Cation Interdiffusion in Polycrystalline Calcium and Strontium Titanate

Abstract

A method has been developed to study bulk lattice interdiffusion between calcium and strontium titanate by fabrication of a diffusion couple y i n g cosintering. The measured interdiffusion coefficients, D(C), indicate that strontium impurity diffusion in calcium titanate occurs at a faster rate than calcium impurity diffusion in strontium titanate. These interdiffusion coefficients are composition independent when the concentration of the calsium cation exceeds that of the strontium cation; otherwise D(C) is strongly composition dependent. Investigations into the effect of cation nonstoichiometry give results that are consistent with a defect incorporation reaction in which excess TiOz, within the solid solubility limit, produces A-site cation vacancies as compensating defects. The interdiffusion coefficients increase with increasing concentrations of Ti02, so it is concluded that interdiffusion of these alkaline-earth cations in their titanates occurs via a vacancy mechanism. [

Kirkendall porosity in barium titanate–strontium titanate diffusion couple

“…Examining the annealed microstructure of La 3+ -donor-doped SrTiO 3 -BaTiO 3 couple [6] suggests that Kirkendall porosity is located in the BaTiO 3 side. By an analogy to the original a-brass-Cu diffusion couple [8], it implies that Ba 2+ diffuses via a vacancy mechanism at a faster rate than Sr 2+ in both SrTiO 3 and BaTiO 3 lattice [5,6]. This is consistent with the present results where the thicknesses of regions (ii) and (iii) by the ratio of $3:1 (i.e.…”

“…Ca 2+ and Sr 2+ , there is no Kirkendall effect. This was concluded [5] for sintered CaTiO 3 -SrTiO 3 couple since no Kirkendall pores were detected.…”

“…The size of Kirkendall pores estimated directly from optical micrographs [6] was $15-20 mm. Such observation [6] is an evidence of the Kirkendall effect, suggesting diffusion via a vacancy mechanism [5,6,8,9] when all species, including Ti 4+ [6,[10][11][12], via V 00 00…”

“…a-brass-Cu [8], Ni-Ti [9], the migration of all species, including both cations and oxygen, across the interface in the inter-diffusion of such complex oxides [6] is required for the effect to be realized. However, no observation of Kirkendall porosity by scanning electron microscopy (SEM) was reported by Butler et al [5] when only inter-diffusion of Ca 2+ and Sr 2+ in CaTiO 3 and SrTiO 3 lattice, respectively, via a vacancy mechanism was found; the former cation was the faster diffusing species. Although Kirkendall-like porosity was again not detected experimentally in another study [7], it was thought to have retarded densification in a sintered BaTiO 3 -SrTiO 3 powder mixture.…”

“…The inter-diffusion of CaTiO 3 -SrTiO 3 [5] and BaTiO 3 -SrTiO 3 [6,7] couples has been reported. The Kirkendall effect was investigated [6] for hot-pressed and annealed BaTiO 3 -SrTiO 3 samples, where Kirkendall porosity was observed, the inter-diffusion (or chemical diffusion) coefficient determined, and a theoretical model developed.…”