Unexpected Behavior of Alkaline Earth Cuprates and Related Compounds: I. Room Temperature Superconductivity. I. High Temperature Ferromagnetism.

Several samples of nominal composition Bi _2Sr_2CuO _6 and one of nominal composition YBa _2Cu_3O_7 were found to be superconducting well above the established critical temperatures with superconducting volume fractions up to 0.2%. Magnetic measurements using a vibrating sample magnetometer were obtained on these samples which were indicative of Type II superconductors. An experiment performed on a thermogravimetric analyzer was interpreted to show the destruction of flux exclusion (Meissner effect) as a function of temperature (up to 70 ^circC). All super-high temperature superconducting samples were multiphased as analyzed by powder X-ray diffraction. The X-ray diffraction pattern for the YBa_2Cu_3 O_7 sample was deconvoluted into sets of lines attributable to four phases. Lattice parameters of one of these phases match those found for a room temperature superconducting phase reported by Nayuran et al. The X-ray diffraction patterns for the nominal Bi_2Sr_2CuO _6 samples were deconvoluted into as many as 5 distinct sets of lines. These lines were assigned to: Bi_2Sr_2 CuO_6, a tetragonal SrCuO _2, an orthorhombic SrCuO _2, Sr_2CuO _3, and a mystery lattice which has a "c" parameter of 47.6A. The samples are unstable but the super -high temperature superconducting effect has been reproduced in this laboratory many times. By comparing the X-ray diffraction patterns obtained in this work to the literature, patterns similarities were found with the tetragonal SrCuO _2 reported by Niu et al. This material is similar to the 110K superconductor (Sr_ {0.7}Ca_{0.3} CuO_2) reported by Azuma et al. Since this phase appeared in the nominal Bi2201, we believe the SrCuO_2 is a likely candidate for the room temperature superconductors observed here. Ferromagnetic behavior was observed in a series of alkaline earth cuprates (Mg,Ca,Sr,Ba). These compounds were prepared under air or nitrogen at temperatures between 800^circC and 1000 ^circC. In general, higher annealing temperatures were required for alkaline earth cuprates of lighter molecular weight. The Curie points ranged from 300^circC for BaCuO _2 to 700^circC for MgCu_2O_3. The ferromagnetic alkaline earth cuprates of Mg,Ca and Sr were all found to be orthorhombic, while the barium cuprate was cubic. Alkali metal doping of the alkaline earth cuprates resulted in ferromagnetic materials with greater retentive fields but lower Curie points than the undoped materials.