Synthesis, structure and oxygen thermally programmed desorption spectra of La_(1-x)Ca_xAl_yFe-(1-y)O_(3- delta) perovskites

摘要

Perovskites La_(1-x)Ca_xAl_yFe_(1-y)O_(3_ delta) (x, y = 0 to 1) were prepared by high-temperature solid-state synthesis based on mixtures of oxides produced by colloidal milling. The XRD analysis showed that perovskites La_(0.5)Ca_(0.50Al_(1-y)Fe_(1-y)O_(3- sigma) with a high Fe content (1 - y = 0.8 -1.0) were of orthorhombic structure, perovskites with a medium Fe content (1 - y = 0.8-0.5) were of rhombohedral structure, and perovskite with the lowest Fe content (1 - y = 0.2) were of cubic structure. Thermally programmed desorption (TPD) of oxygen revealed that chemical desorption of oxygen in the temperature range from 200 to 1000 deg C had proceeded in the two desorption peaks. The low-temperature alpha -peak (in the 200-550 deg C temperature range) was brought about by oxygen liberated from oxygen vacancies; the high-temperature beta -peak (in the 550-1000 deg C temperature range) corresponded to the reduction of Fe~(4+) to Fe~(3+). The chemidesorption oxygen capacity increased with increasing Ca content and decreased with increasing Al content in the perovskites. The Al_(3+) ions restricted, probably for kinetic reasons, the reduction of Fe~(4+) and the high-temperature oxygen desorption associated with it.