Experimental evidence for fractional valence of La in LaAl2 : Electronic structure from x-ray photoelectron spectroscopy and band structure calculations

We confirmed the fractional filling of the $4f$ shell of La in ${\mathrm{LaAl}}₂$ by x-ray photoemission spectroscopy (XPS) and ab initio calculations. The La $3d$ core-level XPS spectra were analyzed by use of the Gunnarsson-Sch\"onhammer theoretical model, which allowed us to determine the valency of La smaller than 3 ($v=2.9$). A similar occupation of the La $4f$ shell results from the band structure calculations. In real unstable La atom charge is transferred in the mixing process from its outer $5d6s$ electron shells to the local $4f$ state, according to $4{f}^{\ensuremath{\nu}}(5{d}^{1\ensuremath{-}\ensuremath{\nu}}6{s}²)$, where $0<\ensuremath{\nu}\ensuremath{\ll}1$. The calculated density of states fit well the valence band XPS spectra. We have shown that the fractional valence of La plays a key role in electric transport properties of ${\mathrm{LaAl}}₂$ and the fractional filling of the $4f$ shell is really at the root of the resistivity anomalies either at the high or low temperatures.