Thermoelectric crystals Bi2Te2.88Se0.12 undoped and doped by CdCl2 or CdBr2 impurities, fabricated and characterized by XRD and Hall effect

Empirical studies on the fabrication of thermoelectric element Bi₂Te_2.88Se_0.12 undoped and doped by 0.08 wt.% CdCl₂ or CdBr₂ have been carried out. Zone melting method was employed to crystallize the solid solutions of the compounds. Structural characteristics of the grown crystals were examined by XRD technique. Measurements of thermoelectric parameters, such as electrical conductivity and Seebeck coefficient, showed a continuous deviation of the composition along the crystal growth direction. Effects of an impurity as a dopant on thermoelectric parameters were studied. Finally, Hall effect system was used to measure free carrier concentration and their mobility at 300 K. Results showed a significant increment on α²σ, due to dopant addition.     

Crystal structure, thermal expansion and electrical properties of a new compound Al0.33DyGe2

A new ternary compound Al_0.33DyGe₂ has been synthesized and studied from 298 K to773 K by means of X-ray powder diffraction technique. The crystal structural refinement of Al_0.33DyGe₂ has been performed by using the Rietveld method. The ternary compound Al_0.33DyGe₂ crystallizes in the orthorhombic of the defect CeNiSi₂-type structure (space group Cmcm, a = 0.41018(2)nm, b = 1.62323(6)nm, c = 0.39463(1)nm, Z = 4 and D_calc = 8.004 g/cm³). The average thermal expansion coefficients α_a, α_b and α_c of Al_0.33DyGe₂ are 1.96 × 10^− 5 K^− 1, 0.93 × 10^− 5 K^− 1 and 1.42 × 10^− 5 K^− 1, respectively. The bulk thermal expansion coefficient α_V is 4.31 × 10^− 5 K^− 1. The resistivity is observed to fall from 387 to 308 µΩ cm between room temperature and 25 K.     

Boron substitution in ternary metal phosphide superconductors

Partial substitution of boron for phosphorus in the ternary phosphide superconductors ZrRuP and LaRu₂P₂ induces highly anisotropic structural changes and suppresses the superconducting transition temperatures. Undoped ZrRuP [hexagonal, a=6.445(4) Å, c=3.765(2) Å] systematically expands for ZrRuP_1−xB_x (x=0.05, 0.10, 0.15, 0.20, 0.40) in the a direction to 6.949(4) Å and shrinks in the c direction to 3.601(3) Å. Superconductivity is still present in the boron-substituted samples, although T_c drops from 10.1 K in ZrRuP to 5.6 K in ZrRuP_0.6B_0.4. LaRu₂P₂ [tetragonal, a=4.032(2) Å, c=10.632(7) Å] remains essentially unchanged along the a axis [a=4.037(1) Å] upon substitution at a starting composition of LaRu₂P_1.6B_0.4, but shrinks along the c axis to 10.477(5) Å. Accordingly, the superconducting T_c drops from 4.0 K for LaRu₂P₂ to 2.0 K for LaRu₂P_1.6B_0.4.     

Influence of Mn-site doped with Ru on the high-temperature thermoelectric performance of CaMnO3−δ

Thermoelectric properties were measured from room temperature to 1000 K to study the effects of doping of Ru for Mn-site on the thermoelectric performance of CaMnO_3−δ. The electrical resistivity shows a sharp decrease upon Ru doping. The Seebeck coefficients of all the samples are negative, and their absolute values remain fairly large even after Ru doping. Among the samples of CaMn_1−xRu_xO_3−δ (x=0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.15, 0.18), CaMn_0.96Ru_0.04O_3−δ has the largest power factor, 1.85×10⁻⁴ W/mK² at 1000 K. An increase or a decrease of x value results in a marked decrease of the power factor. The thermal conductivity and the figure of merit Z of CaMn_0.96Ru_0.04O_3−δ is 2.88 W/mK and 0.64×10⁻⁴ K⁻¹ at 1000 K, respectively.     

