U. R. K. Rao

Attempts to prepare fluorine containing YBa2Cu3O6.5+δ

Abstract Although several authors have reported the replacement of different elements at the yttrium, barium and copper sites of the high temperature superconducting YBa 2 Cu 3 O 6.5+δ , replacement of oxygen by fluorine was claimed by only one group which, however, was refuted by another group. By study of a series of interconnected reactions, it was shown in this communication that starting from BaF 2 , it is not feasible to obtain fluorinated compounds of this series. A non-superconducting cubic modification of the parent compound is reported.

Microwave absorption and EPR studies of a new copper oxyfluoride superconductor synthesised through the ammonium bifluoride route

Abstract The recently reported superconductor Sr 2 CuO 2 F 2.4 is synthesised by a new route of fluorination by NH 4 HF 2 and its superconducting phase is investigated by electron paramagnetic resonance, direct microwave absorption and non-resonant response employing magnetic field modulated microwave absorption. These studies have given clear evidence for the occurrence of a superconducting phase below 40 K. From the magnetic field dependence of the changes in microwave absorption, the lower critical field H c1 (perpendicular to the c -axis) is determined to be 100 G at 10 K. The EPR studies on this oxyfluoride have shown a Cu 2+ signal with 19 F super hyperfine structure due to interaction with two equivalent F nuclei. In view of its low intensity it was assigned to a possible ‘defect site’ in the lattice. The line width of the EPR signal increased in the superconducting phase smearing out the 19 F shfs below 35 K. This is consistent with what is normally expected in superconductors.

TG/DTA evidence of fluorine incorporation into YBa2Cu3O6⋅+δ

TG/DAT evidence to support incorporation of fluorine into superconducting orthorhombic lattice in YBa2Cu3O6⋅+δ is presented. It is inferred that the F atom goes substitutionally in place of O atom in the lattice. Replacement of two oxygen atoms by four fluorine atoms did not alter theTcof the product. By a similar replacement of two of its oxygens by fluorine atoms, the tetragonal modification of the oxide yields a product that remains non-superconducting down to 30 K.

Fluorination of Sr2CuO3 by NH4HF2

Abstract Different amounts of fluorine can be incorporated in the place of oxygen in Sr2CuO3 by reacting it with NH4HF2 in different stoichiometry. The superconducting (SC) behaviour of the product was found to be critically dependent on the amount of F incorporated into the lattice. Too little fluorine content or too much produced a non-superconducting product. The fact that in all stoichiometries of Sr2CuO3 and NH4HF2 in the mixture, SrF2 is invariably formed, indicates that all the products including the superconducting ones are Sr-deflcient. It is argued that sub-stoichiometry in Sr in the product serves the same function as that of interstitial F in producing superconductivity.

Crystallographic and magnetic phase transition in TlMnCl3

The results of our NMR, EPR and magnetic susceptibility measurements in the paramagnetic state of TlMnCl3 are reported here. The NMR paramagnetic shift of thallium is found to be small but positive. Mn2+ EPR line is exchange narrowed. The susceptibility measurements indicate an antiferromagnetic transition. The heat of crystallographic phase transition ΔH, in TlMnCl3 has been measured using differential scanning calorimetry. The crystallographic phase transition appears to be first order and ΔH is unusually low viz. 10 cal mole−1. In the case of KMnF3 ΔH, which is reported here for the first time, is determined to be 2 cal mole−1.

Study of fluorine incorporation into YBa2Cu3O6.5 + δ by X-ray photoelectron spectroscopy

Abstract Earlier, the study of fluorine incorporation into the lattice of YBa 2 Cu 3 O 6.5 + δ by X-ray emission spectroscopy using F K α and O K α was reported. In this communication recent XPS studies on these fluorine-containing Y-123 oxides are ionized. These data lend further support to the conclusions drawn earlier from X-ray emission spectroscopic studies. It is seen from XPS data that there is a significant chemical shift of fluorine, oxygen and Cu 1 S 1 2 peaks up to compositions corresponding to the formation of YBa 2 Cu 3 O 5.0 F 3 beyond which there is no further change.

On the magnetic behaviour of REBa2F7 compounds (RE = Pr, Nd, Sm and Gd)

Abstract Decomposition of the maximum fluorinated 1,2,3 oxide at 800°C in vacuum gave a new phase identified to be YBa 2 F 7 . Four other rare earth analogs of this new compound were prepared and were studied by a SQUID magnetometer and an ac susceptometer. Except for Sm, the other three compounds showed Curie-Weiss behaviour. There is no magnetic ordering in any of these compounds down to 1.5 K.

Evidence for superconductivity in fluorinated La2CuO4 at 35 K: Microwave investigations

In the fluorinated La2CuO4−x prepared using a solid state reaction with NH4HF2 as a fluorinating agent at 550 K at ambient pressure, superconductivity was detected by microwave and EPR techniques with aTc of 35 K.

On the magnetic behaviour of REBa2F7 compounds (RE Eu, Dy, Ho, Er, Tm and Yb)

Abstract The dc magnetic susceptibility of the REBa 2 F 7 compounds was measured by a SQUID magnetometer in the temperature range 5–300 K. Except for Eu, the other compounds show either Curie law or Curie-Weiss law with relatively small values of paramagnetic Curie temperature, θ p , with effective magnetic moments in the range expected of electronic ground state of the trivalent rare earth ion with appropriate J value. The behaviour of Eu is complex.

Fluorination of REBa2VO6 compounds

New compounds of the type REBa2VO4F4 where RE=Y, Eu and Sm have been prepared for the first time by reacting REBa2VO6 with NH4HF2 used as a low-temperature fluorinating agent. Thermogravimetric analysis/differential thermal analysis, chemical analysis, 151Eu Mössbauer and EPMA evidence for the formation of the fluorinated compound is presented.