Transport coefficients of titanium-doped Sb2Te3 single crystals

Titanium-doped single crystals (c_Ti=0-2×10²⁰ atoms cm⁻³) were prepared from the elements Sb, Ti, and Te of 5 N purity by a modified Bridgman method. The obtained crystals were characterized by measurements of the temperature dependence of the electrical resistivity, Hall coefficient, Seebeck coefficient and thermal conductivity in the temperature range of 3-300 K. It was observed that with an increasing Ti content in the samples the electrical resistivity, the Hall coefficient and the Seebeck coefficient increase. This means that the incorporation of Ti atoms into the Sb₂Te₃ crystal structure results in a decrease in the concentration of holes in the doped crystals. For the explanation of the observed effect a model of defects in the crystals is proposed. The data of the lattice thermal conductivity were fitted well assuming that phonons scatter on boundaries, point defects, charge carriers, and other phonons.     

Defect structure of Sb2−xFexTe3 single crystals

Single crystals of Sb_2−xFe_xTe₃ (c_Fe=0-9.5×10¹⁹ cm⁻³) were prepared by Bridgman method. The interpretation of the reflection spectra in plasma resonance region indicates that Fe increases the concentration of holes (acceptor) and each Fe atom incorporated in Sb₂Te₃ structure liberates 0.4-0.5 hole. Observed effect is elucidated by means of point defect model. According to the model Fe atoms enter the structure and form uncharged substitutional defects . Since this defect cannot affect the free-carrier concentration directly, we assume an interaction of the entering Fe-atoms with natives defects leading to a rise in the concentration of antisite defects , to a decrease of concentration, and to an increase in the concentration of holes.     

Electrical properties and sulfur tolerance of La0.75Sr0.25Cr1−xMnxO3 under anodic conditions

Complex metal oxides with composition of La_0.75Sr_0.25Cr_1−xMn_xO₃(x=0.4,0.5,0.6) (LSCM) have been synthesized and examined as anode materials for solid oxide fuel cells (SOFCs). LSCM compositions show excellent tolerance to both reduction and oxidation but the crystal structure transforms from hexagonal in air to orthorhombic in H₂. The volume change associated with this phase transformation is only about 1%, thus having little effect on other properties. The total electrical conductivity increases with the content of Mn, whereas the resistance to sulfur poisoning increases with the content of Cr. Fuel cells using LSCM as the anode show very good performance when pure hydrogen is used as the fuel. However, they do not appear to be stable in fuels containing 10% of H₂S.     

Local environment in Ba2In2−xWxO5+3x/2 oxide ion conductors

The actual oxygen environment of the tungsten dopant in the Ba₂In_2−xW_xO_5+3x/2 solid solution was revealed by combining X-ray absorption spectroscopy at the tungsten L_I and L_III edges and at the indium L_I edge. Whatever the substitution ratio, the tungsten atoms exhibit a regular octahedral environment. When the substitution ratio increases, the oxygen vacancies are progressively filled until their total occupancy for x=2/3. For x?0.3, the perovskite structure is stabilised; the tungsten atoms are randomly distributed in the structure. Although X-ray diffraction revealed a cubic symmetry for these compositions, a local distortion of the indium environment is observed when a tungsten atom is in its surrounding.     

