Dependence of crystal structure, phase transition temperature, chemical state of Fe, oxygen content and electrical conductivity of Ba_2-xLa_xFe₂O_{5 + δ} (x = 0.00-0.15) on la content

Single phase preparation, crystal structure and structure phase transition by temperature of Ba_2-xLa_xFe₂O_{5 + δ} (x = 0.00-0.15), which is regarded to be promising as high electrical conduction material at high temperature, were investigated. Single phase with monoclinic structure at room temperature could be successfully prepared employing sintering under N₂ atmosphere with 10^{- 4} bar of oxygen partial pressure. Abrupt increase of molar volume and discrete decrease of temperature for the structural phase transition from monoclinic to cubic between x = 0.05 and x = 0.10 indicated existence of phase transition by La content. From iodometric titration, it was revealed that the specimens with x less than 0.05 were oxygen deficient while those with x more than 0.10 were oxygen excess from stoichiometric composition of 5.00. By combining Mössbauer spectra and iodometric titration, it was concluded that Ba₂Fe₂O_{5 + δ} contained both Fe^{3 +} and Fe^{2 +}. Concentration of Fe^{2 +} decreased with increasing La content from 0.00 to 0.05. Fe^{2 +} disappeared for the specimens with x larger than 0.10, showing correspondence with the phase transition by La content. For the specimens with x less than 0.05, discrete increase originating from structural phase transition from monoclinic to cubic was observed in temperature dependence of electrical conductivity. For the specimen with x = 0.10, even monoclinic phase showed high electrical conductivity comparable to ones of cubic, which could be originating from the phase transition by La content from oxygen deficient phase to oxygen excess one. Not discrete but continuous variation owing to the structural phase transition from monoclinic to cubic was observed in temperature dependence of electrical conductivity of the specimen with x = 0.10.