M.N. Khan

A spectroscopic study of vanadium-phosphate glasses containing vanadium chloride

The optical properties of vanadium-phosphate glasses containing various amounts of (V2O5)50(P2O5)50−x(VCl3)x, wherex = 0.05, 0.06, 0.08 and 0.1, were measured as a function of VCl3 content. It is found that by adding VCl3 to the melt when the glass is formed, the added chlorine which acts as an oxidizing agent alters the ratio of concentration of vanadium ions and thus the absorption coefficient. Data on absorption in this study show that the power law,α(ω) =B(ħω) −E0)n/ħω is best followed forn = 3, and the absorption is solely due to vanadium oxide. Furthermore, the infrared absorption spectra of this system were investigated in the wave number range 400 to 4600cm−1. The addition of VCl3 to the vanadium-phosphate glasses does not seem to introduce any new absorption band in this range as compared with the spectrum of a pure vanadium-phosphate glass. Furthermore, no significant difference in the absorption spectra was observed by annealing the V2O5-P2O5 glasses up to 200° C.

Superconducting properties of glass ceramics in the Bi-R-Sr-Ca-Cu-O (with R=Pb or Ag) system

The influence of partial substitution of Pb and Ag in the Bi1-xRxSrCaCu1.5Oy (R=Pb or Ag; x=0.2) glass ceramics has been investigated. It was found that the Ag-containing compound exhibited superconductivity with a Tc of 78 K while the Pb-containing compound exhibited a Tc of 87 K. The ultrasonic attenuation was measured at frequencies of 5, 10, 20 and 30 MHz in the temperature range 77 to 300 K. Five relaxation peaks were observed in Bi0.8Pb0.2SrCaCu1.5Oy and Bi0.8Ag0.2SrCaCu1.5Oy compounds. As the frequency of the measurement is increased, the temperature of the peaks shifts to higher values while the height of the peaks decreases. The first three peaks (P1, P2 and P3) may be attributed to the electronic defects (hopping of holes) and the other two could be ascribed to the thermally activated movement of boundaries by interaction with the defects.

Study of superconducting and physical properties of 110 K (Bi, Pb)SrCaCuO superconductors from glass precursor

Abstract High quality samples of 110 K(Bi 1.6 Pb 0.4 ) Sr 1.7 Ca 2.3 Cu 3 O y were prepared in the form of long-length rods by rapid quenching of the melt, which showed a glass transition temperature of 379.77°C, crystallization temperature of 463.4°C and melting temperature of 861.5°C. We also prepared glasses of Bi 1-x R x SrCaCuO y (R = Al and x = 0.5) system at various melting temperatures which on further heat treatment under appropriate conditions, crystallized into superconducting phase(s). It was found that copper ions in the glasses exists mainly in the Cu(I) ionic state and the Cu(I)/Cu(total) ratio increases with glass melting temperature. The control of the Cu(I)/Cu(total) ratio leads to the successful continuous drawing of BiCaSrCuO glass fibers.

Infrared temperature dependent studies of Bi2Sr2CaCu2O8 glass ceramic superconductors

Abstract We report on the investigation of infrared reflectance spectroscopy on Bi2Sr2CaCu2O8 glass ceramics superconductor in the temperature range 20 K to 300 K and between the frequency of 100 to 20000cm−1. The frequency dependent conductivity was calculated by Kramer-Kronig transformation of the reflectance spectra. Analysis by Drude and lattice vibration models reveals the Drude plasma frequency and the reflectance edge, closer to those of a single crystal. All the predicted phonon modes are observed in the measured spectra. We did not find any appreciable change in the frequency and oscillator strength of the phonons in the present temperature range.

Structural and superconducting properties of vanadium substituted Y Ba2 (Cu1−x Vx)3 O9 perovskite oxide system

Abstract The superconducting properties YBaCuVO system has been studied. It was found that upon substituation of copper by vanadium, the critical temperature decreases linearly for the whole compositional range under investigation (0.2 ⩾ × ⩾ 0.001). X-ray diffraction analysis reveals that the structure of these compounds changes systematicaly from orthorhombic which corresponds to oxygen difficient perovskite to tetragonal.

Strength of infrared phonons in R Ba2Cu3O7-x ceramic superconductors

Abstract The infrared reflectance spectra for mixed rare earths and lanthanides ceramic superconductors were measured in the normal state at ambient temperature. The dielectric response function obtained by Kramer-Kronig analysis of the reflectance data is analyzed using a four component dielectric response model. The Drude parameter results show that the oscillator strength of the phonon modes reduced as Yittrium is partially replaced by Europium and Gadolinium. The infrared d.c. conductivity calculated from the oscillator model fit and from K-K analysis, disagree with the four probe d.c. conductivity. The critical temperature of R Ba 2 Cu 3 O 7−x superconductors reduces systematically on partial substitution of Yittrium by lanthanides.

THE PROPERTIES OF RBa2(Cu1−xAgx)3O7 SUPERCONDUCTORS

We present superconducting properties and structural analysis of RBa2(Cu1−xAgx)3O7 (with R = Y, Eu and Gd) ceramic compounds. Transition temperatures improve with a small amount of Ag. Results of powder X-ray diffraction (indicate no detectable substitution of Ag into the RBa2(Cu0.999Ag0.001)3O7 oxide system. We also found that no superconductivity was observed above 10 K for RBa2(Cu0.4Ag0.6)3O7 compounds. The superconductivity is discussed in relation to the results of X-ray diffraction study.

SUPERCONDUCTIVITY AND STRUCTURES OF MBa2Cu3O7−x (WITH M = Y, Eu AND Yb) OXIDES

We have prepared and observed that the zero-resistance temperature transition occurs in the chemical system MBa2Cu3O7 (M = Y, Eu and Yb). The highest Tc (ρ = 0) is 92.50 K for Y, 93.80 K for Eu and 96.00 for Yb. The X-ray diffraction analysis shows that these compounds correspond to a single phase which is an orthorhombic perovskite structure. The results of the infrared (IR) studies on orthorhombic and tetragonal phases of some of these compounds provide evidence for the oxygen rearrangement on going from the orthorhombic to the tetragonal phase.

Spin glass behaviour in the Co2−xZnxTiO4 compound

A.c. susceptibility measurements were carried out on the disordered spinel system Co2−xZnxTiO4 (0<x<1) between 10 and 80 K. Our measurements show three peaks in the Χ versus T curve for Co2TiO4 and CoZnTiO4 compounds. In both compounds two peaks are very close to each other, indicating that the Néel and semi-spin glass temperatures are very close. A third peak indicates the presence of the semi-spin glass to spin glass transition. In Co1.5Zn0.5TiO4 only one peak is observed which indicates spin glass ordering at 26 K. Furthermore, the A-site canting present in Co2TiO4 and CoZnTiO4 has a collinear and magnetic structure, indicating strong A-B coupling. X-ray analysis indicated that compounds of the system Co2ZnxTiO4 synthesized with cubic symmetry. From transport properties it was found that the activation energy and thermoelectric coefficient decrease with increasing concentration of Zn in the system. The mobility of the system calculated from infrared measurements is typically of the order of 10−9cm2V−1s−1.

FAR INFRARED REFLECTIVITY STUDY OF CERAMIC SUPERCONDUCTORS

We report on a study of the far-infrared reflectivity of mixed rare earths and lanthanides ceramic superconductors RBa2Cu3O7 in the normal state. Our results show that the strength of the phonon modes is reduced when yittrium is partially replaced by gadolinium and europium. Also the critical temperature of these mixed materials is reduced as indicated by the four probe technique.