Synthesis and electrical conductivity of perovskite-type PrCo1−xMgxO3

Oxides in the system PrCo_1−xMg_xO₃ (x=0.0, 0.05, 0.10, 0.15, 0.20, 0.25) were synthesized by citrate technique and characterized by powder X-ray diffraction and scanning electron microscope. All compounds have a cubic perovskite structure (space group ). The maximum ratio of doped Mg in the system PrCo_1−xMg_xO₃ is x=0.2. Further doping leads to the segregation of Pr₆O₁₁ in PrCo_1−xMg_xO₃. The substitution of Mg for Co improves the performance of PrCoO₃ as compared to the electrical conductivity measured by a four-probe electrical conductivity analyzer in the temperature range from 298 to 1073 K. The substitution of Mg for Co on the B site may be compensated by the formations of Co⁴⁺ and oxygen vacancies. The electrical conductivity of PrCo_1−xMg_xO₃ oxides increases with increasing x in the range of 0.0-0.2. The increase in conductivity becomes considerable at the temperatures ?673 K especially for x?0.1; it reaches a maximum at x=0.2 and 1073 K. From x>0.2 the conductivity of PrCo_1−xMg_xO₃ starts getting lower. This is probably a result of the segregation of Pr₆O₁₁ in PrCo_1−xMg_xO₃ , which blocks oxygen transport, and association of oxygen vacancies. A change in activation energy for all PrCo_1−xMg_xO₃ compounds (x=0-0.25) was observed, with a higher activation energy above 573 K and a lower activation energy below 573 K. The reasons for such a change are probably due to the change of dominant charge carriers from Co⁴⁺ to Vö in PrCo_1−xMg_xO₃ oxides and a phase transition mainly starting at 573 K.     

Thermochemistry of Li1+xMn2−xO4 (0?x?1/3) spinel

Lithium substituted Li_1+xMn_2−xO₄ spinel samples in the entire solid solution range (0?x?1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x?0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na₂O·4MoO₃ at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x?0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn³⁺ to Mn⁴⁺ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn³⁺ to Mn⁴⁺ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction.     

Zr1−xLnxW2O8−x/2 (Ln=Eu, Er, Yb): Solid solutions of negative thermal expansion-synthesis, characterization and limited solid solubility

Zr_1−xLn_xW₂O_8−x/2 solid solutions (Ln=Eu, Er, Yb) of different substitution fractions x have been synthesized. Their X-ray diffraction (XRD) patterns have been indexed and lattice parameters calculated based on the α-ZrW₂O₈ structure. The coefficients of thermal expansion (CTEs) of these solid solutions were estimated to be −10.3×10⁻⁶ K⁻¹ in temperature range of 30-100 °C. The solubility of lanthanide ions in these solid solutions decreases linearly with the increase in the radius of substituted lanthanide ions. Based on the concentration dependence of phase transition temperatures, a novel method for determination of solubility of the lanthanide ions in Zr_1−xLn_xW₂O_8−x/2 solid solutions has been developed. This method seems to be more sensitive as compared with that based on XRD technique.     

Synthesis of layered cathode material Li[CoxMn1−x]O2 from layered double hydroxides precursors

Cathode materials Li[Co_xMn_1−x]O₂ for lithium secondary batteries have been prepared by a new route—precursor method of layered double hydroxides (LDHs). In situ high-temperature X-ray diffraction (HT-XRD) and thermogravimetric analysis coupled with mass spectrometry (TG-MS) were used to monitor the structural transformation during the reaction of CoMn LDHs and LiOH·H₂O: firstly the layered structure of LDHs transformed to an intermediate phase with spinel structure; then the distortion of the structure occurred with the intercalation of Li⁺ into the lattice, resulting in the formation of layered Li[Co_xMn_1−x]O₂ with α-NaFeO₂ structure. Extended X-ray absorption fine structure (EXAFS) data showed that the Co-O bonding length and the coordination number of Co were close to those of Mn in Li[Co_xMn_1−x]O₂, which indicates that the local environments of the transitional metals are rather similar. X-ray photoelectron spectroscopy (XPS) was used to measure the oxidation state of Co and Mn. The influences of Co/Mn ratio on both the structure and electrochemical property of Li[Co_xMn_1−x]O₂ have been investigated by XRD and electrochemical tests. It has been found that the products synthesized by the precursor method demonstrated a rather stable cycling behavior, with a reversible capacity of 122.5 mAh g⁻¹ for the layered material Li[Co_0.80Mn_0.20]O₂.     

