Chemical Instability of the Cobalt Oxyhydrate Superconductor under Ambient Conditions

The layered sodium cobalt oxyhydrate superconductor Na0.3CoO2*1.4H2O is shown through X-ray diffraction and thermogravimetric studies to be one of a series of hydrated phases of Na0.3CoO2. Further, it is shown that the material is exceptionally sensitive to both temperature and humidity near ambient conditions, easily dehydrating to a non-superconducting lower hydrate. The observation of this stable lower hydrate with c=13.8 angstroms implies that the superconductivity turns on in this system between CoO2 layer spacings of 6.9 and 9.9 angstroms at nominally constant chemical doping.Comment: 10 pages and 4 figure

Theoretical study on the electronic and magnetic properties of double perovskite La 2−x Sr xMnCoO 6 (x = 0,1,2)

In this paper, the electronic and magnetic properties of double perovskite La2−xSrxMnCoO6 (x = 0,1,2) have been studied using the local-spin-density approximation + U method. For the three compositions investigated, the low symmetry P21/n structure yields consistently lower energy than that of the high symmetry \hbox{$Fm\bar{3}m$} structure. The strong electronic correlation and the orbital polarization of Co-d electrons play crucial roles. In agreement with experiments, we find that La2MnCoO6 is a ferromagnetic insulator with both Mn and Co ions in their high-spin states. The tilting of oxygen octahedrons is most significant in this case and is responsible for its insulating behavior; for LaSrMnCoO6, the ground state remains a ferromagnetic insulator with Mn and Co ions in their high-spin states. The optimized P21/n and \hbox{$Fm\bar{3}m$} crystal structures are nearly the same, and the P21/n structure is stabilized by the spontaneous layer-wise antiferro-orbital ordering of Co-d electrons. We also predict that Sr2MnCoO6 is a ferromagnetic metal, and its electronic structure can be viewed as a rigid band shifting from that of LaSrMnCoO6. Due to the strong covalency between transition metal and oxygen ions, the valences of Mn and Co ions differ considerably from those derived from purely ionic model. Also, doping induced holes mainly go to oxygen sites though the density of states near the Fermi energy has strong mixed character. This feature, together with the orbital ordering phenomenon, should be observable via the X-ray near-edge absorption spectroscopy and the polarized X-ray diffraction spectra