Influence of rare-earth ion doping on magnetotransport behavior of potassium doped manganites

The effect of A-site cationic radius <r_A> on structural, electronic and magnetic properties of the perovskite manganites Ln_0.67K_0.33MnO₃ (Ln = La, Pr & Nd) have been investigated. The X-ray diffraction studies show a structural transformation from rhombohedral to orthorhombic with decreasing <r_A>. The ferro to paramagnetic transition decreases from 280 to 110 K and the temperature dependent resistivity shows a metal to insulating transition at room temperature for La_0.67K_0.33MnO₃ while insulating behavior was observed in the case of Pr and Nd substituted samples. Further, La_0.67K_0.33MnO₃ sample exhibits room temperature magnetoresistance, and may be exploited for device applications. The sign of the thermopower is negative at 300 K and its magnitude increases with decreasing <r_A>. The lanthanum based sample shows an upturn in the resistivity data at 30 K and the observed behavior may be attributed to the combined effect of weak localization, electron–electron and electron–phonon scattering. The paramagnetic insulating part of resistivity and thermopower data was analyzed using small polaron hopping mechanism.     

New palladium phosphate complexes: K2PdP2O7 and K3.5Pd2.25(P2O7)2 synthesis, single crystal structure and conductivity

Two new diphosphate complexes containing potassium and palladium, K₂PdP₂O₇ and K_3.5Pd_2.25(P₂O₇)₂, have been synthesized and characterized by single crystal X-ray diffraction. K₂PdP₂O₇ exists with layers formed of linked PdP₂O₇ polyhedra, between which are found the potassium ions. K_3.5Pd_2.25(P₂O₇)₂ with a Pd/P₂O₇ ratio of 1.125:1 crystallizes with tunnels of various sizes in which are found the potassium ions. Conductivity measurements reveal the material to be conducting.     

High temperature thermopower and electrical resistivity studies in the transparent conducting oxide Sn doped MgIn2O4

Sn doping in an n-type transparent conducting oxide MgIn₂O₄ is carried out and its effect on the high temperature transport properties viz. thermopower and electrical resistivity is studied. A solid solution exists in the composition window Mg_1+xIn_2−2xSn_xO₄ for 0 < x ≤ 0.4. The band gap as well as the transport properties increases with increasing Sn concentration. The high temperature resistivity properties indicate degenerate semiconducting behavior for all the compositions. The highest figure of merit obtained is 0.12 × 10⁻⁴ K⁻¹ for the parent compound at 600 K.     

Growth and transport property measurements of rhenium doped tungsten diselenide single crystal

Single crystals of rhenium doped tungsten diselenide i.e. Re_xW_1−xSe₂ (x = 0, 0.0005, 0.001, 0.05, 0.1) are grown by vapour phase technique. The stoichiometry of grown single crystals is confirmed by energy dispersive analysis of X-rays. X-ray powder diffractograms obtained of these compounds were used for lattice parameter determination based on hexagonal system similar to that of host WSe₂.The crystallite size for each sample for different reflection is calculated using Scherrer's formula. Surface morphology as observed under optical microscope reflects that screw dislocation mechanism is responsible for growth of crystals. Electrical properties viz. Hall effect at room temperature, resistivity measurements at low temperature, and high pressure resistivity measurements indicates the semiconducting behaviour of Re_xW_1−xSe₂ (x = 0, 0.0005, 0.001, 0.05, 0.1) single crystals. Thermoelectric power measurements shows p-type nature of host WSe₂ whereas n-type nature of rhenium doped WSe₂ which matches with the results of Hall effect.     

Synthesis,electrical and dielectric characterization of cerium doped nano copper ferrites

The nanosized CuFe_2−xCe_xO₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8) ferrites doped with cerium are synthesized by chemical co-precipitation method. The synthesized materials are characterized by XRD, FTIR, TGA and SEM. XRD analysis of cerium substituted copper ferrites confirms the cubic spinel structure. The average crystallite size calculated by using Scherrer's formula ranges from 37 to 53 nm. The values of cell constant and cell volume vary with the dopant concentration. These variations can be explained in terms of their ionic radii. The DC electrical resistivity, measured by two point probe method, increases with increase in dopant concentration while it decreases with rise in temperature exhibiting semiconductor behaviour. Energy of activation of these ferrites is calculated by using Arrhenius type resistivity plots. Dielectric measurements of the synthesized compounds show exponential decrease in dielectric constant and dielectric loss factor with increase in frequency. This indicates the normal dielectric behaviour of ferrites.     