Formation of crystalline TiO2−xNx and its photocatalytic activity

Amorphous precursors to nitrogen-doped TiO₂ (NTP) and pure TiO₂ (ATP) powders were synthesized by hydrolytic synthesis and sol-gel method (SGM), respectively. Corresponding crystalline phases were obtained by thermally induced transformation of these amorphous powders. From FT-IR and XPS data, it was concluded that a complex containing titanium and ammonia was formed in the precipitate stage while calcination drove weakly adsorbed ammonium species off the surface, decomposed ammonia bound on surface of precipitated powder and led to substitution of nitrogen atom into the lattice of TiO₂ during the crystallization. The activation energies required for grain growth in amorphous TiO_2−xN_x and TiO₂ samples were determined to be 1.6 and 1.7 kJ/mol, respectively. Those required for the phase transformation from amorphous to crystalline TiO_2−xN_x and TiO₂ were determined to be 129 and 142 kJ/mol, respectively. A relatively low temperature was required for the phase transformation in NTP sample than in ATP sample. The fabricated N-doped TiO₂ photocatalyst absorbed the visible light showing two absorption edges; one in UV range due to titanium oxide as the main edge and the other due to nitrogen doping as a small shoulder. TiO_2−xN_x photocatalyst demonstrated its photoactivity for photocurrent generation and decomposition of 2-propanol (IPA) under visible light irradiation ().     

Thermoelectric properties of Ag-doped n-type (Bi2Te3)0.9-(Bi2−xAgxSe3)0.1 (x=0-0.4) alloys prepared by spark plasma sintering

Ag-doped n-type (Bi₂Te₃)_0.9-(Bi_2−xAg_xSe₃)_0.1 (x=0-0.4) alloys were prepared by spark plasma sintering and their physical properties evaluated. When at low Ag content (x=0.05), the temperature dependence of the lattice thermal conductivity follows the trend of (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1; while at higher Ag content, a relatively rapid reduction above 400 K can be observed due possibly to the enhancement of scattering of phonons by the increased defects. The Seebeck coefficient increases with Ag content, with some loss of electrical conductivity, but the maximum dimensionless figure of merit ZT can be obtained to be 0.86 for the alloy with x=0.4 at 505 K, about 0.2 higher than that of the alloy (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1 without Ag-doping.     

New synthesis of excellent visible-light TiO2−xNx photocatalyst using a very simple method

An excellent visible-light-responsive (from 400 to 550 nm) TiO_2−xN_x photocatalyst was prepared by a simple wet method. Hydrazine was used as a new nitrogen resource in this paper. Self-made amorphous titanium dioxide precursor powders were dipped into hydrazine hydrate, and calcined at low temperature (110 °C) in the air. The TiO_2−xN_x was successfully synthesized, following by spontaneous combustion. The photocatalyst was characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscope (TEM), UV-Vis diffuse reflectance spectrometer (DRS), and X-ray photoelectron spectroscopy (XPS). Analysis of XPS indicated that N atoms were incorporated into the lattice of the titania crystal during the combustion of hydrazine on the surface of TiO₂. Ethylene was selected as a target pollutant under visible-light excitation to evaluate the activity of this photocatalyst. The newly prepared TiO_2−xN_x photocatalyst with strong photocatalytic activity and high photochemical stability under visible-light irradiation was firstly demonstrated in the experiment.     

Evolution of structure and magnetic properties in electron-doped double perovskites, Sr2−xLaxMnWO6 (0?x?1)

Powder neutron and X-ray diffraction studies show that the double perovskites in the region 0?x?1 exhibit two crystallographic modifications at room temperature: monoclinic P2₁/n and tetragonal I4/m, with a boundary at 0.75<x<0.9. Magnetic susceptibility measurements indicate that for x=0 and 0.5 Sr_2−xLa_xMnWO₆ orders antiferromagnetically (AFM) at 15 and 25 K, respectively, for 0.75?x<1.0, a contribution of weak ferromagnetism (FM), probably due to canted-AFM order, increases with increasing x. The end point compound SrLaMnWO₆ shows the strongest FM cluster effect; however, no clear evidence of magnetic order is discernable down to 4.2 K. X-ray absorption spectroscopy (XAS) confirms Mn²⁺ and mixed-valent W^6+/5+ formal oxidation states in Sr_2−xLa_xMnWO₆.     

