C. R. Venkateswara Rao

Synthesis and properties of bitetraselenafulvalene

Abstract Copper(II)- and palladium(II)-catalyzed homo-coupling reactions of trimethylstannyltetraselena-fulvalene proceed smoothly to produce bitetraselenafulvalene (biTSF) in good yield. The X-ray analysis of biTSF shows an interesting crystal structure. The CT-complex and radical cation salts derived from biTSF indicate fairly high electric conductivities.

Growth and characterization of bulk-textured Bi2Sr2Ca1-xYxCu2Odelta by the float zone technique

The immersed-heater floating zone technique has been adopted to grow bulk-textured Bi2Sr2Ca1-xYxCu2Odelta (x=0 to 0.3). The growth temperature increases with the yttrium dopant concentration. The samples grown at low pulling rate are well-textured and superconducting. X-ray diffraction and inductively coupled plasma analyses have confirmed the phase formation. The superconducting transition temperature increases up to x=0.1 and then decreases for higher yttrium dopant concentration.

Growth of large size single crystals and whiskers of Bi2Sr2CaCu2O8 by step-cooling method

Single crystals of large surface area with dimensions of 10×3×0.2 mm3 have been grown from excess CuO flux by adopting a step-cooling method. Crystals have also been grown from a stoichiometric melt and are thicker and smaller in size (2×1.5×0.5 mm3). Laue and powder X-ray diffraction studies confirm the single crystallinity and orthorhombic structure of the crystals, respectively. Single crystalline whiskers of length 4–6 mm, which are highly deficient in Sr and Ca, and platelet crystals of the Ca-free phase have also been observed for excess CuO flux. AC susceptibility studies show superconducting transition of about 82 and 78 K for crystals grown from CuO flux and stoichiometric melt, respectively.

Hydriding of the mixed-valent compound CeRhIn: structural and magnetization studies

Abstract Mixed-valent compound, CeRhIn, having the hexagonal Fe 2 P-type structure and a moderate value of the Kondo temperature ( T K ∼150 K), is found to form two distinct hydride phases. The higher hydride phase in CeRhInH y system with y ≅1.55 is found to be quite unstable and readily transforms to a lower hydride with y ≅0.55. From structural and magnetization studies, it is found that in the lower hydride CeRhInH 0.55 (i) the hexagonal structure is retained, with a slightly expanded unit cell, and (ii) the cerium continues to be in the mixed-valent state, though a definite shift towards Ce 3+ is indicated, as seen by a reduction in the T K value to ∼90 K. It is suggested that the hydride formation results in increasing the d-band occupancy of Rh, due to partial transfer of electrons from hydrogen. This, in turn, decreases the hybridization between the Ce-4f electrons and those on the neighboring Rh atoms, which is responsible for a shift of Ce valence towards trivalent state.

The hydriding behaviour of U(Fe1−xNix)Al system (0≤x≤0.75) and magnetic studies on U(Fe1−xNix)AlH0.8

Abstract The hydriding behaviour of the solid-solution series of compounds, U(Fe 1− x Ni x )Al, has been investigated. In the parent pseudoternary compounds, as Ni is gradually substituted for Fe, the magnetic correlations grow. As a result, the magnetic properties change from exchange enhanced Pauli-paramagnetic and spin fluctuating ( x =0), via weakly magnetic with no long range magnetic ordering down to 4 K (0 x ≤0.3) to ferromagnetic (0.35≤ x ≤0.75) and eventually to antiferromagnetic for x ≥0.9. The present hydriding report covers compositions up to the ferromagnetic regime. It is found that hydrogen absorption does not take place for x below ≅0.7. The lowest nickel containing composition, which forms a well defined hydride phase, viz., U(Fe 0.3 Ni 0.7 )Al, absorbs y =0.8 H atoms per formula unit. This matches exactly with the lower hydride phase of pure UNiAl. No other hydride phase is formed either above or below y =0.8. The magnetization studies show that U(Fe 0.3 Ni 0.7 )AlH 0.8 has much higher values of the magnetic ordering temperature ( T C ), paramagnetic Curie temperature ( θ P ) and the uranium magnetic moment, relative to the unhydrided composition. These findings are in consonance with the large increase in the 57 Fe Mossbauer isomer-shift value, on hydriding. It is suggested that the electron charge transfer from H to the Fe 3d band (inferred from the Mossbauer studies) weakens the 5f–3d hybridization, thus enhancing the ferromagnetic correlations. The observed large increase in the a -axis cell parameter also implies a reduction in U–U hybridization, further justifying the observed increase in T C , θ P and U moment values.

U(Fe1−xNix)AlHy system: new hydride phases, structural and magnetic properties

Abstract We report the hydriding characteristics of U(Fe1−xNix)AlHy, with x=0.85. It is found that (i) U(Fe0.15Ni0.85)Al can be hydrided to a maximum of y≈2.4 and the hydride shows antiferromagnetic ordering with T N ≈70 K , and (ii) new single phase orthorhombic hydrides can be stabilized for x⩾0.85. From our hydriding and magnetic studies on all the compositions in this series of compounds, including our earlier work, it is found that irrespective of the magnetic state of the parent composition, the hydrides with y⩾2 result in U-5f-electron localization and antiferromagnetic ordering. Structural and magnetic studies have also been carried out on the orthorhombic hydride phases and these are found not to order down to 2 K .

Growth of large size twin free YBa2Cu3O7-δ single crystals by a temperature gradient technique

Abstract Untwinned single crystals of supercomducting YBa 2 Cu 3 O 7-δ (YBCO), have been grown by the temperature gradient technique. Three different types of crystals (both untwinned and twinned) have been grown in the same growth run by taking advantage of the combined effect of flux creeping and a large temperature gradient. The as-grown crystals are orthorhombic in structure. The twin free crystals annealed at 500°C in an oxygen atmosphere show a 90 K superconducting transition under alternating current (AC) susceptibility studies. The effect of temperature gradient and the growth features of three types of crystals are discussed.

Growth of CaLnBaCu3O7- delta (Ln=La, Pr and Nd) single crystals by the flux technique

Single crystals of CaLnBaCu3O7- delta (Ln=La, Pr and Nd) were grown by the flux technique from a BaO-CuO eutectic melt. The morphological features of the grown crystals were studied using optical microscopy and SEM. Phase formation and single crystallinity were confirmed by X-ray diffraction studies. Superconductivity was characterized by four-probe electrical resistivity measurements.

Growth and characterisation of Bi2Sr2Ca1Cu2Oy by the floating zone method

Abstract The floating Zone Melting (FZM) technique using the immersed strip heater arrangement was adopted to grow Bi 2 Sr 2 Ca 1 Cu 2 Q y crystals. High quality single crystals have been obtained from the grown boule. The crystals were investigated with an optical microscope to understand the growth mechanism. The growth direction is perpendicular to the (001) plane. The crystals have an orthorhombic structure with cell parameters a = 5.40 A, b = 5.44 A and c = 30.74 A. The composition of the grown crystal is Bi 2.04 Sr 1.8 Ca 1.16 Cu 2 O 8+y . Superconducting transition of crystals occur at 82 K.

Preparation and characterisation of BiSrCaCuO through glassy route

Abstract Highly dense superconducting material of BiSrCaCuO with high volume fraction of 80 K phase has been obtained through amorphous glass prepared from the stoichiometric composition 2212 by splat quenching technique. The crystallisation process has been studied by X-ray powder diffraction analysis on as-prepared and annealed samples. The resistivity and a.c. susceptibility measurements show the existence of 2212-80 K phase. A low volume fraction of 110 K phase has also been observed for the sample annealed at 850°C for 40 h.