Upadhyayula V Varadaraju

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.

Polycrystalline samples of isostructural MSb2O4 (M = Co, Ni) have been prepared by solid state synthesis and lithium-storage is investigated as possible anode materials for lithium-ion batteries. The reaction mechanism of lithium with MSb2O4 (M = Co, Ni) is explored by galvanostatic cycling, cyclic voltammo-gram and ex situ studies. Both CoSb2O4 and NiSb2O4 exhibit similar electrochemical behavior and show reversible capacity of 490 and 412 mAhg -1 respectively in the first cycle. Reversible alloying de-alloying of Lix Sb takes place in an amorphous matrix of M (Co, Ni) and Li2O during electrochemical cycling. © 2012 Elsevier Ltd. All rights reserved.

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.

The oxygen-deficient phase of the high T c superconductor, YBa2Cu3O7, was oxygen-enriched using the fluidization technique to give good superconducting properties. The normal method of oxygen treatment at 900°C for 24 h and at 600°C for 24 h has been reduced to just one treatment at 600°C for 12 h by the fluidization technique to achieve almost the same strength of superconducting signal for the YBa2Cu3O7 powder, which establishes the attractiveness of the latter route for the large-scale preparation of superconducting material. The particle sizes were in the range 0-90, 90-180 and 180-420 μm. The fluidized particles were crystalline with orthorhombic distortion. T c onset, estimated using the a.c. magnetic susceptibility method, was 91·3 K. The volume fraction of superconducting material in the product was 83·7-85·3%, one of the highest values reported so far for YBa2Cu3O7. © 1989 Indian Academy of Sciences.

Resistance and thermopower measurements have been made on a series of compounds, YBa2Cu3-xZnx07_Yl, with x - 0.025, 0.05, 0.1, 0.15 and 0.2. The superconducting transition temperature decreases as the zinc concentration increases. In a range of temperatures below TM, the mid point of the transition, the resistance shows an exponential temperature dependence fitting the phenomenological formula proposed by Ausloos et ai From the plot of logarithm of resistivity vs. (TM- T)1/2/T, one deduces a value of the average dimension of the Josephson junction to be a few tens of A, suggesting the microtwin boundaries to be the location of the junctions. The thermopower shows a peak always just above Tc. This conclusively shows that phonon drag is not the cause of the peak. The temperature dependence of the thermopower appears to resemble closely the earlier observations of Srinivasan et al. on yttrium barium copper oxide. Single-particle tunneling measurements carried out for two concentrations, x = 0 and 0.05, appear to indicate that the energy gap parameter scales with Tc, and 2Δ/kTchas an approximate value of 5.5. © 1989, Taylor & Francis Group, LLC. All rights reserved.

Structure and T//c data on LnBa//2Cu//3O//7 (Ln equals Dy, Ho, Er, Tm, Yb and Y) compounds are reported and discussed. No reentrant behavior has been noted in these high-T//c superconductors for Ln equals Er and Yb until 4. 2 K. Four-probe resistivity was measured in the range 77-300 K employing ultrasonically soldered In-contacts. An inductance bridge with carrier-frequency amplifier was used to quickly check the samples for superconductivity at and above 77 K. A calibrated test set-up was employed to measure the superconductivity transition by the magnetic susceptibility technique.

Systematic substitutional studies in the Bi2.1Sr1.93Ca0.97-xRxCu2O8+y (R=Pr,Gd,Er; 0≤x≤0.3 in steps of 0.05 and 0.4≤x≤0.97 in steps of 0.1) system were carried out in order to determine the effect of the magnetic moment and ionic radius of the rare-earth ion on the Tc suppression rate. X-ray-diffraction studies indicate that the solid solubility of Gd and Er exists up to x=0.97 whereas that of Pr is limited to x=0.6 under the preparative conditions employed. Resistivity and ac susceptibility studies have shown that superconductivity persists up to x=0.4 and a metal-semiconductor transition occurs for x>0.4. The most interesting observation is that the rate of Tc suppression for the superconducting phases is found to be identical for all rare earths. We have explained that hole filling rather than Abrikosov-Gor'kov pair breaking is responsible for the decrease in Tc. The insulating phases with 0.5≤x≤0.97 exhibit the phenomenon of Mott's variable-range-hopping mechanism (VRH). The physical parameters related to VRH such as localization length (α-1), hopping range (R), and activation energy (W) for conduction, have been evaluated and discussed in detail. © 1994 The American Physical Society.

Studies on MTl2 Sr2 Ca2 Cu3 Oy (M = Rare Earth, Y, Sc, Pb, Bi and Th) show that superconductivity in the range 40-77K can be achieved in this system depending on the M-ion. The minimum amount of rare earth required to show a Tc ∼ 70K appears to be 25 mole percent. X-ray studies confirm the single phase material responsible for high Tc superconductivity to be 1212 type. It appears that Pb, Bi and Th substitute at the Tl- site whereas the rare earths, Y and Sc prefer the Ca- site. © 1990.

The specific heat of samples of Y1-xPrxBa2Cu3O7-y for x = 0.1 to 0.5 has been measured from 4.5 to 65 K using a quasi-adiabatic calorimeter. A plot of C/T vs T2 shows a linear behavior below 20 K. The excess specific heat due to praseodymium in all the compounds is obtained by subtracting the specific heat of YBa2Cu3O7-x from the that of doped compound. The excess specific heat shows a peak near 19 K and its height decreases with increase in x. The excess specific heat could be interpreted on the basis of Schottky anomalies due to Pr3+ and Pr4+ ions, the peak at 19 K arising from the former. The fraction of Pr3+ ion obtained from a fit to the specific heat curves decreases from 0.30 for x = 0.1 to 0.05 for x = 0.5. © 1988.

The layer-type compound, Ca0·85Sr0·15CuO2, which is the prototype of the cuprate family of high T c oxide superconductors is an electronic insulator, and has a low ordered moment at 300 K indicating that Cu2+ ions are highly correlated. Doping with lithium (up to x=0·15) decreases the room temperature resistivity drastically (from 103 ohm·cm to 0·6 ohm·cm) but metallic behaviour is not induced below 300 K. The magnetic moment on Cu2+ remains low but our studies did not show any antiferromagnetic ordering below 300 K in any of the Li-doped phases. © 1991 Indian Academy of Sciences.