Se-Ga23Se phase equilibria: a regular associated solution model for the Ga-Se liquid

Se vapor pressures were measured by the dew point method for the binary system Ga_1·xSe_x, 0.60 < x < 1.00 (Ga₂Se₃-Se), between 700 and 1050°C. The Se vapor pressure was found to decrease rapidly near x = 60 atomic% Se. The T-X and P-X liquidus curves are thermodymanically modelled by a regular associated solution model for the liquid phase, with the associate, Ga₂Se₃. The best fit to the data was obtained with 4000 cal/mole of Ga₂Se₃ for the regular solution interaction parameter and 0.10 for the degree of dissociation in the stoichiometric liquid (x = 0.60).     

Über Kristallstruktur und elektrische Eigenschaften der Wismutselenide Bi2Se2 und Bi2Se3

The properties of bismuth triselenide (Bi₂Se₃) are already known to a certain extent through the work of several authors, while it was still an open question whether there exists an individual solid phase of BiSe. Further information on this subject could be obtained by the successful growth and investigation of single crystals of both Bi₂Se₃ and Bi₂Se₂. X-ray analysis by means of goniometry, Weißenberg, Laue, and Debye-Scherrer diagrams confirmed the known crystal structure of Bi₂Se₃ (ditrigonal scalenohedral;D _3d ⁵ ?R—m; with the hexagonal axes:a=4·15 Å andc=28·55 Å, and 3 molecules per unit cell). As to Bi₂Se₂ it can be shown that it belongs to the same class but to a different space group (D _3d ¹ ?P— 1m orD _3d ³ ?P—m 1; hexagonal axes:a=4·15 Å,c=22·84 Å, unit cell: 3 molecules, if the formula Bi₂Se₂ is adopted). Common to both is a subcell with the dimensions:a′=a=4·15 Å andc′=5·71 Å. The temperature dependence of electrical conductivity and Hall coefficient was measured on several specimens having different crystal orientations. The most striking difference is the high anisotropy of Bi₂Se₃ (σ _a σ _c =10) as compared with Bi₂Se₂ (σ _a /σ _c <2). All specimens turned out to ben-type. The room temperature carrier concentration observed was:n (Bi₂Se3)=8·10¹⁸ cm^?3 andn (Bi₂Se₂)=4·10²⁰ cm^?3, the carrier mobility:μ(Bi₂Se3)=2·10³ cm²/V·s andμ(Bi₂Se₃)=20 cm²/V·s.     

P−T phase diagram for ferromagnetic semiconductor CdCr2Se42

The P?T phase diagram associated with the ferromagnetic semiconductor CdCr₂Se₄ was determined between 600 and 964°C for the first time from the P_{Se2(or Cd)} ? T diagram for the stability of the compound disclosed by annealing experiments under controlled Se₂ and Cd vapor pressures, and from the phase equilibria in the CdSeCrSeSe system examined by usual annealing experiments. The following characteristic features are also clarified. (1) The maximum temperature for the stability of CdCr₂Se₄ is about 900°C. (2) CdCr₂Se₄ dissolves into Se melt and is in equilibrium with the Se-rich melt which has a CdSe content in excess of CdSe·Cr₂Se₃. The solubility decreases rapidly as temperature falls from 900 to 860°C. (3) Cr₂Se₃ is the only compound in the CrSe system in equilibrium with CdCr₂Se₄ above about 860°C. Both Cr₃Se₄ and Cr₂Se₃ exist below this temperature. (4) The P_Se2 range for the stability of CdCr₂Se₄ is estimated as a measure of the concentration ranges of native defects due to nonstoichiometry. It increases as temperature decreases and is about 5 × 10⁹ at 600°C.     

