Mass and Charge Transport Properties of Al Doped La2NiO4+δ
Authors: Jeon Sang-Yun , Im Ha-Ni , Song Sun-Ju 
Published: February 2020

Mixed oxide-ion and electron conducting ceramic material La2NiO4+
d(LNO) have been reported to exhibit impressive oxide ion and p-type electronic conductivity, and high catalytic activity which make them an excellent component of some electrochemical devices such as fuel cell cathode, oxygen permeation membrane. The hyperstoichiometric LNO oxides show high oxygen diffusion coefficient and high oxygen ion conductivity, which are attributes of their crystal structure.

The extent of oxygen nonstoichiometry significantly affects the electrochemical properties of LNO systems. The nonstoichiometric oxygen content may be varied by aliovalent doping or by substituting the host cation with smaller size cations[1,2]. The conduction mechanism in LNO-based system has been explained on the basis of both, the band conduction by delocalized electrons and polaron-hopping by localized electrons[3,4].

The objective of this work is to study the effect of a donor cation (Al3+) on oxygen nonstoichiometry and resultant thermodynamic properties of LNO system. In this work we have prepared a non-transition metal cation Al3+-doped LNO system, La2Ni0.95Al0.05O4.025+
d, and presented an equilibrium defect model for this system. The variation of oxygen nonstoichiometry was measured by coulometric titration and the absolute value of oxygen nonstoichiometry was calculated from thermogravimetric analysis. Various thermodynamic quantities for LNAO were calculated from the experimental data and were compared with those for LNO in our previous work.

Fig.1 shows pO2 dependence of oxygen nonstoichiometry at different temperature. As can be seen, the values of δ increased with decreasing temperature and with increasing pO2. The inset of Fig. 1 shows the relationship of log δ vs. log pO2, as can be seen, at isothermal condition the dependence of δ on the pO2exponent (m) is slightly less than 1/6. It is clear from the above observation that the oxygen exponent decreases with increasing oxygen activity, suggesting a positive deviation of the LNAO system from the ideal solution behavior[4].

The best estimated values of Nv obtained from fitting the Eq. (1) with fitting parameters Kox, Kf, and Nv to the δ vs. log(pO2) data in Fig.(1). The density of states of the valence band varies from 1.87x1020 to 2.55x1020 in 800-1000 oC range. And , the effective mass mh
*of holes is 1.02 - 1.21 times the rest mass mo. This indicates the occurrence of band-like conduction and allows the effect of conduction by a small degree of polaron hopping to be ignored. The behavior of holes starts deviates from that of ideal solution at a very low δ value and rh
• increases with the increasing δ and reaches to ~7 at rh
•≈0.08 and T=900°C. The calculation of excess partial molar quantities showed that incorporation of interstitial oxygen is less favorable in La2Ni0.95Al0.05O4.025
 
+
 
δ in comparison to La2NiO4
 
+
 
δ.

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