M. N. Khan

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

The system Zn1−xCoxMn1−xFexCrO4 is tetragonal in the range 0.0⩽x⩽0.4 and cubic in the range 0.5⩽x⩽1. Electrical resistivity temperature behaviour obeys Wilsons law for all the compounds and thermoelectric coefficient values vary between 135 to 226μV 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 Zn1−x2+Fex−y3+-Cox2+ [Mn1−x3+Fey3+COx−y2+Cr3+]O4.

X-ray, electrical conductivity, and infrared studies of oxide spinels containing cobalt, titanium and manganese

X-ray, electrical conductivity and infrared studies of the system Co2−xTi1−xMn2xO4 were carried out with a view to investigate the cation distribution. Compounds 0 ⩽ x ⩽ 0.6 showed cubic symmetry, compounds 0.8 ⩽ x ⩽ 1 are tetragonal. X-ray intensity calculation indicated the presence Of Co2+ and Mn3+ at both octahedral and tetrahedral sites. A plot of activation energy against composition shows a break where crystal symmetry changes. The electrical conductivity-temperature behaviour obeys the Raschhinrichsen law. The mobility of holes calculated from infrared and conductivity data is of the order Of 10−9 cm2 V−1 sec−1. X-ray intensity calculations and conductivity measurements suggest the ionic configuration of the system to be Co1−x2+ Mnx3+ [Co2−2x2+ Mnx3+ Tix4+]O42−.

X-ray, electrical conductivity and magnetic hysteresis studies of the chalcogenide system Zn1−xCuxCr2Se4

X-ray diffraction studies performed here indicated solid-solution formation between ZnCr2Se4 and CuCr2Se4, and all the compounds could be indexed on the basis of cubic spinel lattice. The compositions between 0 ⩽x ⩽ 0.8 showed semi-conducting behaviour, while CuCr2Se4 (x=1.0) exhibited metallic conductivity. Seebeck measurements showd P-type behaviour for 0 ⩽x ⩽ 1. Magnetic hysteresis indicated a ferromagnetic nature for all the compounds except ZnCr2Se4 (x=0). The shapes ofχ/χi plotted againstT exhibited single domain behaviour, and Curie temperatures showed an increase with increasing copper concentration.

The role of rare-earth ions in electronically conducting glasses

Abstract The rare-earth (lanthanide) ions play an unusual role in glasses, particularly the phosphate and germanate glasses which conduct electricity by a carrier hopping process between reduced and normal valency metal ion sites. As a result of the near-empty or incompletely filled 4f electron shells in the lanthanides, the electrons in the 4f shell behave very much as they would in a crystal. As a result, the optical absorption edges of glasses containing significant contents of lanthanide materials are much sharper than for normal glassy materials. The esr absorption curves are substantially modified as is the electrical conductivity when even moderately small quantities of lanthanide are incorporated into the glasses. Examples are given of phosphate and germanate glasses containing praseodymium, lutetium and other metals.

THE INFLUENCE OF Pb, Ni AND V DOPING ON THE CHARACTERISTICS OF HIGH-TcBi-Si-Ca-Cu-O SUPERCONDUCTORS

We report on the studies of ac susceptibility, ultrasonic attenuation, superconductivity and some calorimetric measurements on Pb-doped for Bi-site as well as Ni and V substitution for Cu in Bi-Sr-Ca-Cu-O superconductors. It was confirmed that the Bi(Pb)-based glass ceramics (i.e. crystallized glasses) exhibit superconductivity with Tc above 105 K higher than Bi(Ni) with Tc 68 K and Bi(V)-based glasses with Tc 40K. X-ray diffraction analysis revealed that the structure of these materials is Bi-2223 type with a=4.067 A and C=30.768 A. It was found from the temperature dependence of ac complex susceptibility that the coupling nature at grain boundaries of superconducting crystals in the samples obtained was weak, and was improved by increasing the annealing time. Data are discussed for varying Tc of different substituents. Our results on the ultrasonic attenuation revealed a series of relaxation peaks with low activation enthalpies which indicate that electronic defects may be the cause for the relaxation processes.

Cation distribution investigation of the system Li0.5Fe x Ga2.50-xO4 by X-rays, electrical conductivity and Mossbauer studies

AbstractStructural, electrical and Mossbauer studies were carried out for the system Li0.5FexGa2.50-xO4. All the compounds with 0 ⩽ x⩽ 2.5 crystallised with cubic spinel structure. Lattice constant values calculated from XRD analysis were found to increase with increasing x. X-ray intensity calculations indicated that Li1+ occupies only the octahedral site and Ga3+ and Fe3+ ions occupy both octahedral and tetrahedral sites. Activation energy and thermoelectric coefficient values decreased with increasing values of x. All the compounds studied were p-type semiconductors and possess low mobility values of 10−7-10−9 cm2V−1 s−1. Mossbauer data show the presence of iron in the Fe3+ state and the isomer shift values for all the compositions of the system are within the range of high spin ferric compounds. The probable ionic configuration for the system is suggested as:

The mechanical and structural properties of Y1Ba2Cu3O7−x superconductors

{\text{Ga}}_{{\text{1 - }}\alpha }^{{\text{3 + }}} {\text{Fe}}_\alpha ^{{\text{3 + }}} [Li_{0.5}^1 {\text{Fe}}_{{\text{x - }}\alpha }^{{\text{3 + }}} {\text{Ga}}_{{\text{2}}{\text{.5 - x + }}\alpha }^{\text{3}} ] {\text{O}}_{\text{4}}^{{\text{2 - }}}