Preparation, thermal expansion, high pressure and high temperature behavior of Al2(WO4)3

The titled compound Al₂(WO₄)₃ was synthesized by a conventional solid state reaction and characterized by powder XRD. It crystallizes in an orthorhombic (Pbcn, No. 60) lattice, with unit cell parameters as 12.582(2), 9.051(1), 9.128(2) Å, and V = 1039.5(3) (Å)³. The compound was found to show negative thermal expansion (NTE) behavior in the temperature range of 25 to 850°C. The average linear NTE coefficient (₁), in this temperature range, was –1.5 × 10^–6 K^–1. The effect of pressure at ambient temperature, was studied by a Bridgman Anvil (BA) apparatus, to reveal that there is no irreversible phase transition up to 8 GPa. The effect of high pressure and high temperature on this compound was studied by a Toroid Anvil (TA) apparatus. This compound has a limited stability under high pressure and temperature, as it undergoes a decomposition to AlWO₄ and WO_3–x with a partial oxygen loss. As an off-shoot of this work, certain new modifications of WO_3–x under pressure and temperature were observed, viz., monoclinic, tetragonal and an orthorhombic modifications at 5 GPa/1400°C, 3 GPa/900°C and 1.8 GPa/1030°C, respectively. The detailed XRD studies of the products are presented here.     

Structural and electrical studies on thin films of solid electrolyte Ag6I4WO4

An electrolytic method of preparation of thin films of solid electrolyte Ag₆I₄WO₄ on a silver substrate is described. Films of different quality were obtained when the electrolysis was carried out at different temperatures and current densities. X-ray diffraction studies were carried out to confirm the compound formation. Scanning electron micrographs were taken in order to study the surface characteristics of these films. Investigations on the a.c. electrical conductivity of films prepared at different electrolysis temperatures (10 to 80° C) and two current densities (2 and 10 mA cm^–2) are also reported in the temperature range 30 to 130° C. The films deposited at 65° C gave values of room temperature conductivity and the activation energy for Ag⁺ ion conduction as 0.04 ^–1 cm^–1 and 0.132 eV, respectively.     

Structural study of superionic conducting glasses Agl-AgPO3 by X-ray diffraction

Atomic structures of (Agl) _x (AgPO₃O₃)_1–x glasses for x=0.0, 0.1, 0.2, 0.3 and 0.5 have been investigated by X-ray diffraction. Coordination numbers and atomic distances in the near-neighbour region were determined by the least-squares variational method. The coordination numbers of P-O, P-P and O-O pairs are unchanged with x, which suggests no modification of the connectivity of the PO₄ tetrahedral chains by doping with Agl. The coordination number of I^– around Ag⁺ linearly increases from 0 to 1.9 ± 0.2 with increase in x, while the coordination number of O^2– around Ag⁺ linearly decreases from 5.1±0.2 to 2.5±0.2. This also suggests that the Agl gets into the PO₄ chains while keeping the local environment of the Agi itself.     

X-ray,electrical conductivity and infrared studies of the system Zn1−x CO x Mn1−x Fe x CrO4

The system Zn_1–x Co _x Mn_1–x Fe _x CrO₄ is tetragonal in the range 0.0x0.4 and cubic in the range 0.5x1. Electrical resistivity temperature behaviour obeys Wilson's law for all the compounds and thermoelectric coefficient values vary between 135 to 226V K^–1. All compounds exhibit P-type semiconductivity and possess low mobility values (10^–9 cm² V^–1 sec^–1). The infrared spectra show the presence of two strong absorption bands around 500 and 600 cm^–1. The probable ionic configuration for the system is suggested to be Zn _1–x ²⁺ Fe _x ^–y3+ -Co _x ²⁺ [Mn _1–x ³⁺ Fe _y ³⁺ CO _x ^–y2+ Cr³⁺]O₄.     

Structure, Phonon Vibration, and Spin Correlation of LaBa2Cu3?xCoxOy

Polycrystalline samples LaBa₂Cu_3–xCo _x O _y (0 x 1.0) were synthesized by solid state reaction method. The structure, phonon vibration, conduction, and spin correlation were investigated by means of X-ray diffraction, infrared spectra, resistivity, and electron spin resonance. It is found that there are orthorhombic–tetragonal and tetragonal–orthorhombic structural transitions with Co doping, and the conduction behavior changes from metallic to semiconducting. With the increase of Co content, the Cu(1)—O(1) phonon mode around 531 cm^–1 softens, the Cu(2)—O(2) phonon mode around 657 cm^–1 hardens, and the Cu(1)—O(4) mode around 583 cm^–1 is nearly unchanged. The Cu²⁺ spins tend to localize with Co doping. The changes in structure, phonon vibration, and spin correlation with Co doping are analyzed and discussed.     

