134Ba diffusion in polycrystalline BaMO3 (M = Ti, Zr, Ce)

Abstract

Cation diffusion in functional oxide materials is of fundamental interest, particularly in relation to interdiffusion of cations in thin film heterostructures and chemical stability of materials in high temperature electrochemical devices. Here we report on 134Ba tracer diffusion in polycrystalline BaMO3 (M = Ti, Zr, Ce) materials. The dense BaMO3 ceramics were prepared by solid state sintering, and thin films of 134BaO were deposited on the polished pellets by drop casting of an aqueous solution containing the Ba-tracer. The samples were subjected to thermal annealing and the resulting isotope distribution profiles were recorded by secondary ion mass spectrometry. The depth profiles exhibited two distinct regions reflecting lattice and grain boundary diffusion. The grain boundary diffusion was found to be 4-5 orders of magnitude faster than the lattice diffusion for all three materials. The temperature dependence of the lattice and grain boundary diffusion coefficients followed an Arrhenius type behaviour, and the activation energy and pre-exponential factor demonstrated a clear correlation with the size of the primitive unit cell of the three perovskites. Diffusion of Ba via Ba-vacancies was proposed as the most likely diffusion mechanism.

96Zr Tracer Diffusion in AZrO3 (A = Ca, Sr, Ba)

“…The reasons could be attributed to more space for the migration, and lower Coulomb's repulsion between the two cations A-A or A-B (see Figure 5). A similar trend has been previously observed for Ba 2+ diffusion in cubic Ba-based perovskites [18]. The activation energy for Zr 4+ diffusion in CZ is the lowest, meaning that the environment of the intermediate diffusion configurations is different compared to cubic perovskites.…”

“…Recently, we have reported on a systematic investigation of Ba diffusion in BaMO3 (M = Ti, Zr, Ce) [18], and here, we present a corresponding systematic investigation of 96 Zr cation tracer diffusion in polycrystalline AZrO3 (A = Ba, Sr, Ca), measured by secondary ion mass spectrometry (SIMS). The experimental data were obtained by annealing dense AZrO3 ceramics coated with thin films of 96 ZrO2 in the temperature range 1300-1500 °C.…”

“…The main contribution to point defects in the compounds is the frozen-in point defects after processing at higher temperature, and the change in the point defect concentration during tracer anneal at considerably lower temperature is minor [17,18,29] . The metal interstitial defects are not energetically favored, and less likely to be present [30,32].…”

“…We have recently argued that the activation energy barrier for Ba 2+ diffusion in BZ is mainly dominated by the enthalpy of migration [18]. Including the fact that AZrO 3 materials are refractory in terms of sintering, and the defect chemistry is dominated by frozen-in point defects especially on the A-site after sintering, the contribution of the enthalpy of formation to the activation energy is assumed negligible at 1300-1500 • C. The experimental activation energies for Zr 4+ diffusion in all three materials given in Figure 4 imply that increased A-cation size energetically favors Zr 4+ mobility in BZ compared to SZ.…”

“…AZrO 3 materials belong to the family of A II B IV O 3 perovskites among which cation diffusion has only been investigated in ATiO 3 (A = Ba, Sr), and recently in BZ and BaMO 3 (M = Ti, Zr, Ce) [9][10][11][12][13][14][15][16][17][18]. Systematic studies of factors influencing the cation diffusion, such as A-and B-site cation size, unit cell volume, and concentration of point defects in the materials, are still lacking.…”