X-ray diffraction study of ternary layered compounds in the PbSe-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

The pseudobinary system PbSe-Bi₂Se₃ has been studied by x-ray diffraction using cleaved single-crystal specimens and powders, and the compositions of the layered compounds existing in this system have been refined. The monoclinic cell parameters of the compounds Pb₅Bi₆Se₁₄ and Pb₅Bi₁₂Se₂₃, belonging to the [(PbSe)₅] _m [(Bi₂Se₃)₃] _n homologous series, have been determined.     

Electrical Properties of CuInCr<Subscript>2</Subscript>Se<Subscript>5</Subscript>-Based Solid Solutions

Data are presented on the electrical properties of single-crystal and polycrystalline CuInCr₂Se₅-based solid solutions.     

Cathodo- and photoluminescence of Cu<Subscript>3</Subscript>Ga<Subscript>5</Subscript>Se<Subscript>9</Subscript> single crystals

The cathodo- and photoluminescence spectra of Cu₃Ga₅Se₉ single crystals have been measured at different excitation intensities and temperatures. The results indicate that the radiative recombination of nonequilibrium charge carriers occurs primarily through impurity levels due to anion and cation vacancies. The anion and cation vacancy concentrations can be controlled by thermal and laser anneals.     

Phase equilibria in the Ag<Subscript>2</Subscript>Se-As<Subscript>2</Subscript>Se<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

We have studied phase equilibria in the pseudoternary system Ag₂Se-As₂Se₃-Bi₂Se₃ and constructed the 300-, 600-, and 800-K isothermal sections, a number of partial phase diagrams, and the liquidus projection of this system. The AgAsSe₂-AgBiSe₂ and As₂Se₃-AgBiSe₂ joins are shown to be pseudobinary, and the Ag₃AsSe₃-AgBiSe₂ and AgAs₃Se₅-AgAsSe₂ joins are pseudobinary below the liquidus. Several in- and univariant peritectic, eutectic, and eutectoid equilibria and a broad region of AgBiSe₂-based solid solutions are identified. The homogeneity region of the AgBiSe₂-based phase has the largest extent along the AgAsSe₂-AgBiSe₂ join: 40 mol % (650 K) for the high-temperature form of AgBiSe₂ and 20 mol % (300 K) for its low-temperature form.     

Phase transitions and transport properties of Cu<Subscript>3</Subscript>Co<Subscript>0.5</Subscript>Se<Subscript>2</Subscript> crystals

We have synthesized a solid solution with the composition Cu₃Co_0.5Se₂ and investigated its structural phase transitions by high-temperature X-ray diffraction on a D8 Advance diffractometer. The results demonstrate that, in the range 100–773 K, the solid solution undergoes two phase transitions: at 560 K, the low-temperature, orthorhombic phase (α) transforms into an intermediate, primitive cubic phase (β); at 765 K, the β-phase transforms into a high-temperature phase (γ, sp. gr. Fm3m). The thermoelectric power, electrical conductivity, and Hall coefficient of the solid solution were measured in the temperature range 80–350 K. The observed temperature variation of its thermoelectric power and conductivity can be accounted for under the assumption that its conduction band has an additional subband due to the cobalt atoms.     

X-ray diffraction study of compounds in the Ag<Subscript>2</Subscript>S-Cu<Subscript>2</Subscript>S system

Ag_1.55Cu_0.45S and Ag_0.93Cu_1.07S single crystals have been grown by the Bridgman method, and their polymorphic transformations have been studied by high-temperature x-ray diffraction.     

Formation of solid solutions in the TiO<Subscript>2</Subscript>-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The phase formation and reaction kinetics in the TiO₂-Cr₂O₃ system have been studied by x-ray diffraction and electron microscopy. The Cr₂O₃ solubility in TiO₂ has been accurately determined, and the rate parameters of the formation of solid solutions in this system have been evaluated. The results demonstrate that Cr₂O₃ dissolves in rutile and not in anatase. Cr₂O₃ markedly reduces the temperature of the anatase-rutile phase transition.     