Dielectric, magnetic and spectroscopic properties of Li2O–WO3–P2O5 glass system with Ag2O as additive

Li₂O–WO₃–P₂O₅ glasses containing small concentrations of Ag₂O from 0 to 1 mol% were prepared. A number of studies viz., chemical durability, dielectric studies (constant ′, loss tan δ, a.c. conductivity σ_ac over a range of frequency and temperature), spectroscopic (infrared, optical absorption ESR spectra) and magnetic susceptibility studies of these glasses, have been carried out. The interesting variations observed in all these properties with the concentration of Ag⁺ ions have been analyzed in the light of different oxidation states and environment of tungsten ions in the glass network.     

Zinc Doping Profile in AlGaAs/GaAs Heteroepitaxial Structures Grown by Metalorganic Chemical Vapor Deposition

The Zn profile in Al _x Ga_{1 – x} As/GaAs (x = 0.2–0.4) quantum-well heteroepitaxial structures doped during growth by metalorganic chemical vapor deposition is modeled with allowance made for the diffusional broadening of the nominal doping profile. Experimentally determined carrier distributions in the heterostructures are used to refine the diffusion coefficient of Zn at a growth temperature of 770°C. The average value of D _Zn is determined to be 6.0 × 10^–14 cm²/s. The position of the p–n junction in Al _x Ga_{1 – x} As/GaAs heterostructures is assessed as a function of the nominal Zn profile and growth rate. The ways of optimizing the doping profile are outlined.     

Structural,thermal and transport studies on Ag1 − x Cuxl (0.05 ⩽ x ⩽ 0.25) solid electrolyte

Preparation and characterization studies on polycrystalline samples of Ag_{1 – x}Cu_xl wherex=0.05, 0.1, 0.15, 0.2 and 0.25, respectively, have been reported. Samples were analysed using powder X-ray diffraction (XRD) and differential scanning calorimetric (DSC) techniques in order to identify the compositions and phase transition temperatures. A.c. electrical conductivity studies were carried out on pelleted specimens of various compositions in the frequency range 65.5 kHz to 1 Hz and over the temperature range 293–412 K. DSC results obtained in the temperature range 373–473 K have shown that the ß- to -phase transition temperature is enhanced from 426 K to 438 K whenx is increased from 0.05 to 0.25. XRD results have indicated that there is a shift ind-spacing when the Cul content is increased, suggesting changes in the crystal structure. Typical XRD patterns recorded for the composition Ag_0.95Cu_0.05l at three different temperatures (room temperature, 373 and 473 K, respectively) have confirmed that both face-centred cubic and hexagonal phases would be present at room temperature and at 373 K as well, whereas at 473 K the structure would be purely body-centred cubic in nature. A.c. impedance analysis of the above samples appears to suggest that their electrical conductivity, predominantly due to the migration of Ag⁺ ions, lies in the order of 10^–4S cm^–1 at room temperature.     

Transport Properties of Heavy Fermion Compounds: YbIn1−x Cu4+x and YbIn1−y Ag y Cu4

The intermetallic compounds of Yb with In and Cu (YbIn_1–x Cu_4+x ) and Yb with In, Ag, and Cu (YbIn_1–y Ag _y Cu₄) exhibit interesting magnetic and transport properties. Of the compounds of Yb with In and Cu the compound with x=0, namely YbInCu₄, has attracted particular attention, because—while being a Curie–Weiss paramagnet—it undergoes a first-order isostructural phase transition at T _v =approx. 40 to 80 K and atmospheric pressure. Below T _v the ytterbium in this compound is in a mixed-valence state and the compound as a whole is sometimes called a light heavy-fermion system. Above T _v , the compound is known as a Curie–Weiss paramagnet of localized magnetic moments and, below T _v , a Pauli paramagnet in a nonmagnetic Fermi-liquid state. In the present paper the results of measurements of the thermal conductivity of polycrystalline samples, YbIn_1–x Cu_4+x with x=0,0.015, 0.095, and 0.17 and YbIn_1–y Ag _y Cu₄ with y=0, 0.3, 0.7, and 1.0, are reported. The thermal conductivity is separated into the phonon thermal conductivity ( _ph ) (i.e., related to the heat carried by phonons) and into the electronic thermal conductivity ( _e ) (related to the heat carried by electrons). The electrical resistivity of the compounds was measured to determine the temperature dependence of the Lorenz number. The results show that in the temperature interval 4.2 to 300 K the latter quantity behavior follows the theoretical predictions for heavy fermion materials.     