Synthesis and properties of the Co7Se8−xSx and Ni7Se8−xSx solid solutions

We report the synthesis and elementary properties of the Co₇Se_8−xS_x (x=0-8) and Ni₇Se_8−xS_x (x=0-7) solid solutions. Both systems form a NiAs-type structure with metal vacancies. In general, the lattice parameters decrease with increasing x, but in the Ni₇Se_8−xS_x system c increases on going from x=5 to 7. Magnetic susceptibility measurements show that all samples exhibit temperature-independent paramagnetism from 25-250 K. Samples within the Co₇Se_8−xS_x system, as well as Ni₇Se₈ and Ni₇SeS₇, were found to be poor metals with resistivities of ∼0.20 and ∼0.06 mΩ cm at 300 K, respectively. The Sommerfeld constant (γ) was determined from specific heat measurements to be ∼13 mJ/mol_CoK² and ∼7 mJ/mol_NiK² for Co₇Se_8−xS_x and Ni₇Se_8−xS_x, respectively.     

Synthesis and characterization of metal substituted AlxCr1−x(acetylacetonate)3 single-source precursors for their application to MOCVD of thin films

A detailed study, involving the synthesis of a single-source precursor containing two metal ions sharing the same crystallographic site, has been undertaken to elucidate the use of such a single-source precursor in a CVD process for growing thin films of oxides comprising these two metals, ensuring a uniform composition and distribution of metal ions. The substituted complexes Cr_1−xAl_x(acac)₃, where acac = acetylacetonate, have been prepared by a co-synthesis method, and characterized using UV–Vis spectroscopy, TGA/DTA measurements, and single crystal X-ray diffraction at low temperature. All the studied compositions crystallize in the monoclinic space group P2₁/c with Z = 4 in the unit cell. It was observed that the ratio (Al:Cr) of the site occupancy for the metal ions, obtained from single crystal refinement, is in agreement with the results obtained from complexometric titrations. All the solid state structures have the metal in an octahedral environment forming six-membered chelate rings. M–O acac bond lengths and disorder in the terminal carbon have been studied in detail for these substituted metal–organic complexes. One composition among these was chosen to evaluate their suitability as a single-source precursor in a LPMOCVD process (low-pressure metal–organic chemical vapour deposition) for the deposition of a substituted binary metal oxide thin film. The resulting thin films were characterized by X-ray diffraction, scanning electron microscopy, and infrared spectroscopy.     

X-ray and neutron powder diffraction studies of Ba(NdxY2−x)CuO5

Ba(R,R′)₂CuO₅ (R,R′=lanthanides and Y) plays an important role as a flux-pinning agent in enhancing the superconducting properties of the Ba₂(R,R′)Cu₃O_6+x (R,R′=lanthanides and Y) coated conductors. Using X-ray diffraction and neutron diffraction, we found that the Ba(Nd_xY_2−x)CuO₅ solid solution adopts two structure types. In the Nd-rich region (1.8?x?2.0), the materials are of brown color (commonly referred to as the ‘brown phase’), and the structure is tetragonal with space group I4/mbm (no. 127). In the Y-rich region (0.0?x?1.4), the materials are green (commonly referred to as the ‘green phase’) and the structure is orthorhombic with space group Pnma (no. 62). A two-phase region (1.4<x<1.8) exists between the orthorhombic and tetragonal phases. The crystal chemistry and crystallography of the orthorhombic ‘green phase’ series, Ba(Nd_xY_2−x)CuO₅ (isostructural to BaY₂CuO₅), are discussed in this paper.     

Thermoelectric properties of p-type pseudo-binary (Ag0.365Sb0.558Te)x-(Bi0.5Sb1.5Te3)1−x (x=0-1.0) alloys prepared by spark plasma sintering

In this paper, pseudo-binary (Ag_0.365Sb_0.558Te)_x-(Bi_0.5Sb_1.5Te₃)_1−x (x=0-1.0) alloys were prepared using spark plasma sintering technique, and the composition-dependent thermoelectric properties were evaluated. Electrical conductivities range from 7.9×10⁴ to 15.6×10⁴ Ω⁻¹ m⁻¹ at temperatures of 507 and 318 K, respectively, being about 3.0 and 8.5 times those of Bi_0.5Sb_1.5Te₃ alloy at the corresponding temperatures. The optimal dimensionless figure of merit (ZT) of the sample with molar fraction x=0.025 reaches 1.1 at 478 K, whereas that of the ternary Bi_0.5Sb_1.5Te₃ alloy is 0.58 near room temperature. The results also reveal that a direct introduction of Ag_0.365Sb_0.558Te in the Bi-Sb-Te system is much more effective to the property improvement than naturally precipitated Ag_0.365Sb_0.558Te in the Ag-doped Ag-Bi-Sb-Te system.     

