Mass and Charge Transport Properties of Al Doped La2NiO4+δ

Mixed oxide-ion and electron conducting ceramic material La₂NiO_4+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, La₂Ni_0.95Al_0.05O_4.025+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 N_v obtained from fitting the Eq. (1) with fitting parameters K_ox, K_f, and N_v 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 m_h^*of holes is 1.02 - 1.21 times the rest mass m_o. 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 r_h^• increases with the increasing δ and reaches to ~7 at r_h^•≈0.08 and T=900°C. The calculation of excess partial molar quantities showed that incorporation of interstitial oxygen is less favorable in La₂Ni_0.95Al_0.05O_{4.025 + δ} in comparison to La₂NiO_4 + δ.

Reference

1. Nakamura T, Yashiro K, Sato K, Mizusaki, J. Solid State Ionics, 180, 368 (2009).
2. Naumovich EN, Patrakeev MV, Kharton VV, Yaremchenko AA, Logvinovich DI, Marques FMB. Solid State Sciences, 7,1353 (2005).
3. H. S. Kim, and H. I. Yoo, Solid State Ionics, 232, 129  (2013).
4. H. S. Kim, and H. I. Yoo, Phys. Chem. Chem. Phys., 12, 4704 (2010).