Upadhyayula V Varadaraju

Many Zr-containing phases of the sodium zirconium phosphate (NZP) type structure have been evaluated as host materials for the immobilization of radionuclides. Phosphates crystallizing in eulytine type structure are identified as promising crystalline hosts matrices for the immobilization of rare earth cations and are tested for the first time. The soxhlet tests performed on these two kinds of phosphates with different crystal structure show that the Ln3+ in eulytine and Zr4+ in the NZP phases show very low leach rates of the order of 10-5 to 10-6gm2 day even in powdered samples. The leach rates attain levelling after 4-5 days. It is found that the leaching process is controlled by both diffusion and dissolution, The leaching process of the individual species is found to depend on the charge density of the respective ions present in the matrix.

Magnetization measurements have been performed on superconducting YNi2B2C at different temperatures below Tc with maximum applied fields ranging from 200 Oe to 15 kOe. The experimental data were analyzed using Kim and exponential field dependent critical state models. It is found that Kim model provides a better estimation of Jc.

The photoelectrochemical behaviour of oxide bronzes of the type A6Nb8Ti2O30 (A = Ba. Sr) has been studied. They show good photoactivity with photopotentials of the order of 700-800 mV. The flat band potential has been estimated to be - 0·9 V vs SCE. The band gap estimated from the spectral response is 3·1 eV. © 1994 Indian Academy of Sciences.

Synthesis and characterization of electrical and magnetic properties of ilmenite phases of the type MnTi1-xNbxO3have been carried out. Single phase materials could be obtained for 0.0≤x≤0.25. The electrical conductivity increases with increasing Nb content. Magnetic susceptibility studies show that the phases exhibit 2D antiferromagnetic behavior. The magnetic susceptibility data has been analyzed using Fisher's specific heat to determine the long range ordering temperature. © 1998 Academic Press.

Hollandite oxides of the type Bi1.7-xPbxV8O16 have been synthesized. The electrical resistivity studies show that the conductivity improves upon Pb substitution. The feasibility of Li intercalation in the system has been established. Magnetic susceptibility studies on the pure and Li intercalated phases show that except for pure Bi1.7V8O16, all phases exhibit Pauli paramagnetism. No superconductivity is encountered down to 12 K in any of the phases. © 1998 Elsevier Science B.V. All rights reserved.

The resistivity (ρ{variant}) and thermopower (S) of semiconducting phases in the system Bi2Sr2Ca1-xYxCu2O8+δ and Bi2Sr2-xLaxCuO6+δ have been measured in the temperature ranges 300-700 K and 300-1000 K respectively. The values of ρ{variant} and S at any temperature increase with increase in x. Both systems show thermally activated hopping at low temperatures (300-450 K) with the activation energy increasing with x. The figure of merit (S2σ/λ) peaks to a maximum value of 1.9×10-5 WK-2m-1 at x=0.7 in the Bi-2212 system. © 1994.

The structural and magnetic properties of HoErFe17-xGax (x = 0-7) were studied by means of X-ray diffraction and magnetization measurements. Powder X-ray diffraction studies show that the compounds transform from the hexagonal Th2Ni17 structure (x ≤ 4) to rhombohedral Th2Zn17 structure (x ≥ 5) with increasing Ga concentration. The substitution of Ga for Fe leads to an increase in lattice constants a, c and unit-cell volume v. The Curie temperature is found to first increase and then decrease with increasing Ga concentration, attaining a maximum value of 535 K at about x = 4, which is 207 K higher than that of HoErFe17. Both iron sublattice anisotropy and rare-earth sublattice anisotropy are found to be modified by the introduction of the Ga atoms and have been confirmed through X-ray diffractograms of magnetically aligned samples. © 1998 Elsevier Science B.V. All rights reserved.

HoErFe17-xGaxCy were synthesized by arc melting technique. The temperature dependence of the ac magnetic susceptibility (0.1 Oe; 313 Hz) was carried out in the temperature range 13 - 300 K using a SUMITOMO ac susceptometer. Based on the observations, a magnetic phase diagram was established.

Samples with the stoichiometric composition Bi2Sr2-xKxCa1-yNayCu 2Oz with x = 0.1 and y = 0.1, 0.2, 0.3 and 0.4 were prepared by the solid state reaction method. The role of Na and K doped at the Ca and Sr sites simultaneously in the Bi-2212 composition was studied. From the x-ray diffraction data it was found that the structure is single-phase Bi-2212. From the resistivity data the Tc(0) varies from 63 to 67 K for different dopant levels. The diamagnetic nature of the samples was confirmed by ac susceptibility measurements. The oxygen content was determined by the two-step iodometric titration technique. The present results on the effect of double doping are compared with those of single dopants published earlier by our group. The microstructures of the samples were examined by scanning electron microscopy.

The search for new magnetic materials with better thermal and magnetic performance led to the discovery of new intermetallic phases, R(Fe, M)12 and R3(Fe, M)29 which are known as 1:12, and 3:29 compounds. In both the compounds, the presence of a third element M (=Ti, V, Cr, Mn, Mo, W) is necessary for the stabilization of the phase. This article discusses recent investigations of structural and magnetic properties of substitutionally and interstitially modified 2:17 and 3:29 compounds.