Valence state of Sm in single crystalline EuO thin films

Abstract

Samarium has two stable valence states, 2+ and 3+, which coexist in many compounds forming spatially homogeneous intermediate valence states. We study the valence state of samarium when incorporated in a single crystalline EuO thin film which crystallizes in a $fcc$-structure similar to that of the intermediate valence SmO, but with a larger lattice constant. Due to the increased lattice spacing, a stabilization of the larger Sm${}^{2+}$ ion is expected. Surprisingly, the samarium incorporated in Sm${}_{\mathrm{x}}$Eu${}_{1-\mathrm{x}}$O thin films shows a predominantly trivalent character, as determined by x-ray photoelectron spectroscopy and magnetometry measurements. We infer that the O${}^{2-}$ ions in the EuO lattice have enough room to move locally, so as to reduce the Sm-O distance and stabilize the Sm${}^{3+}$ valence.