2 TRANSITION-METAL-SUBSTITUTED SUPERCONDUCTING MERCURY-BASED OXYCARBONATES, HG(1-X)M(X)SR(4)CU(2)(CO3)O-6+DELTA (M=CR AND MO)

Two new superconducting mercury based oxycarbonates have been synthesi zed by introducing a transition element on the mercury site: Hg(1-x)M( x)Sr(4)Cu(2)(CO3)O-6+delta with x = 0.5 and 0.6 for M = Mo and Cr, res pectively. They crystallize in the tetragonal system with a = 3.8747(1 ) Angstrom and c = 16.1555(6) Angstrom for M = Cr, and a = 3.8448(1) A ngstrom and c = 16.3173(7) Angstrom for M = Mo. In most of the crystal s, the HREM studies show a very regular stacking of the layers along c , characteristic of the intergrowth of the 1201-type structure with th e Sr2Cu(CO3)O-2 structure. The molybdenum phase Hg0.5Mo0.5Sr4Cu2(CO3)O -6+delta exhibits the highest T-c of 74 K, that has been encountered u p to date for isotypic oxycarbonates, with a sharp transition, suggest ing that the parent phase (Hg, Mo)Sr2CuO4+delta does not superconduct. On the opposite, the chromium phase Hg0.4Cr0.6Sr4Cu2(CO3)O-6+delta is characterized by a T-c of 37 K, smaller than that of the parent phase (Hg, Cr)Sr2CuO4+delta.