Physical properties and Ce-valence of the filled skutterudite CePt$_4$Ge$_{12}$

Filled skutterudite compounds $M$Pt$_4$Ge$_{12}$ with a Pt-Ge framework structure show intriguing physical ground states (conventional ($M$ = Sr, Ba) and unconventional superconductivity ($M$ = Pr), heavy-fermion behavior ($M$ = Sm), magnetic ordering), similar to the well-known transition-metal pnicogen skutterudites. Here, we report on the electronic, magnetic, and transport properties of CePt$_4$Ge$_{12}$ [1]. High-resolution X-ray absorption spectroscopy (XANES) measurements at the cerium L$_{\mathrm{III}}$ edge demonstrate that Ce in this compound has a temperature-independent valence close to three. However, magnetic susceptibility, thermopower, Hall effect, and electronic specific heat reveal broad maxima at $T_{\mathrm{max}}$ = 65-80\,K, suggesting the presence of valence fluctuations. The Sommerfeld coefficient $\gamma$ = 105 mJ\,mol$^{-1}$\,K$^{-2}$ indicates moderately enhanced band masses for CePt$_4$Ge$_{12}$. We discuss these findings and conclude that CePt$_4$Ge$_{12}$ represents a system at the border between intermediate valence (IV) and Kondo lattice behavior.\\[4pt] [1] R.Gumeniuk et al. J.\ Phys.: Condensed Matter 23 (2011) 456601.