OXYGEN NONSTOICHIOMETRY AND THERMODYNAMIC QUANTITIES OF PEROVSKITE-TYPE La_(1-x)Sr_xFeO_(3-δ) (x=0.2, 0.5, 0.8)

摘要

In this work, the defect structure analysis of La_(1-x)Sr_xFeO_(3-δ) (x=0.2, 0.5, 0.8) was presented. Thermogravimetric measurements were performed to determine the change in oxygen nonstoichiometry (Δδ) with oxygen partial pressure (pO_2) in 10~(-19) ≤ (pO_2/atm) ≤ 0.21 and temperature in 750 ≤ (T/℃) < 900 range. La_(1-x)Sr_xFeO_(3-δ) showed a clear electronic stoichiometric point around δ≈3-x/2. The relative partial molar enthalpy (h_o-h_o~o) and entropy (s_o-S_o~o) of oxygen were calculated from δ-pO_2-T relation by using Gibbs-Helmholtz equation. The negative sign of h_o-h_o~o and s_o-s_o~o indicated that the incorporation of oxygen was an exothermic process and showed that the experimentally observed variations in h_o-h_o~o and s_o-s_o~o with δ matched well with the statistical thermodynamic model proposed by Mizusaki. The defect diagram analysis showed that in n-type regime Fe~(2+) concentration varied with (pO_2)~(-1/4) whereas in p-type regime Fe~(4+) concentration varied with (pO_2)~(1/4).