Electrical properties of LiBSe ternary system

Amorphous products were obtained in the LiBSe ternary system by quenching melts of Li₂Se, B and Se mixture prepared at 100°C in sealed silica tubes. The vitreous region was slightly lower selenium composition than that of the Li₂SeB₂Se₃ tie-line. The amorphous products were lithium ionic conductors and most of them showed contributions to their total conductivity. The amorphous product of composition Li₂₅B₃₆Se₃₉ has the least electronic contribution to its total conductivity of 6.0 × 10⁻⁶ S/cm at room temperature. A new crystalline compound and crystalline LiBH₄ were also obtained in LiBSe ternary. Both of them were lithium ionic conductors having conductivities of about 1 × 10⁻⁶ S/cm at room temperature.     

Photoelectron spectra of Ga20, In20 and Tl20

The HeI photoelectron spectra of Ga₂O, In₂O, and Tl₂O, as well as atomic In, have been obtained. Some spectral features of the recently published photoelectron spectrum of Al₂O differ noticeably from the heavier suboxides. The assignment of spectral features is supported by fully relativistic DVM SSC X_α calculations, which have been extended to include Al₂O. Best agreement is obtained between calculation and experiment for linear Al₂O, but for bent heavier metal suboxides. In general, the spectral features are indicative of ionic bonding, analogous to the behavior of group IIIB monohalides. Along the sequence Al₂O:Ga₂O:In₂O:Tl₂O, the highest occupied orbital becomes less and less localized on the metal atoms, with a rather dramatic decrease in metal character occurring at Tl₂O. The latter effect is attributable to a change in relative energies of the charge-adjusted metal and oxygen atomic orbitais and relativistic effects., A decrease in the relative ionization cross-section for molecular orbitais which have a large metal ns contribution is correlated with a similar behavior of the relative (ns)⁻¹cross-section for the corresponding atoms.     

Abhängigkeit der effektiven Masse von der Ladungsträgerkonzentration im Bi2Se3

High quality Bi₂Se₃ crystals with 50 to 65 At% Se have been produced. As a consequence of a Se-doping, the carrier density varies between 3 · 10¹⁹ to 8 · 10¹⁷ cm^?3. Based on optical reflection and transmission measurements in the near infrared the dispersion of the refractive indexn(λ) has been calculated. The plasma resonance frequencyΝ _Pl lies within the energy region from 0.05 to about 0.1 eV. Fromn(λ) andΝ _Pl the effective mass of the electrons in Bi₂Se₃ has been evaluated. Near the band edge a value ofm _⊥c ^*+0.02m ₀ has been found. It depends on the orientation of the crystals and on the carrier concentration. The results of the electrical and optical investigations give reason to assume an isotropic mean free time of the electrons. The “six-valley-model” ofHashimoto for the energy-surfaces in Bi₂Se₃, is confirmed.     

Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3

We show that by Ca doping the Bi₂Se₃ topological insulator, the Fermi level can be fine tuned to fall inside the band gap and therefore suppresses the bulk conductivity. Non-metallic Bi₂Se₃ crystals are obtained. On the other hand, the Bi₂Se₃ topological insulator can also be induced to become a bulk superconductor, with T_c∼3.8 K, by copper intercalation in the van der Waals gaps between the Bi₂Se₃ layers. Likewise, an as-grown crystal of metallic Bi₂Te₃ can be turned into a non-metallic crystal by slight variation in the Te content. The Bi₂Te₃ topological insulator shows small amounts of superconductivity with T_c∼5.5 K when reacted with Pd to form materials of the type Pd_zBi₂Te₃.     

Multiphonon absorption in As2S3As2Se3 glasses

Multiphonon absorption in As₂S₃ and As₂Se₃ glasses is well explained by a molecular model in terms of combination bands of high frequency vibrational modes of pyramidal AsY₃ and bent AsYAs groups (Y = SorSe). Multiphonon absorption coefficients in mixed As₂S₃As₂Se₃ glasses are nearly additive in terms of the pure components, suggesting a high degree of non-random mixing.     