Synthesis,structure and electrical studies of praseodymium doped barium zirconium titanate

Praseodymium (Pr) doped barium zirconium titanate with nominal composition (Ba_1−xPr_x)(Zr_0.52Ti_0.48)O₃ (x = 0.1 and 0.2) were synthesized using solid state reaction method. X-ray analysis conform the formation of cubic phase Pr-doped barium zirconium titanate along with minor pyrochloric phase. The increase in grain size after primary investigation reveals the influence of Pr ions on the domain structure and its microstructure. In order to correlate the effect of the chemical composition with the conduction mechanism, different AC electrical parameters have been addressed. The frequency dependant tangent loss of the sample was less for both the ceramics. The temperature dependence results show that the dielectric parameters and resistivity increases as Pr-content in the ceramic increases; this is attributed to the grain size and dipole dynamics. Complex impedance (Z*) plots show frequency dependent behavior as the response for the grain resistance mechanisms. This mechanism has been represented by an equivalent circuit. The temperature dependence of the electrical conductivity and Seebeck coefficient showed n-type non-degenerated semiconductor in the measured temperature range. The temperature dependent conductivity measurement suggests a novel negative temperature coefficient of resistance behavior of the samples. Furthermore, the frequency dependent conductivity plot shows increasing behavior.     

New ternary intermetallics RE5Ru3Al2 (RE = La, Ce, Pr): Synthesis, crystal structures, magnetic and electric properties

Intermetallics RE₅Ru₃Al₂ (RE = La, Ce, Pr) were prepared by arc melting of the elemental components with subsequent annealing at 820 K. The crystal structures were determined from single-crystal (Ce₅Ru₃Al₂, Pr₅Ru₃Al₂) and powder (La₅Ru₃Al₂) X-ray diffraction at room temperature. The compounds belong to new structural types: space group R3, Z = 6, a = 13.9270(3) Å, c = 8.3260(2) Å for Ce₅Ru₃Al₂ and space group I2₁3, Z = 4, a = 9.95419(6) Å and 9.8084(3) Å for La₅Ru₃Al₂ and Pr₅Ru₃Al₂, respectively. The trigonal structure of Ce₅Ru₃Al₂ is a distorted variant of the cubic La₅Ru₃Al₂ and Pr₅Ru₃Al₂ structures. An interesting feature of the Ce₅Ru₃Al₂ are the short (2.5299(16) Å and 2.5969(15) Å) Ce-Ru distances. Magnetic measurements revealed the Pauli paramagnetic behavior in La₅Ru₃Al₂ and the Curie-Weiss paramagnetism in Pr₅Ru₃Al₂. The latter compound likely exhibits a kind of magnetically ordered state below 24 K. In turn, Ce₅Ru₃Al₂ remains paramagnetic down to 4.2 K and shows signs of mixed valence states of Ce ions. Electrical resistivity measurements indicated simple metallic conductivity in La₅Ru₃Al₂ and Pr₅Ru₃Al₂, and a more complex metallic behavior in Ce₅Ru₃Al₂.     

Microstructure and thermoelectric properties of n-type 95%Bi2Te3-5%Bi2Se3 alloy produced by rapid solidification and hot extrusion

n-type SbI₃-doped 95%Bi₂Te₃+5%Bi₂Se₃ compounds were prepared by a rapid solidification and extrusion at the temperature range 420-480 °C using an extrusion ratio of 25:1. The microstructure and thermoelectric properties of the compounds were investigated as a function of extrusion temperature. The fabricated powder consists of homogeneous Bi₂Te₃+Bi₂Se₃ solid solution and the relative density of over 99% was obtained by hot extrusion. The values of Seebeck coefficient for the compounds hot extruded at 420, 450, and 480 °C were −160.8, −170.2, and −165.7 μV K⁻¹, respectively. The values of electrical resistivity (ρ) for the compounds hot extruded at 420, 450, and 480 °C were 0.49, 0.57, and 0.51×10⁻⁵ Ω m, respectively. The maximum power factor value of the compounds hot extruded at 480 °C was 53.8×10⁶ μW cm⁻¹ K⁻².     