Solid solutions in the Ag<Subscript>2</Subscript>Se-PbSe-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

The phase equilibria in the pseudoternary system Ag₂Se-PbSe-Bi₂Se₃ have been studied using differential thermal analysis, x-ray diffraction, and microhardness measurements. The results have been used to construct the T-x phase diagram along the AgBiSe₂-PbSe join and the 900-K section of the ternary phase diagram. The AgBiSe₂-PbSe system contains a continuous series of cubic solid solutions with the NaCl structure. Their lattice parameter is an almost linear function of composition (a = 5.822–6.125 Å). The formation of the solid solutions stabilizes the high-temperature phase of AgBiSe₂: PbSe dissolution in this compound markedly reduces its polymorphic transformation temperature (590 K), down to room temperature at ? 10 mol % PbSe. In the Ag₂Se-PbSe-Bi₂Se₃ system, the γ-phase exists in a broad region around the AgBiSe₂-PbSe pseudobinary join.     

Composition and properties of compounds in the PbSe-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

To identify the compounds existing in the PbSe-Bi₂Se₃ system, Bridgman-grown ingots and annealed polycrystalline samples have been characterized by X-ray diffraction. The results indicate that the first solidified portion of the Bridgman-grown ingots consists of a PbSe-based solid solution with a cubic structure. Further directional solidification involves peritectic reactions that lead to the formation of the ternary compounds Pb₅Bi₆Se₁₄ and Pb₅Bi₁₂Se₂₃, which have monoclinic structures. The structural data have been used to refine the phase diagram of the PbSe-Bi₂Se₃ system. The thermoelectric properties of Pb₅Bi₆Se₁₄, Pb₅Bi₁₂Se₂₃, and Pb5Bi18Se32 have been studied using annealed polycrystalline samples. Their Hall coefficient and resistivity have been measured in the temperature range 80–670 K, and their thermoelectric power, electrical conductivity, and thermal conductivity have been measured from 80 to 370 K. The ternary compounds in the PbSe-Bi₂Se₃ system have low lattice thermal conductivity, which can be understood in terms of the key features of their crystal structure.     

Redox processes in fine-particle TiO<Subscript>2</Subscript>-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> oxides

We have studied the stability of the Cr⁶⁺ ion in fine-particle TiO₂-Cr₂O₃ oxides during storage after calcination in air. The results indicate that, during storage under normal conditions for 720 days, Cr⁶⁺ is reduced to Cr³⁺. The redox process is due to partial surface hydration of the Cr₂O₃ and TiO₂ crystallites.     

Photosensitivity of thin-film structures based on CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> and CuIn<Subscript>5</Subscript>Se<Subscript>8</Subscript> ternary semiconductor compounds

Polycrystalline n-type CuIn₃Se₅ and CuIn₅Se₈ films with thicknesses from 0.4 to 1 μm have been grown by pulsed laser ablation of bulk p-CuIn₃Se₅ and n-CuIn₅Se₈ crystals in vacuum. The temperature dependences of the resistivities of these crystals are determined by deep donor levels with energies E _D ? 0.2–0.3 eV. Photosensitive thin-film structures based on these films have been created for the first time and their photosensitivity spectra have been measured. The possibility of using thin CuIn₃Se₅ and CuIn₅Se₈ films in broadband photoconverters is demonstrated.     

Oxidation kinetics of chromium in fine-particle TiO<Subscript>2</Subscript>-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Oxides

The mass transport during the oxidation of Cr₂O₃ in mixtures of fine-particle TiO₂ and Cr₂O₃ in air has been studied at temperatures from 800 to 1000°C by magnetochemical analysis. The results indicate that the process leads to the oxidation of up to ten surface atomic layers. The mass transport is shown to be a stochastic, steplike process.     

Phase relations and thermoelectric properties of alloys in the Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

Using differential thermal analysis and x-ray diffraction, we have shown that the Bi₂Te₃-Bi₂Se₃ system contains a continuous series of solid solutions in a narrow temperature range and a compound of composition Bi₂Te₂Se below the solidus line. The liquidus and solidus lines determined using zone-melted samples differ little from those reported in the literature for equilibrium samples. The Bi₂Te_3?x Se _x solid-solution phase extends to ～-14 mol % Bi₂Se₃ (Bi₂Te_2.58Se_0.42). The thermoelectric power of the alloys drops sharply near the boundary of the two-phase region. Within the homogeneity range of Bi₂Te₂Se (33.3 mol % Bi₂Se₃), the thermoelectric power factor has a minimum, while the thermoelectric power has a small maximum.     

New forms of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> and Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> prepared by reacting InAs and GaAs substrates with selenium

Ga₂Se₃ and In₂Se₃ prepared through heterovalent substitution during thermal annealing of single-crystal gallium arsenide and indium arsenide substrates in selenium vapor in a quasi-closed system have been characterized by electron diffraction, scanning electron microscopy, and X-ray microanalysis. Cubic phases of In₂Se₃ (a ₀ = 1.1243 nm and a ₀ = 1.6890 nm) and Ga₂Se₃ (a ₀ = 1.0893 nm and a ₀ = 1.6293 nm) have been identified for the first time.     