Structural, spectroscopic and photoluminescence studies of LiEu(WO4)2−x(MoO4)x as a near-UV convertible phosphor

A series of lithium europium double tungsto-molybdate phosphors LiEu(WO₄)_2−x(MoO₄)_x (x=0, 0.4, 0.8, 1.2, 1.6, 2.0) have been synthesized by solid-state reactions and their crystal structure, optical and luminescent properties were studied. As the molybdate content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm was found to increase and reach a maximum when the relative ratio of Mo/W is 2:0. These changes were found to be accompanied with the changes in the spectral feature, which can be attributed to the crystal field splitting of the ⁵D₀→⁷F₂ transition. As the molybdate content increases the emission intensity of the 615 nm peak also increases. The intense red-emission of the tungstomolybdate phosphors under near-UV excitation suggests them to be potential candidate for white light generation by using near-UV LEDs. In this study the effect of chemical compositions and crystal structure on the photoluminescent properties of LiEu(WO₄)_2−x(MoO₄)_x is investigated and discussed.     

Structure and conductivity of strontium-doped cerium orthovanadates Ce1−xSrxVO4 (0≤x≤0.175)

A-site substituted cerium orthovanadates, Ce_1−xSr_xVO₄, were synthesised by solid-state reactions. It was found that the solid solution limit in Ce_1−xSr_xVO₄ is at x=0.175. The crystal structure was analysed by X-ray diffraction and it exhibits a tetragonal zircon structure of space group I4₁/amd (1 4 1) with a=7.3670 (3) and c=6.4894 (1) Å for Ce_0.825Sr_0.175VO₄. The UV-vis absorption spectra indicated that the compounds have band gaps at room temperature in the range 4.5-4.6 eV. Conductivity measurements were performed for the first time up to the strontium solid solution limit in air and in dry 5% H₂/Ar with conductivity values at 600 °C ranging from 0.3 to 30 mS cm⁻¹ in air to 30-45 mS cm⁻¹ in reduced atmosphere. Sample Ce_0.825Sr_0.175VO₄ is redox stable at a temperature below 600 °C although the conductivity is not high enough to be used as an electrode for solid oxide fuel cells.     

Crystal structures of RPt3−xSi1−y(R=Y, Tb, Dy, Ho, Er, Tm, Yb) studied by single crystal X-ray diffraction

The crystal structures of ternary compounds RPt_3−xSi_1−y(R=Y, Tb, Dy, Ho, Er, Tm, Yb) have been elucidated from X-ray single crystal CCD data. All compounds are isotypic and crystallize in the tetragonal space group P4/mbm. The general formula RPt_3−xSi_1−y arises from defects: x≈0.20, y≈0.14. The crystal structure of RPt_3−xSi_1−y can be considered as a packing of four types of building blocks which derive from the CePt₃B-type unit cell by various degrees of distortion and Pt, Si-defects.     

Solvothermal Synthesis of Superfine Li1−xMn2O4−σ Powders

Superfine Li_1−xMn₂O_4−σ powders were successfully synthesized by the alcohol-thermal method using 0.01 mol of MnO₂, 0.01mol of LiOH·H₂O, and 0.06mol of NaOH as starting materials at 160-200°C. The products are characterized by XRD, TEM, ED, BET, and ICP. Results show that the Li_0.74Mn₂O_3.74 powder prepared at 200°C has an average size of 180 nm with BET surface areas of 16.44 m²/g. A possible formation mechanism is proposed. It was concluded that the alcohol acts not only as the solvent but also as the reducing agent in the synthesis of Li_1−xMn₂O_4−σ powders. The effects of reaction temperature and the contents of NaOH and LiOH on the formation of single phase Li_1−xMn₂O_4−σ were investigated.