Cation Transport and Surface Reconstruction in Lanthanum Doped Strontium Titanate at High Temperatures

Abstract

Tracer diffusion experiments were carried out in synthetic air at 1573 K in SrTiO3(100) and (110) single crystals, which were either undoped or doped with up to 1 at.% La, respectively. Tracer sources of 139La and 142Nd were applied by ion implantation. The resulting depth profiles were measured by SIMS. The reconstruction of the surface was monitored ex-situ using microscopic and spectroscopic methods including SEM, EPMA, and AFM. The measured tracer diffusivities show no dependency on orientation. The tracer diffusion takes place via cation vacancies. Under oxidizing conditions the dopant is compensated by Sr vacancies. Hence the diffusion is increasing strongly with La concentration. The observed time dependency of the diffusivities may be related to a space charge layer postulated by the current defect chemistry model for donor doped SrTiO3. At high dopant concentrations annealing leads to segregation of bulk La to the surface. La is not significantly incorporated into the secondary crystallites at the surface which consist almost entirely of Sr and O.

Sr diffusion in undoped and La-doped SrTiO3single crystals under oxidizing conditions

“…5. The results for the 1 at% La-doped samples show that, similar to the results for Ti and La, 28,29 no systematic dependence on the crystal orientation is observed. This is not surprising, considering the cubic crystal lattice of SrTiO 3 .…”

“…5. The results for the 1 at% La-doped samples show that, similar to the results for Ti and La, 28,29 no Fig. 4 86 Sr and 88 Sr depth profiles recorded with SIMS on a 0.02 at% La-doped SrTiO 3 (100) single crystal coated with a thin 86 SrTiO 3 layer.…”

“…Lastly, the determined Sr diffusion coefficients are about a decade lower than La diffusion coefficients for the same material at 1 at% La-doping and 1573 K. 28 Hence, the La diffusion coefficient is comparable to the Sr vacancy diffusion coefficient which is supported by the relation D La E 0.5 D V 00 Sr following from the combination of eqn. ( 3) and (10).…”

“…A similar behaviour was observed with implanted La tracers. 28 In contrast, identical experiments with Nd and Ti tracers 28,29 did not lead to distorted profiles. The depth distribution of the vacant lattice sites generated by the irradiation possesses a similar shape as the implanted ions, but the maximum is shifted to a lower depth.…”

“…Another value for the Sr vacancy diffusivity was deduced indirectly from the depth-dependent O diffusion coefficient. 17,27 In this work, following investigations of the La, Nd, and Ti diffusion in La-doped SrTiO 3 under oxidizing conditions, 28,29 we present the results of Sr tracer diffusion experiments.…”