Valence Fluctuations In The 3d+3 Modulated Yb3co4ge13 Remeika Phase

Yb3Co4Ge13 is the first example of a Remeika phase with a 3D + 3 [space group P43n(alpha,0,0)000(0,alpha,0)000(0,0,alpha)000; a = 8.72328(1) angstrom, alpha = 0.4974(2)] modulated crystal structure. A slight shift of the composition towards higher Yb-content (i.e. Yb3.2Co4Ge12.8) leads to the disappearance of the satellite reflections and stabilization of the disordered primitive cubic [space group Pm3n, a = 8.74072(2) angstrom] Remeika prototype structure. The stoichiometric structurally modulated germanide is a metal with hole-like charge carriers, where Yb-ions are in a temperature-dependent intermediate valence state varying from +2.60 to +2.66 for the temperature range 85-293 K. The valence fluctuations have been investigated by means of temperature dependent X-ray absorption spectroscopy, magnetic susceptibility and thermopower measurements.