Understanding muon diffusion in perovskite oxides below room temperature based on harmonic transition state theory

Abstract

In positive muon spin rotation and relaxation (${\mu }^{+}$SR) spectroscopy, positive muons (${\mu }^{+}$) implanted into solid oxides are conventionally treated as immobile spin-probes at interstitial sites below room temperature. This is because each ${\mu }^{+}$ is thought to be tightly bound to an oxygen atom in the host lattice to form a muonic analogue of the hydroxy group. On the basis of this concept, anomalies in ${\mu }^{+}$SR spectra observed in oxides have been attributed in most cases to the intrinsic properties of host materials. On the other hand, global ${\mu }^{+}$ diffusion with an activation energy of $\sim$0.1~eV has been reported in some chemically-substituted perovskite oxides at cryogenic temperatures, although the reason for the small activation energy despite the formation of the strong O$\mu$ bond has not yet been quantitatively understood. In this study, we investigated interstitial ${\mu }^{+}$ diffusion in the perovskite oxide lattice using KTaO${}_3$ cubic perovskite as a model system. We used the ${\mu }^{+}$SR method and density functional theory calculations along with the harmonic transition state theory to study this phenomenon both experimentally and theoretically. Experimental activation energies for global ${\mu }^{+}$ diffusion obtained below room temperature were less than a quarter of the calculated classical potential barrier height for a bottleneck ${\mu }^{+}$ transfer path. The reduction in the effective barrier height could be explained by the harmonic transition state theory with a zero-point energy correction; a significant difference in zero-point energies for ${\mu }^{+}$ at the positions in the O$\mu$ bonding equilibrium state and a bond-breaking transition state was the primary cause of the reduction. This suggests that the assumption of immobile ${\mu }^{+}$ in solid oxides is not always satisfied since such a significant decrease in diffusion barrier height can also occur in other oxides.

11 pages, 7 figures. v2: Corrected a typo in ${\omega }_3$ for the ES state in Table III; all other results and conclusions remain entirely unaffected

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Cited by in corpus (2)

• Nonmagnetic Ground State in RuO${}_2$ Revealed by Muon Spin Rotation
• Muon spin relaxation in mixed perovskite (LaAlO${}_3$)${}_x$(SrAl${}_{0.5}$Ta${}_{0.5}$O${}_3$)${}_{1-x}$ with $x\simeq 0.3$