Electromotive force of a COCO2 sensor in COCO2H2H2O atmospheres and simultaneous determination of partial pressures of CO and CO2

The electromotive force (EMF) of the COCO₂ sensor using Na₂CO₃ and NASICON (Na₃Zr₂Si₂PO₁₂) as solid electrolytes has been examined in COCO₂Ar atmospheres. The EMF is related to the partial pressures of CO and CO₂ and proportional to log(P²_CO2P⁻¹_CO). The simultaneous use of the oxygen sensor of stabilized zirconia gives the EMF proportional to log(P_CO2P⁻¹_CO). The EMF's of two sensors permit to determine individually partial pressures of CO and CO₂. The existence of H₂ with high concentration does not affect the EMF's. This fact proves the applicability of the two-sensor system to the monitoring and the controlling of reducing atmospheres in industrial processes.     

Magnetic and transport properties of Mn-doped Bi2Se3 and Sb2Se3

We report on the single crystal growth and thermoelectric and magnetic properties of Mn-doped Bi₂Se₃ and Sb₂Se₃ single crystals prepared by the temperature gradient solidification method. The composition and crystal structure were determined using electron probe microanalysis and θ–2θ powder X-ray diffraction studies, respectively. The lattice constants of several percent Mn-doped Bi₂Se₃ and Sb₂Se₃ were slightly smaller than those of the undoped sample due to the smaller Mn atomic radius (1.40 Å) than those of Bi (1.60 Å) and Sb (1.45 Å). Mn-doped Bi₂Se₃ and Sb₂Se₃ showed spin-glass and paramagnetic properties, respectively.     

Structural and electronic properties of zincblende phase of Tl x Ga1−x As y P1−y quaternary alloys: First-principles study

Using the first-principles band-structure method, we have calculated the structural and electronic properties of zincblende TlAs, TlP, GaAs and GaP compounds and their new semiconductor Tl _x Ga_1?x As _y P_1?y quaternary alloys. Structural properties of these semiconductors are obtained with the Perdew and Wang local-density approximation. The lattice constants of Tl _x Ga_1?x As, Tl _x Ga_1?x P ternary and Tl _x Ga_1?x As _y P_1?y quaternary alloys were composed by Vegard’s law. Our investigation on the effect of the doping (Thallium and Arsenic) on lattice constants and band gap shows a non-linear dependence for Tl _x Ga_1?x As _y P_1?y quaternary alloys. The band gap of Tl _x Ga_1?x As _y P_1?y , E _g (x, y) concerned by the compositions x and y. To our awareness, there is no theoretical survey on Tl _x Ga_1?x As _y P_1?y quaternary alloys and needs experimental verification.     

Phase stability and structure of alkali doped-beta-gallia rutile intergrowths

The incorporation of alkali cations into the tunneled structure of Ga₄TiO₈ was investigated and compared to predictions based on atomistic computer simulations. Samples were prepared as A_xGa_4−xTi_1−xO₈, A=Li, Na, and K, x ≤ 0.7, and as Na_xGa_4+xTi_2−xO₁₀ (x = 0.7, 0.85, and 1.0) using solid-state reactions at 1050–1350 °C. The sodium-containing tunneled structure, Na_xGa_4−xTi_1−xO₈, formed via solid-state reaction, but the potassium and lithium analogs did not. Instead, these systems formed mixed-phase assemblages, which are discussed in reference to compatibility triangles in the Li₂O–Ga₂O₃–TiO₂ and K₂O–Ga₂O₃–TiO₂ systems. Experimental results were compared to the results of energy minimization calculations using the General Utility Lattice Program (GULP). For the lithium-containing system, the computer simulations correctly predicted the formation of a mixed-phase assemblage containing LiGa₅O₈, Ga₂O₃, and TiO₂. For the sodium- and potassium-containing system, the computer simulations suggested that mixtures of the single-cation oxide components should be the stable phase assemblages, in contradiction with experimentally observed results. Energy minimization calculations conducted on structurally different Na_xGa_4+xTi_2−xO₁₀ and Na_xGa_4+xTi_3−xO₁₂ phases indicated that those based on the n = 6 and n = 7 β-gallia rutile intergrowth structures have lower lattice energies than the experimentally observed sodium titanogallate structures reported previously in literature.     

