Hydration and proton conductivity in LaAsO4

Abstract

Incorporation and transport of protonic defects have been studied in nominally undoped and 1 and 3 mol% Sr-doped LaAsO₄ prepared by a co-precipitation route. AC impedance of the materials was measured as a function of temperature (1150 to 400 °C), p_O2 (1 to 1 × 10⁻⁵ atm) and p_H2O (0.025 to 3 × 10⁻⁵ atm). The bulk conductivities generally decrease with decreasing temperature and moreover with decreasing p_H2O within the whole temperature range. At the highest temperatures, a small decrease in the conductivity with decreasing p_O2 was also observed. The defect structure of Sr-doped LaAsO₄ appears to be dominated by oxygen vacancies in the form of pyroarsenate ions, As₂O⁴⁻₇, in dry atmospheres at high temperatures and by protonic defects in the form of hydrogen arsenate ions, HAsO²⁻₄, in wet atmospheres. A significant isotope effect shows that protons contribute to the total conductivity at all temperatures under wet conditions and predominate at temperatures below ∼850 °C. The remaining contributions are attributed to oxide ions and electron holes. The extracted hydration thermodynamics are comparable to those determined for other LnXO₄ (X = P, V, Nb, Ta) compounds, and the enthalpy of mobility of protons (86 ± 5 and 88 ± 5 kJ mol⁻¹ for the 1 and 3 mol% doped samples, respectively) follows an apparent trend for the isostructural LaXO₄ (X = P, As, V) series with the enthalpy of mobility of protons decreasing with increasing radius of the X-site cation. However, the partial proton conductivities of Sr-doped LaAsO₄ are lower than those determined for acceptor doped LaPO₄ and LaVO₄ for which the possible reasons are discussed.