Electret effect in Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> crystals

The short-circuit thermally stimulated depolarization current (TSDC) through lead germanate single crystals has been measured as a function of temperature (100–800 K) and time. The results indicate that poling of the crystals at elevated temperatures (t ≥ 150°C) produces an electret state related to the spacecharge mechanism of polarization. Relatively mild poling conditions (E = 0.75 kV/cm, t = 150°C) produce a well-defined, long-lived electret with an abnormally large electret charge (1400 μC/cm²). The electret discharge leads to heating of the crystal, in the form of a peak in the range 520–570 K, whose height exceeds that of the pyroelectric peak near T _C by two to three orders of magnitude. From the TSDC data, we evaluated the stored electret charge, its density, and the activation energy of traps for electret charges, and made preliminary conclusions as to the nature of the traps. We demonstrate that neglect of the electret discharge current (which depends on a number of factors) when determining the pyroelectric coefficient p ^σ above 280 K leads to a significant scatter in data.     

Optical and X-ray photoelectron spectroscopy of PbGeO<Subscript>3</Subscript> and Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> single crystals

Pb₅Ge₃O₁₁ crystals are found to exhibit pale yellow colouration while PbGeO₃ are colourless. X-ray photoelectron spectroscopy (XPS) measurements show lead deficiency in both the crystals. The results also reveal a stronger ionic character for PbGeO₃ as compared to Pb₅Ge₃O₁₁ crystal. The binding energy of Ge3d core level in the case of Pb₅Ge₃O₁₁ crystal is found to be smaller than the binding energy of germanium oxide, thereby indicating the incomplete oxidation of Ge ions in the crystal lattice. On gamma ray irradiation, the transmission of both the crystals is observed to deteriorate uniformly over the entire wavelength range, which has been attributed to the oxidation of some of the lattice Pb ions. On gamma irradiation the changes observed in O1s core level energies for both the crystals are seen to be consistent with the changes noted in the Pb4f _7/2 and Ge3d spectra. Interestingly, the results reveal oxidation of surface Ge atoms with atmospheric oxygen under gamma irradiation.     

X-ray diffraction study of the Cu<Subscript>2</Subscript>Se-In<Subscript>2</Subscript>Se<Subscript>3</Subscript>-Cr<Subscript>2</Subscript>Se<Subscript>3</Subscript> system near CuInCr<Subscript>2</Subscript>Se<Subscript>5</Subscript>

The structure of three compounds in the Cu₂Se-In₂Se₃-Cr₂Se₃ system near CuInCr₂Se₅ is determined by single-crystal x-ray diffraction: CuInCr₄Se₈ (I), Cu₂In₂Se₄ (II), and Cu_0.5In_0.5Se (III). I has a cubic (spinel type) structure: a = 10.606(4) Å, Z = 4, sp. gr. F43m. II has a pseudotetragonal (sphalerite type) structure: a = 5.774(2) Å, c = 11.617(6) Å. The structure of II was solved in a reduced unit cell with a = 5.774(2) Å, b = 5.774(2) Å, c = 7.095(6) Å, = 113.95(5)°, = 113.95(5)°, = 90.00(4)°, Z = 1, sp. gr. P1. III has a triclinic cell (disordered structure of II): a = 4.088(1) Å, b = 4.091(2) Å, c = 4.101(1) Å, = 60.05(1)°, = 60.08(1)°, = 89.98(4)°, Z = 1, sp. gr. P1. The Cu and In atoms in I sit in inequivalent tetrahedral sites, and the Cr atoms reside in octahedral interstices of the close packing of Se atoms. The bond lengths are In–Se = 2.538(6), Cr(1)–Se(1) = 2.514(7), Cr(1)–Se(2) = 2.576(8), and Cu–Se = 2.437(5) Å. In II, all of the atoms sit in tetrahedral sites; the mean bond lengths are In–Se = 2.578(6) and Cu–Se = 2.44(1) Å. In III, the Cu and In atoms are fully disordered in the same tetrahedral site; the mean Cu(In)–Se bond length is 2.508(6) Å.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1435–1439.Original Russian Text Copyright © 2004 by Antsyshkina, Sadikov, Koneshova, Sergienko.     

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.     

Electrical,Thermal and Spectroscopic Characterization of Bulk Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Topological Insulator

We report the electrical (angular magneto-resistance and Hall), thermal (heat capacity) and spectroscopic (Raman, X-ray photoelectron, angle-resolved photoelectron) characterization of a bulk Bi₂Se₃ topological insulator, which was grown by self-flux method through solid-state reaction from high-temperature (950^°C) melt and slow cooling (2^°C/h) of constituent elements. Bi₂Se₃ exhibited metallic behaviour down to 5 K. Magneto-transport measurements revealed linear up to 400 and 30% magneto-resistance (MR) at 5 K under a 14-T field in perpendicular and parallel field directions, respectively. We noticed that the MR of Bi₂Se₃ is very sensitive to the angle of the applied field. The MR is maximum when the field is normal to the sample surface, while it is minimum when the field is parallel. The Hall coefficient (R _H) is seen nearly invariant with a negative carrier sign down to 5 K albeit having near-periodic oscillations above 100 K. The heat capacity (C _p) versus temperature plot is seen without any phase transitions down to 5 K and is well fitted (C _p = γ T + β T ³) at low temperature with a calculated Debye temperature (?? _D) value of 105.5 K. Clear Raman peaks are seen at 72, 131 and 177 cm^?1 corresponding to A\(_{\mathrm {1g}}^{1}\), E\(_{\mathrm {g}}^{2}\) and A\(_{1\mathrm {g}}^{2}\), respectively. Though two distinct asymmetric characteristic peak shapes are seen for Bi 4f_7/2 and Bi 4f_5/2, the Se 3d region is found to be broad, displaying the overlapping of spin-orbit components of the same. Angle-resolved photoemission spectroscopy (ARPES) data of Bi₂Se₃ revealed distinctly the bulk conduction bands (BCB), surface state (SS), Dirac point (DP) and bulk valence bands (BVB), and 3D bulk conduction signatures are clearly seen. Summarily, a host of physical properties for the as-grown Bi₂Se₃ crystal are reported here.     

