Structural and physical properties of the NaxCoO2 center dot yH(2)O superconducting system

In this paper, we study the doping effects of vanadium (V) in La1.85Sr0.15Cu1-xVxO4+ and La2Cu1-xO4+delta systems. It is found that, in the La1.85Sr0.15Cu1-xVxO4+delta system, the maximum doping concentration of V for Cu is 0.15 and a tetragonal-to-orthorhombic phase transition occurs when x = 0.10. The doping of V leads to the decrease of the superconducting transition temperature, T-c. In the La2Cu1-xVxO4+delta system, the maximum doping concentration is 0.045 and the V-doped compounds have an orthorhombic structure. The lattice parameter c decreases and a increases lightly as x increases. No superconductivity is found in the as-prepared La2CuO4 compound without V doping. However, with the addition of V, La2Cu1-xVxO4+delta prepared under the same conditions shows superconductivity with T-c (onset) similar to40 K. The average excess oxygen delta in La2Cu1-xVxO4+delta increases from 0.005 to 0.011 as x increases from 0.0 to 0.045. We propose that the occurrence of superconductivity in this system is attributed to the macroscopic phase separation induced by the addition of V4+.