Phase change and optical band gap behavior of Ge-Te-Ga thin films prepared by thermal evaporation

The amorphous Ge_11.4Te_86.4Ga_2.2 chalcogenide thin films were prepared by thermal evaporation onto chemically cleaned glass substrates. Properties measurements include X-ray diffraction (XRD), Scanning electron microscopy (SEM), Differential scanning calorimetry (DSC), Four-point probe and VIS-NIR transmission spectra. The allowed indirect transition optical band gap and activation energy of samples were calculated according to the classical Tauc equation and Kissinger's equation, respectively. The results show that there is an amorphous-to-crystalline phase transition of Ge_11.4Te_86.4Ga_2.2 thin film. The investigated film has high crystallization temperature (∼200 °C) and activation energy (2.48 eV), indicating the film has good amorphous stability. The sheet resistance of the crystalline state is ∼10 Ω/and the amorphous/crystalline resistance ratio is about 10⁵. Besides, a wide optical band gap (0.653 eV) of Ge_11.4Te_86.4Ga_2.2 is obtained, indicating that the material possesses a low threshold current from amorphous-to-crystalline state for phase-change memory application.     

Growth, structural, optical, thermal and mechanical properties of glycine zinc chloride single crystal

The nonlinear optical single crystals of glycine zinc chloride were grown by the slow cooling method from aqueous solution. The title compound was synthesized and purified by repeated recrystallization process. Grown crystals were characterized by X-ray diffraction, FT-IR and FT-Raman spectral analysis. The range and percentage of optical transmission were ascertained by recording UV-Vis-NIR spectrum. Thermal properties were investigated by DTA and TGA analyses. Its second harmonic generation relative efficiency was measured by Kurtz and Perry powder technique using Nd:YAG laser and was observed to be 0.5 times that of potassium dihydrogen orthophosphate. Its mechanical hardness was estimated by Vickers microhardness method.     

Nickel-boron nanolayer evolution on boron carbide particle surfaces during thermal treatment

This study is focused on reduction of Ni₂O₃ and B₂O₃ in the Ni-B nanolayer on B₄C particle surfaces and understanding of the nanolayer composition and morphology changes. Initially, the nanolayer contains Ni₂O₃, B₂O₃, and amorphous boron. After 400 °C thermal treatment in a H₂-Ar atmosphere, Ni₂O₃ is reduced to nickel; the nanolayer morphology is maintained and the coated particles demonstrate magnetism. As the thermal treatment temperature is increased to 550 °C, B₂O₃ is reduced to boron, which reacts with nickel and forms Ni₂B. Simultaneously, the nanolayer evolves into nanoparticles. Thermal treatment temperature increase to 700-900 °C only causes Ni₂B particle growth but does not fundamentally change the composition or phase.     

Formation and stability of anatase phase of phosphate incorporated and carbon doped titania nanotubes

Phase transformation analysis of the phosphate containing and carbon doped titania nanotubes, prepared by a simple anodization method, reveals complete transformation from amorphous to anatase phase in air between 360 and 400 °C. Activation energies for formation of anatase phase are evaluated and compared for the two types of titania nanotubes. A detailed analysis of the phase transformation characteristics and stability of the anatase phase is reported.     