Anisotropic physical properties of Mn<Subscript>5</Subscript>Ge<Subscript>3</Subscript> crystals

We have prepared chemically homogeneous Mn₅Ge₃ single crystals and evaluated the anisotropy in their electrical resistivity and linear expansion coefficient. These properties have been shown to vary anomalously with temperature near the ferromagnetic transition of Mn₅Ge₃. In the temperature range 320–900 K, its resistivity remains constant.     

Photocurrent and Optical Transmission Spectra of Sn- and Pb-Doped (As<Subscript>2</Subscript>S<Subscript>3</Subscript>)<Subscript>0.3</Subscript>(As<Subscript>2</Subscript>Se<Subscript>3</Subscript>)<Subscript>0.7</Subscript> Glass Films

The photocurrent and optical transmission spectra of thin (As₂S₃)_0.3(As₂Se₃)_0.7 glass films doped with Sn and Pb are reported. The strongest photoresponse is offered by the films doped with 0.010– 0.015 at % Sn or Pb. Low doping levels are shown to have a significant effect on the peak-response wavelength and band gap of the films.     

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.     

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.     

Low-frequency photodielectric phenomena in polycrystalline Pb<Subscript>3</Subscript>O<Subscript>4</Subscript> layers

Photopolarization phenomena in the polycrystalline layers of Pb₃O₄ with a binder have been studied. The samples exhibited a positive photocapacitive effect, an increase in dielectric losses under illumination, and a low-frequency dispersion of dielectric parameters. Possible mechanisms of relaxation processes in the dark and under illumination and the nature of relaxators are discussed.     

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.     

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.     

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.     

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.     

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.     

Synthesis and High-Temperature Heat Capacity of Dy<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript> and Ho<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates have been prepared by solid-state reactions in several sequential firing steps in the temperature range 1237–1473 K using stoichiometric mixtures of Dy₂O₃ (or Ho₂O₃) and GeO₂. The heat capacity of the synthesized germanates has been determined as a function of temperature by differential scanning calorimetry in the range 350–1000 K. The experimentally determined C _p (T) curves of the dysprosium and holmium germanates have no anomalies and are well represented by the Maier–Kelley equation. The experimental C _p (T) data have been used to evaluate the thermodynamic functions of the Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates: enthalpy increment H°(T)–H°(350 K), entropy change S°(T)–S°(350 K), and reduced Gibbs energy Ф°(T).     

Synthesis and High-Temperature Heat Capacity of Sm<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript> and Eu<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Sm₂Ge₂O₇ and Eu₂Ge₂O₇ germanates have been prepared by solid-state reactions via multistep firing of stoichiometric mixtures of Sm₂O₃ (Eu₂O₃) and GeO₂ in air at temperatures from 1273 to 1473 K. The molar heat capacity of the samarium and europium germanates has been determined by differential scanning calorimetry in the range 350–1000 K and the C _p (T) data have been used to evaluate their thermodynamic properties.     

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.     

Impedance analysis of Pb<Subscript>2</Subscript>Sb<Subscript>3</Subscript>LaTi<Subscript>5</Subscript>O<Subscript>18</Subscript> ceramic

Polycrystalline sample of Pb₂Sb₃LaTi₅O₁₈, a member of tungsten- bronze (TB family, was prepared using a high temperature solid- state reaction technique. XRD analysis indicated the formation of a single-phase orthorhombic structure. The dielectric studies revealed the diffuse phase transition and the transition temperature was found to be at 52° C. Impedance plots were used as tools to analyse the sample behaviour as a function of frequency. Cole-Cole plots showed Debye relaxation. The activation energy was estimated to be 0·634 eV from the temperature variation of d.c. conductivity. The nature of variation of d.c. conductivity with temperature suggested NTCR behaviour.     

Giant Magnetoresistance in Topological Insulator Bi<Subscript>1.98</Subscript>Nd<Subscript>0.02</Subscript>Se<Subscript>3</Subscript> Single Crystal

Doped topological insulators (TI) Bi_2?x Nd _x Se₃ single crystals were prepared by the self-flux method. The phase structure, magnetic properties, and electrical transport properties of the samples were studied. The X-ray diffraction (XRD) patterns of the sample indicate an incorporation of Nd into the Bi₂Se₃, and the crystal can be easily cleaved with silvery surface. The Bi_2?x Nd _x Se₃ sample shows a giant magnetoresistance (MR) with different magnetic field. The positive magnetoresistance (MR) can reach 190 % at the field of 9 Tesla when the field is perpendicular to ab-plane of the crystal. In addition, at low magnetic fields, the MR exhibits a weak antilocalization (WAL) cusp.