Origin of the eclectronoc conductivity in La2CuO4 synthesized by solution combustion method

Abstract

Lanthanum cuprate (La2CuO4) is a well-known stable layered perovskite oxide with K2NiF4 structure, consisting perovskite layers with alternating rock-salt layers. In this thesis, we reported the synthesis of La2CuO4 by solution combustion technique using urea as a fuel. The assynthesized and calcined La2CuO4 were characterized by X-ray diffraction technique. Compound La2CuO4 was calcined at different temperature ranges 900oC, 950oC, 1000oC, 1050oC and 1100oC. Conductivity of all compound were measured using four-probe technique. Assynthesized La2CuO4 doesn’t show conductivity and other compounds are conducting with a different extent. Highest conductivity was observed in La2CuO4 at 1000oC calcination temperature. Conductivity probe - atomic force microscopy (CP-AFM) studies of three samples of compound La2CuO4 calcined at 900oC, 1000oC and 1100oC were performed. CP-AFM results shows that the grain size of La2CuO4 crystallites increases with the increase of heat treatment temperature from 900°C to 1100°C. TUNA current micrographs of compound La2CuO4 in CPAFM concludes conductivity is not properly channelized and present in the grain boundaries. Also, CP-AFM give assurance of maximum conductivity of La2CuO4 calcined at 1000oC by looking in TUNA current micrograph. For identifying the reason of conductivity in La2CuO4, we again synthesized compound with different precursor amounts. On increasing amount of urea in La2CuO4, increase in conductivity was observed in four-probe technique. Thus, Carbon is responsible for conductivity in La2CuO4. Also, for identifying change in size of lattice, we were focused on shifting of peak by adding NaCl. The shifting of highest intensity peak in La2CuO4 at 1000oC towards higher diffraction angle showing compression in lattice size and supports the highest conductivity result at that temperature by four-probe and CP-AFM. Shrinkage in lattice size helps in attaining the conductivity in the temperature range of 900-1000oC. Above 1000oC, lattice size again expands and hence the decrease in the conductivity.