Preparation and photovoltaic properties of anodically grown Ag2O films

The semiconducting and photovoltaic properties of p-type Ag₂O films grown anodically on silver electrodes were studied, in view of possible applications in solar energy conversion. Films were grown in different alkaline solutions; the best results were obtained for 0.02M Ag₂SO₄ + 0.17M NH₄OH + 5.7 × 10^–3M Ba(OH)₂ saturated with Ag₂O powder, stirred mechanically at room temperature. Film thicknesses of up to 10m were thus obtained for the first time in anodically grown Ag₂O. Photovoltaic spectra taken at 300 K give a bandgap ofEg = 1.42 ± 0.04 eV. Evaporated gold on Ag₂O appears to be ohmic while aluminium and platinum are rectifying. The barrier height of Ag/Ag₂O is 0.90 ± 0.02 eV, that of Al/Ag₂O is 0.93 ± 0.02 eV, and that of platinum 0.94 ± 0.02 eV. The best cells give an open-circuit voltage,V _oc, of over 150 mV, and a short circuit current,I _sc = 100A cm^–2 under 50 mW cm^–2 illumination.     

Direct current electrical conduction in lead tungsten phosphate glasses

Measurements have been made on d.c. electrical conductivity of semiconducting lead tungsten phosphate glasses (X mol% WO₃–(60–x) mol% PbO–40 mol% P₂O₅;x= 10, 20, 30, 40, 50 and 60) of six different compositions over a temperature range of 240 to 500 K. It is shown that the conduction can be described by a small polaron hopping model. When the WO₃ content is 30 mol% the d.c. conductivity _dc decreases and the activation energyW increases with increasing WO₃ content and the glass samples exhibit predominant ionic conduction, however, when the WO₃ content is > 30 mol%, _dc increases andW decreases with the increase of tungsten ion concentration and the glass samples exhibit electronic conduction. The electronic conduction in these glasses havingx > 30 mol% seems to be adiabatic. Greaves variable range hopping has been found to be valid. The value of the electron wavefunction decay constant a is of the order of 16 nm^–1. A minimum in the d.c. conductivity has been observed when the PbO concentration is 30 mol%.     

Phase relationships in the system Ni-W-O and thermodynamic properties of NiWO4

The phase diagram of the Ni-W-O system at 1200 K was established by metallographic and X-ray identification of the phases present after equilibration at controlled oxygen potentials. The oxygen partial pressures over the samples were fixed by metered streams of CO+CO₂ gas mixtures. There was only one ternary oxide, nickel tungstate (NiWO₄), in the Ni-W-O system at a total pressure of 1 atm, and this compound decomposed to a mixture of Ni+WO_2.72 on lowering the oxygen potential. The Gibbs' free energy of formation of NiWO₄ was determined from the measurement of the e.m.f. of the solid oxide galvanic cell, Pt, Ni+NiWO₄+WO_2.72/CaO-ZrO₂/Ni+NiO, Pt and thermodynamic properties of tungsten and nickel oxides available in the literature. For the reaction, NiO(s)+WO₃(s)NiWO₄(s) G°=–10500–0.708 T (±250) cal mol^–1.     

Rapidly quenched sodium tungsten bronzes

Rapid quenching was applied to the sodium tungsten bronzes, Na _x WO₃, of differentx values. The crystallization processes of the coloured flakes obtained were analysed by differential thermal analysis and X-ray powder diffraction. It was found that an amorphous bronze phase is present in the quenched Na_0.71WO₃, whereas quenched Na_0.84WO₃ was composed of tungsten crystal and Na₂O-WO₃ glass. This difference is explained by the following equilibrium present in the melt of Na _x WO₃: 3xNa₂WO₄+(6–4x)WO₃+xW6Na _x WO₃ This equilibrium is shifted to the right whenx is small (x<0.7), while it is shifted to the left whenx is large (x>0.8).     

Metal–Nonmetal Transition in Rubidium Tungsten Bronze RbxWOy

An oxygen concentration dependent metal–nonmetal (MN) transition was observed for Rb_0.23WO _y with 2.80 < y < 3.08. As 2.80 < y < 3.0, the room temperature resistivity (_RT) of the Rb_0.23WO _y is about 5 × 10^–4 cm. While in the case of y > 3.04, the _RT of the Rb_0.23WO _y exhibits a four orders of magnitude increase with a value of 5 cm. Correspondingly, the lattice constant along c-direction slightly shortens as oxygen concentration increases from 2.80 to 3.08. The observed results suggest that the hybridization between W 5d (t_2g) and O 2p orbitals might be responsible for the MN transition. In addition, similar measurements were performed for Rb _x WO_3.04 and Rb _x WO_2.85 with 0.19 < x < 0.27. No rubidium concentration dependent MN transition was observed, indicating the electronic structure of the host WO _y is not modified significantly by varying the soluble rubidium concentration.     