The effect of addition of gallium on the thermal stability and crystallization kinetic parameters of GaxSe100−x glass system

Differential scanning calorimetry (DSC) technique was used to study the kinetics of amorphous to crystalline transformation for Ga_xSe_100−x glass system (x=0, 2.5 and 5 at%). The kinetic parameters of Ga_xSe_100−x glass system under non-isothermal conditions are analyzed by the model-free and model-fitting models at different constant heating rates (5-50 K/min). A strong heating rate dependence of the effective activation energy of crystallization was observed. The analysis of the present data shows that the effective activation energy of crystallization is not constant but varies with the degree of crystallization and with temperature as well. The crystallization mechanisms examined using the local Avrami exponents indicate that one mechanism (volume nucleation with one-dimensional growth) is responsible for the crystallization process for heating rates 5-50 K/min for Se glass and two mechanisms (volume nucleation with two- and one-dimensional growth) are working simultaneously during the amorphous-crystalline transformation of the Ga_2.5Se_97.5 and Ga₅Se₉₅ glasses (5-50 K/min). The reaction model that may describe crystallization process of all the compositions of Ga_xSe_100−x glass system is Avrami-Erofeev model (g(α)=[−ln(1−α)]^1/n) with n=2 for Se glass. While for Ga_2.5Se_97.5 and Ga₅Se₉₅ glasses, the values of n are equal to 3 and 2 for the heating rates 5-20 and 35-50 K/min, respectively. A good agreement between the experimental and the reconstructed (α-T) curves has been achieved. The transformation from amorphous to crystalline phase in Ga_xSe_100−x glass system demonstrates complex multi-step involving several processes.     

Geometries and electronic structures of Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript>, Ga<Subscript>3</Subscript>Se<Subscript>2</Subscript> and their anions. Theoretical insights

We hereby report a theoretical study on the equilibrium geometries, electronic structures and harmonic vibrational frequencies of Ga₂Se₃, Ga₃Se₂ and their anions. The ground and low-lying excited states of Ga₂Se₃⁻, Ga₂Se₃, Ga₃Se₂⁻ and Ga₃Se₂ are studied at the B3LYP and/or MP2 and CCSD(T) levels in conjunction with 6-311+G(d) and 6-311+G(2df) one particle basis sets. Ga₂Se₂⁻ adopts the C_2v kite geometry while Ga₂Se₃ has a ‘V’ geometry. Ga₃Se₂⁻ has a three-dimensional ‘D_3h ’ geometry and Ga₃Se₂ prefers the three-dimensional ‘C_2v ’ structure. Electron detachment energies from the ground electronic states of the anions to several neutral states are reported and discussed. At CCSD(T)//MP2 level, the adiabatic electron affinity (AEA) of Ga₂Se₃ is calculated to be 3.23 eV when using the 6-311+G(2df) basis set and that of Ga₃Se₂ is 2.77 eV with the 6-311+G(d) basis set. The findings of this research are analyzed and compared with gallium oxide and sulfide analogues.     

Photoconductivity in amorphous thin films of Ge22Se78−xBix

Temperature and intensity dependence of photoconductivity is studied in amorphous thin films of Ge₂₂Se_78−x Bi_x with x = 0, 2 and 10. Transient photoconductivity measurements have also been made on the same samples. Our results show that photosensitivity decreases as Bi concentration is increased from x = 0 to x = 2. However, at high concentration of Bi(x = 10), photosensitivity again increases. Transient photoconductivity also show a different behaviour at low and high concentration of Bi. Results have been explained in terms of defect states produced due to Bi incorporation in GeSe system.     

