YbPtGe sub(2) : A multivalent charge-ordered system with an unusual spin pseudogap

We performed a study of the structural and physical properties of YbPtGe sub(2). This compound is a multivalent charge-ordered system presenting an unusual spin pseudogap below 200 K. The crystal structure of YbPtGe sub(2) is refined from single-crystal and powder high-resolution synchrotron x-ray diffraction data at different temperatures. Analysis of the structural features of YbPtGe sub(2), together with a combined study of Yb L sub(III) x-ray absorption spectroscopy, magnetic susceptibility chi (T), thermopower S(T), super(171)Yb and super(195)Pt NMR indicate half of the Yb atoms to be in an intermediate valence state with an electronic configuration close to 4 f super(13) (Yb super(3+)), while for the remaining Yb atoms the 4f super(14) (Yb super(2+)) configuration with almost no valence fluctuations is most likely. A drastic drop of the magnetic susceptibility and a decrease of the isotropic shift super(195)K sub(iso) (T)with decreasing temperature in the temperature range of 50-200 K evidence the opening of a spin pseudogap with an activation energy of Delta /k sub(B) ~ 200 K. Surprisingly, transport properties do not show clear evidence for the opening of a charge gap, thus excluding a standard Kondo-insulator scenario. Possible origins for this unusual electronic (valence) behavior are discussed.