IR Spectra of La1.85Sr0.15Cu1?xMxO4?δ(M = Zn, Ni, Mg)

Polycrystalline samples La_1.85Sr_0.15Cu_1–xZn_xO_4– (0 x 0.3), La_1.85Sr_0.15Cu_1–xNi_xO_4– (0 x 0.3), and La_1.85Sr_0.15Cu_1–xMg_xO_4– (0 x 0.3) were synthesized by the solid-state reaction method. The crystal structure and phonon vibration were investigated by means of X-ray diffraction (XRD) and infrared spectrum. Zn, Ni, and Mg doping results in the lattice parameter c decreasing and a (b) increasing. The change of the phonon modes around 504 cm^–1 and 681 cm^–1 can be satisfactorily interpreted in terms of the change of the crystal structure and the itinerant nature of charge carrier in CuO₂ sheet. The relation between the structure and phonon vibration is discussed.     

Oxidation of reduced Y-doped semiconducting barium titanate ceramics

Oxidation-induced microstructural changes in reduced yttrium-doped barium titanate (Ba_{1 – x}Y_xTi_{1 – x}⁴⁺Ti_x³⁺ O₃) are studied using samples sintered in a reducing atmosphere ( = 10^–4Pa) and then oxidized in air at 1150 and 1350°C. The results indicate that oxidation leads to precipitation of Ba₆Ti₁₇O₄₀ and, at relatively high doping levels, Y₂Ti₂O₇. These phases increase the electrical resistance of the outer layer of the grains in the ceramics.Translated from Neorganicheskie Materialy, Vol. 41, No. 1, 2005, pp. 93–100.Original Russian Text Copyright © 2005 by Vyunov, Kovalenko, Belous, Belyakov.     

Modification of electron spectrum and properties of HTSC during doping

A mechanism for the influence of doping on the electron spectrum of La_2–xSr_xCuO₄ and Nd_2–xCe_xCuO₄ is proposed. In the framework of simple model taking into account the local nature of HTSC electron properties, the appearance of midgap states and the existence of critical concentration values (0.15 and 0.25) are explained. The effect of doping-induced changes of electron structure on transport and superconducting properties of La_2–xSr_xCuO₄ and Nd_2–xCe_xCuO₄ is considered. The dependence of the Hall resistance on doping concentration,x, is discussed. It is shown that in Nd_2–xCe_xCuO₄ the pair level of negativeU-centers arises at the top of the valence band only atx=0.15, and the dielectric-metal transition takes place. The superconductivity in Nd_2–xCe_xCuO₄ occurs in the hole-conductive phase.     

IR Spectra of La1.85Sr0.15Cu1?xMxO4?δ(M = Zn, Ni, Mg)

Polycrystalline samples La_1.85Sr_0.15Cu_1–xZn_xO_4– (0 x 0.3), La_1.85Sr_0.15Cu_1–xNi_xO_4– (0 x 0.3), and La_1.85Sr_0.15Cu_1–xMg_xO_4– (0 x 0.3) were synthesized by the solid-state reaction method. The crystal structure and phonon vibration were investigated by means of X-ray diffraction (XRD) and infrared spectrum. Zn, Ni, and Mg doping results in the lattice parameter c decreasing and a (b) increasing. The change of the phonon modes around 504 cm^–1 and 681 cm^–1 can be satisfactorily interpreted in terms of the change of the crystal structure and the itinerant nature of charge carrier in CuO₂ sheet. The relation between the structure and phonon vibration is discussed.     

Stability and doping of infinite-layer compound (Ca1−x Srx) CuO2 at ambient pressure

The stability region of the infinite-layer compound (Ca_1–x Srx)CuO₂ has been investigated in detail along the composition line CaCuO₂-SrCuO₂ in the SrO-CaO-CuO system. The samples were prepared at 956C in air at ambient pressure. Single phase of the infinite-layer structure compound is found to lie at the intersection of two different phase regions and its stability region is extremely narrow nearx = 0.16. Temperature also has pronounced effect on the stability of the infinite layer structure. A trial of doping into the infinite layer compound (Ca_1–x Srx)CuO₂ was performed by partial substituting (Ca, Sr) with lanthanum.     

Infrared study of relevance of Nd2CuO4 structure to superconductivity of La-Sr-Cu-O system

A comparative study of the infrared spectra of Nd₂Cu_1–xAg_xO₄ and La_1.8Sr_0.2Cu_1–xAg_xO₄ is reported. It is shown that the appearance of an absorption peak at 680 cm^–1 is due to local distortions around the Ag impurities, and the appearance of this peak need not imply a structural transformation from Nd₂CuO₄ type of K₂NiF₄ type. It is shown that Nd₂CuO₄ structure is not important for the superconductivity of La-Sr-Cu-O system.