Investigations of binary lithium-zinc,lithium-cadmium and lithium-lead alloys as negative electrodes in organic solvent-based electrolyte

The thermodynamic properties of the binary LiZn, LiCd and LiPb alloy systems were investigated in an organic solvent-based electrolyte at ambient temperature. Titration curves showed several intermediate phases existing in each lithium alloy system. There was a wide composition range in Li_yCd (1.5 ⩽ y ⩽ 3), which showed a very low equilibrium potential (below 60 mV) against pure lithium. There was a longer potential plateau in the LiPb system corresponding to Li_yPb (1 ⩽ y ⩽ 3) at about 450 mV against pure lithium. The preliminary charge-discharge and polarization results showed rather fast kinetics for Li_yZn (0.67 ⩽ y ⩽ 1), Li_yCd (1.5 < y < 2.0), and Li_yPb (1 < y < 3).     

An analysis of the thermoelectric efficiency of n-(Bi,Sb)2(Te,Se,S)3 solid solutions within an isotropic scattering model

A study is reported on the thermoelectric properties of n-type solid solutions Bi₂Te_3?y Se_y (y=0.12, 0.3, 0.36), Bi_2?x Sb_xTe_3?y Se_y (x=0.08, 0.12; y=0.24, 0.36), and Bi₂Te_3?z S_z (z=0.12, 0.21) as functions of carrier concentration within the 80-to 300-K range. It has been established that the highest thermoelectric efficiency Z is observed in the Bi₂Te_3?y Se_y (y=0.3) solid solution containing excess Te at optimum carrier concentrations (0.35×10¹⁹ cm^?3) and at temperatures from 80 to 250 K. The increase in Z in the Bi₂Te_3?y Se_y solid solution compared with Bi_2?x Sb_xTe_3?y Se_y and Bi₂Te_3?z S_z is accounted for by the high mobility μ₀, an increase in the effective mass m/m ₀ with decreasing temperature, the low lattice heat conductivity κ_L, and the weak anisotropy of the constant-energy surface in a model assuming isotropic carrier scattering.     

Bi2O3 rods deposited under atmospheric pressure by means of halide CVD on c-sapphire

Bismuth Oxide (Bi₂O₃) rods are successfully prepared on δ-Bi₂O₃ films under atmospheric pressure by means of halide chemical vapour deposition using BiI₃ and O₂ as a starting material. The deposition of Bi₂O₃ rods strongly depends on the deposition temperature, the input partial pressure of BiI₃ and O₂ and the method for supplying O₂ gas. Bi₂O₃ rods can be obtained at [O₂]/[BiI₃] ratios of 500 and N₂:O₂=50:250. The length of the Bi₂O₃ rods increases proportionally from 2 to 30 μm, while their diameters of between 0.2 and 0.5 μm do not depend on the deposition time.     

Electro- and thermoreflectance of Bi2Te3 and Bi2Se3

Electroreflectance measurements in Bi₂Te₃ and Bi₂Se₃ with the electric field vector of the incident light both inclined and perpendicular to the C-axis have been made at room temperature. The structures found by other workers in the reflection measurements are observed in the present experiment, together with new structures at 0.91 eV, 1.18 eV, 1.78 eV, and 2.61 eV in Bi₂Se₃ which are not related to formerly observed transitions. From these measurements, the selection rules for direct optical transitions in Bi₂Te₃ and Bi₂Se₃ are studied. Thermoreflectance measurements are also made at both room and liquid-nitrogen temperatures. The positions of the peaks obtained in the present work are compared with the electroreflectance and reflection data.     

Effects of Li2CO3 on the sintering behavior and piezoelectric properties of Bi2O3-excess (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics doped with Li₂CO₃ and Bi₂O₃ as sintering aids were manufactured, and their micro structural, dielectric and piezoelectric properties were investigated. All specimens could be well sintered at a low-temperature of 1080 °C. The bulk density of the specimens doped with a small amount of Li₂CO₃ was enhanced. The dielectric and piezoelectric properties of ceramics were investigated with different amounts of Li₂CO₃ substitutions. High electrical properties of d₃₃ = 167 pC/N, k_p = 0.34, P_r = 40 μC/cm² and E_c = 38 kV/cm were obtained from the specimen containing 0.1 mol% of Li₂CO₃ sintered at 1080 °C.