High-Field Magneto-Conductivity Analysis of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystal

We report the high-field (up to 14 Tesla) magneto-conductivity analysis of Bi₂Se₃ topological insulator grown via the self-flux method. The detailed experimental investigations including crystal growth as well as the electrical, thermal, and spectroscopic characterizations of the resultant Bi₂Se₃ single crystal are already reported by some of us. The current letter deals with high-field magneto-conductivity analysis in terms of Hikami Larkin Nagaoka (HLN) model, which revealed that the electronic conduction is dominated by both surface state-driven weak anti-localization (WAL), as well as the bulk weak localization (WL) states. Further, by applying the HLN equation, we have extracted the fitting parameters, i.e., phase coherence length (l_φ) and the pre-factor (α). Here, the magneto-conductivity data is fitted up to ± 5 Tesla, but in order to extract reliable fitting parameters, the same is fitted at much lower magnetic fields, i.e., up to ± 1 Tesla. The value of the HLN coefficient (α), extracted from the HLN equation exhibited values close to ? 1.0, indicating both WAL and WL contributions. On the other hand, the extracted \(l_{\varphi }\) is seen to decrease from 11.125 to 5.576 nm as the temperature is increased from 5 to 200 K, respectively. Summarily, the short letter discusses primarily the temperature-dependent magneto-conductivity analysis of pristine Bi₂Se₃ single crystal by the HLN model.     

Extruded thermoelectric materials based on Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> solid solutions

Extruded n-type materials based on Bi₂Te₃-Bi₂Se₃ alloys containing 6 to 40 mol % Bi₂Se₃ have been investigated using microstructural analysis and thermoelectric measurements at room temperature and in the range 100–400 K. Their electrical properties have been compared to those of single-crystal analogs. Compositions have been found at which the extruded materials offer the highest thermoelectric performance in different temperature ranges.     

Photoelectric properties of ordered-vacancy Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> single crystals

We have studied the photoconductivity spectrum, thermally stimulated current, current-light characteristics, and temperature-dependent photocurrent in Bridgman-grown ordered-vacancy Ga₂Se₃ crystals. The observed temperature quenching of photoconductivity and two regions of its thermal activation in Ga₂Se₃ crystals are interpreted in terms of a multicenter recombination model which incorporates an s-channel of active recombination, r-centers of photosensitivity, and traps for nonequilibrium majority carriers.     

Crystal structure of Gd<Subscript>14</Subscript>Cu<Subscript>48</Subscript>Ga<Subscript>3</Subscript> and Tb<Subscript>14</Subscript>Cu<Subscript>48</Subscript>Ga<Subscript>3</Subscript>

The compounds Gd₁₄Cu₄₈Ga₃ and Tb₁₄Cu₄₈Ga₃ have been synthesized, and their structures have been determined by powder x-ray diffraction (Gd₁₄Ag₅₁ type).     

Thermodynamic study of the Cu-As-S system by EMF measurements with Cu<Subscript>4</Subscript>RbCl<Subscript>3</Subscript>I<Subscript>2</Subscript> as a solid electrolyte

The Cu-As-S system has been studied at temperatures from 300 to 370 K using emf measurements with Cu₄RbCl₃I₂ as a Cu⁺ ion conducting solid electrolyte, and its subsolidus phase diagram has been mapped out, including the ternary compounds Cu₃AsS₄, Cu₆As₄S₉, Cu₄As₂S₅, Cu₃AsS₃, and CuAsS. From the emf data, we have calculated the partial molar thermodynamic functions (D[`(G)]\Delta \bar G, D[`(H)]\Delta \bar H, and D[`(S)]\Delta \bar S) of the copper in the alloys and the standard thermodynamic functions of formation and standard entropies of the ternary compounds.     

Crystal growth and elastic properties of In<Subscript>2</Subscript>Se<Subscript>3</Subscript>

Perfect α-In₂Se₃ single crystals have been grown, and ultrasound velocities, v _i (i = 1–7), have been measured in single-crystal α-In₂Se₃ in various directions for different polarizations. We have determined the components of its elastic tensor (C _ij ) and calculated its elastic characteristics: elastic compliance, Young’s modulus, shear modulus, linear and volume compressibilities, bulk modulus, and Poisson’s ratio.