IMPROVEMENT OF THE SUPERCONDUCTING PROPERTIES OF THE LA2-XBAXCUO4 SYSTEM (0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.15) BY LOW-TEMPERATURE FLUORINATION

The influence of Ba substitution for La and of the fluorination on the thermal conductivity, the electrical resistivity, the thermoelectric power and the specific heat of La2-xBaxCuO4 has been investigated. In the La2-xBaxCuO4 system, either the electron-phonon interactions or phonon-defect interactions may explain the temperature dependence of the thermal conductivity. The intercalation of the fluorine atom into the lattice leads to a superconducting transition with an onset higher than 30 K for all compositions investigated. The effect of fluorine intercalation in these oxides is similar to an increase in barium concentration. Indeed the presence of fluorine leads to an increase of the free carriers concentration and of the lattice disorder. For the particular composition x congruent-to 0.125, the fluorination seems to suppress the low-temperature structural transition (LTO-->LTT) and to eliminate all the anomalies usually observed in the transport properties.