A NEW MERCURY-BASED HIGH-T-C CUPRATE HG0.7V0.3SR2-XLAXCUO4+DELTA

A new barium-free mercury-based superconductor Hg0.7V0.3Sr2-xLaxCuO4+d elta (x = 0.1-0.8) has been synthesized. T-c(onset) of this compound i s similar to 50 K and does not vary appreciably with x. While the lant hanum-free (x = 0) sample contains predominantly various impurity phas es, samples with x = 0.2 and 0.3 are almost single phase. X-ray diffra ction analysis reveals that the crystal structure has a tetragonal sym metry with lattice parameters a = 3.8133-3.8266 Angstrom and c = 8.736 1-8.7944 Angstrom. A neutron powder diffraction (NPD) study on the x = 0.2 sample shows that the O(3) site is four-fold disordered in the Hg -O plane having a total occupancy (delta) similar to 0.8. The variatio n of the resistivity with temperature shows a metallic character for a ll the samples. The as-prepared samples are in the overdoped region wh ich has been established by thermoelectric-power measurements and anne aling treatments, T-c is found to increase with Cu-O(1) and Cu-O(2) bo nd distances whereas it is inversely related to (1/rho(300)) d rho/dT.