Photoemission study of the electronic structure of the Kondo lattices Yb2Pt6X15 ( X =Al, Ga)

The electronic structure of Yb-based Kondo lattices ${\mathrm{Yb}}_{2}{\mathrm{Pt}}_{6}{X}_{15}$ ($X$=Al, Ga) has been investigated by means of photoemission spectroscopy using hard x-ray ($h\ensuremath{\nu}=5.95$ keV) and vacuum ultraviolet ($h\ensuremath{\nu}=182$ eV) synchrotron radiation. The Yb $3d$ spectra of ${\mathrm{Yb}}_{2}{\mathrm{Pt}}_{6}{X}_{15}$ showed both ${\mathrm{Yb}}^{2+}$- and ${\mathrm{Yb}}^{3+}$-derived structures, directly indicating valence fluctuation. The Yb valences of ${\mathrm{Yb}}_{2}{\mathrm{Pt}}_{6}{\mathrm{Al}}_{15}$ were estimated to be $2.89\ifmmode\pm\else\textpm\fi{}0.01$ at 250 K and $\ensuremath{\sim}2.83$ at 20 K, while those of ${\mathrm{Yb}}_{2}{\mathrm{Pt}}_{6}{\mathrm{Ga}}_{15}$ were $\ensuremath{\sim}2.34$ at 300 K with almost no temperature dependence. With changing $X$ ions from Al to Ga, the Pt $5d$ and Pt $4f$ peaks were shifted to shallower binding energies, and the ${\mathrm{Yb}}^{3+}$ $4f$ peaks were shifted to the deeper binding energies. The $X$ dependences of the Yb valence and the Kondo temperature of ${\mathrm{Yb}}_{2}{\mathrm{Pt}}_{6}{X}_{15}$ are discussed based on the ${\mathrm{Yb}}^{3+}$ $4f$ hole level relative to the Fermi level ${E}_{F}$ and Pt-derived density of states at ${E}_{F}$.