Effect of substrate temperature on ac conduction properties of amorphous and polycrystalline GaSe thin films

X-ray diffraction analysis of GaSe thin films used in the present investigation showed that the as-deposited and the one deposited at higher substrate temperature are in amorphous and polycrystalline state, respectively. The alternating current (ac) conduction properties of thermally evaporated films of GaSe were studied ex situ employing symmetric aluminium ohmic electrodes in the frequency range of 120-10⁵ Hz at various temperature regimes. For the film deposited at elevated substrate temperature (573 K) the ac conductivity was found to increase with improvement of its crystalline structure. The ac conductivity (σ_ac) is found to be proportional to (ω^s) where s < 1. The temperature dependence of ac conductivity and the parameter, s, is reasonably well interpreted by the correlated barrier-hopping (CBH) model. The maximum barrier heights W_m calculated from ac conductivity measurements are compared with optical studies of our previous reported work for a-GaSe and poly-GaSe thin films. The distance between the localized centres (R), activation energy (ΔE_σ) and the number of sites per unit energy per unit volume N(E_F) at the Fermi level were evaluated for both a-GaSe and poly-GaSe thin films. Goswami and Goswami model has been invoked to explain the dependence of capacitance on frequency and temperature.     

Proton transport and structural relations in hydroxyl-bearing BaTiO3 and its doped compositions synthesised by wet-chemical methods

Hydrothermally synthesised powders of BaTiO₃ and its Fe- or Nd-doped analogues contain hydroxyl groups in the lattice substitutional to oxide ion, as confirmed from TGA/DTA, IR spectral analysis of D₂O-treated powders, EGA-MS, the contraction in lattice constant with heat treatment by XRD and surface examination by XPS. Electrical resistivity measurements were carried out on the pellets from 298 to 1000 K by ac impedance spectroscopy and dc methods in dry or moist air and 8% H₂+Ar environments. The electrical conductivity observed for unsintered pellets between 298 and 500 K, is in the range of 10⁻³ to 10⁻⁷ S/cm and can be attributed to the extrinsic hydroxyls in BaTiO₃. The acceptor-doped composition, BaTi_0.9Fe_0.1O_3−δ: 2δ(OH) exhibits higher electrical conductivity than BaTiO₃ or the donor-doped Ba_0.9Nd_0.1TiO_3−δ: 2δ(OH) in moist air. The hydrothermally prepared powders heat treated below 1000 K having cubic symmetry at room temperature, possess higher proton conductivity and reabsorption capability for hydroxyls on exposure to moisture than the powders sintered at 1673 K (tetragonal symmetry). The conductivity at 298-500 K is due to the mobility of proton along OHO octahedra in the perovskite lattice. The conduction at 550-1000 K is a combined effect of proton as well as oxygen vacancy mobility in BaTiO₃ and Ba_0.9Nd_0.1TiO₃; electron hole (Ti⁴⁺, Ti³⁺, Fe³⁺, Fe²⁺) participation is the additional contribution in acceptor-doped composition in this temperature range.     

Crystal structure and superconductivity of rubidium tungsten bronzes RbxWO3 prepared by a hybrid microwave method

The rubidium tungsten bronzes Rb_xWO₃ have been prepared from Rb₂CO₃, WO₃ and W powders using hybrid microwave method. The single hexagonal phase samples can be obtained as actual rubidium content x in the range of 0.21-0.33, and their lattice parameters a and c linearly drop and rise with the increase of rubidium content respectively. For samples with x = 0.14, 0.16, 0.18, the superconducting transition temperature T_c from resistivity measurements does not change with the rubidium content, while T_c from susceptibility measurements shows a decrease from 5.3 K for x = 0.14 to 4.8 K for x = 0.18. The charge density wave (CDW) transition appears in Rb_0.21WO₃, Rb_0.23WO₃ and Rb_0.25WO₃ at about 200-260 K. The CDW transition is most obvious in Rb_0.23WO₃ which shows the lowest degree of crystallization among the samples.     

Structural Anomalies of 1223 Hg(Tl)–Ba–Ca–Cu–O Superconductors in the Temperature Range 100–300 K

The crystal structure of Hg-based 1223 phases, Hg_1–x Tl _x Ba₂Ca₂Cu₃O₈₊, with different oxygen content and Hg/Tl substitution having critical temperature from 114 to 133 K has been investigated by the X-ray powder diffraction technique over the temperature range from room temperature to 100 K. Rietveld analysis results indicate the presence of two different structure anomalies at temperatures 138 and 165K, respectively. The changes in lattice parameters, fractional coordinates, and individual atom thermal parameters near the observed anomalies are discussed.