Chemical tracer diffusion of Sr and Co in polycrystalline Ca-deficient CaMnO_3−δwith CaMn₂O₄precipitates

Abstract

Diffusivity on the A- and B-site of polycrystalline perovskite CaMnO with Ca deficiency and spinel CaMnO (marokite) as a secondary phase was studied using chemical tracers and secondary ion mass spectrometry (SIMS) complemented by electron probe microanalysis (EPMA). Thin films containing Sr and Co chemical tracers were deposited on the polished surface of the polycrystalline composite sample followed by annealing at 800-1200 °C for 96 h. Diffusion profiles for each tracer were determined with SIMS, followed by calculation of diffusion coefficients by fitting to appropriate models. The Sr tracer showed mainly lattice diffusion, with an activation energy of 210 ± 30 kJ mol, whereas the Co tracer showed a combination of lattice and enhanced grain-boundary diffusion, with activation energies of 270 ± 30 kJ mol and 380 ± 40 kJ mol, respectively. The diffusivities may be used to predict interdiffusion and lifetime of junctions between n-type CaMnO or CaMnO/CaMnO composites and metallization interlayers or p-type leg materials in oxide thermoelectrics. In particular, the relatively high effective diffusivity of Co in polycrystalline CaMnO may play a role in the reported fast formation of the secondary phase (CaCoMnO) between p-type CaCoO and n-type CaMnO in a direct p-n thermoelectric junction.

Artificially Designed Compositionally Graded Sr-Doped NaTaO3 Single-Crystalline Thin Films and the Dynamics of Their Photoexcited Electron–Hole Pairs

“…In fact, when 20 nm-thick Sr-doped NTO films with upward and downward Sr composition gradients were similarly fabricated on 80 nm-thick NTO films, respectively, such a strong segregation nature of Sr atoms led to a completely inverted composition gradient of Sr in the film that had been designed to have the downward Sr composition gradient (Supporting Information, Figure S9). The diffusive nature of Sr ions has been reported in many oxide film systems such as (LaSr)­CoO 3 , and (SrCa)­MnO 3 , in contrast to the case of compositionally graded Ba 1‑x Sr x TiO 3 films with almost no Sr diffusion, including our previous result as well as those reported by other research groups. ,, It is known that the diffusion of atoms/ions in a crystal can be enhanced through even very small lattice defects and grain boundaries in the crystal, and this diffusion mechanism holds true for Sr ions. A similar lattice defect-enhanced diffusivity of O 2– ions from an oxide substrate was reported in SrTiO 3 films during vapor deposition such as PLD. , In the present case, the volatile nature of Na is likely to cause a large amount of A-site defects in Sr–NTO films, as already discussed above to explain the charging effects in the XPS measurement, probably enhancing the diffusivity of Sr ions in Sr–NTO films.…”