STRUCTURAL AND SUPERCONDUCTING PROPERTIES OF THE LA1.8-XSMXSR0.2CUO4 SYSTEM (0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1)

The phase diagram of the La1.8-xSmxSr0.2CuO4 system has been investigated by solid state reaction at 1150 degrees C in an oxygen flow. In these conditions, two phases have been found: (i) T-phase which exhibits a homogeneity range for 0 less than or equal to x less than or equal to 0.45; (ii) T*-phase which is obtained as single phase only for x=0.8. The thermal dependence of the unit cell parameters allows to correlate the occurrence of an orthorhombic-->tetragonal structural transition to the absence of superconductivity above 4.2 K for the compounds with x=0.45 and 0.8. The tilting of the [CuO6] octahedra (T-phase) or [CuO5] square-based pyramids (T*-phase) provoked by these structural changes plays an important role on the electrical properties of the compounds. Moreover, the overoxygenation of the T*-phase LaSm0.8Sr0.2CuO4 induced by chlorination treatment, leads to the disappearance of the structural transition and the occurrence of a superconducting behaviour below 35 K.