown that the power factor χ, which is proportional to the product of the effective mass of the density of states m/m m/m ₀ and the charge carrier mobility μ₀ in the form (m/m ₀)^3/2μ₀, increases with increasing pressure mainly due to the increase in the mobility and also depends on the solid solution composition. In the composition with substitution Te → Se + S (x = y = 0.09), the peculiarity of the dependence of m/m ₀ on P in the pressure range corresponding to maximal values of the power factor can be explained by the existence of an electronic topological transition. The increase in the power factor under pressure in n-type Bi₂Te_{3 ? x ? y} Se _x S _y solid solutions combined with similar data for p-type Bi_{2 ? x} Sb _x Te₃ solid solutions obtained earlier, including the estimations of possible changes in the thermal conductivity with increasing pressure, give grounds to design thermoelements with improved value of the thermoelectric figure-of-merit, which can be 50–70% at pressures of 2–4 GPa.     

Thermal conductivity of single crystals of the Ca1 − x Y x F2 + x solid solution

The thermal conductivity of single crystals of the solid solution of yttrium fluoride in calcium fluoride Ca_{1 ? x} Y _x F_{2 + x} with the fluorite structure (x ≤ 0.20) and the Ca_0.27Y_0.73F_2.73 phase with the tisonite structure has been studied by the absolute steady-state longitudinal heat flow method in the temperature range 50–300 K. It has been established that the thermal conductivity drops sharply with increasing yttrium trifluoride concentration, especially in the low-temperature region.     

Influence of the composition of TlGa1 − x Er x Se2 crystals on their dielectric characteristics and parameters of localized states

Frequency dependences of the real (?′) and imaginary (?″) parts of the complex permittivity, the dielectric loss tangent (tanδ), and the ac conductivity (σ_ac) in frequency range f = 5 × 10⁴?3.5 × 10⁷ Hz have been investigated for TlGa_{1 ? x} Er _x Se₂ crystals of various compositions. It has been established that the relaxation dispersion of ?′ and ?″ takes place for the studied crystals. The influence of the erbium content in the crystals on their dielectric coefficients has been studied. The ac conductivity of the TlGa_{1 ? x} Er _x Se₂ single crystals in the high-frequency range obeys the law σ_ac ～ f ^0.8, which is characteristic of the hopping mechanism of charge transfer over the states localized in the vicinity of the Fermi level. Parameters of the states localized in the band gap of TlGa_{1 ? x} Er _x Se₂ and the influence of the composition of the crystals on these parameters have been evaluated.     

Effect of irradiating ion fluence on optical properties of ZnO1−x:Nx thin films

Swift heavy ion (SHI) irradiation is an effective technique to modify the optical properties of the materials. In the present investigation, the effect of 100?MeV?Ag⁷⁺ SHI irradiation fluence on the optical properties of ZnO_1?x:N_x thin films was studied. The post irradiation spectroscopic characterizations such as UV–VIS reflectance spectroscopy, Raman spectroscopy and photoluminescence (PL) spectroscopy analysis were carried out. The studies imply that when the SHI passes through the solid, the higher electronic stopping power of ions can weaken oxygen bonds in ZnO, resulting in the formation of donor defects such as oxygen vacancies and zinc interstitials. The formation of donor defects has been acknowledged through the increase in bandgap with irradiating ion fluence. The blue shift observed from the Raman spectra for the 3?×?10¹³ ions/cm² fluence-irradiated films implies the existence of compressive stress in the films. The PL analysis acknowledges the formation of donor defects upon irradiation. Furthermore, it conveys that the presence of N atoms in ZnO lattice leads to the formation of a less number of defects as compared with undoped ZnO while irradiation.     

Tunneling measurements on (La1−x Ce x )Al2 single crystals with wax barriers

In the investigation of the Kondo superconductor (La_1–x Ce _x )Al₂ a new technique is developed to fabricate tunnel junctions of the type: bulk sample — wax barrier — evaporated counterelectrode. Pb, Al, Ag, Au and Mg are used as counterelectrodes. Measurements employing Pb and Al yield the order parameter of nominally pure LaAl₂: ₀=(0.47±0.03) meV=1.63kT _c. The large influence of localized states within the gap is confirmed.     

Radiation effects on the optical properties of Se85−x Te15Sb x thin films

The optical absorption of the as-deposited and γ-irradiated (doses of 3, 10, 12, 15 Mrad) thermally evaporated Se_85?x Te₁₅Sb _x (x=0, 6, 9) films was measured. The refractive index n and the extinction coefficient k of the films were calculated using an analytical method developed by Zheng et al. The Urbach relation was used in the fundamental edge region to calculate the width of the band tail states. The behavior of the composition with high concentration of antimony (x=9) was attributed to the produced Se–Sb bonds and an excess of Te–Te bonds, which break the chain structure of the Se–Te system and generate defects in the film. The related optical parameters were also calculated and discussed.