YbPtGe2: A multivalent charge-ordered system with an unusual spin pseudogap

We performed a study of the structural and physical properties of YbPtGe2. This compound is a multivalent charge-ordered system presenting an unusual spin pseudogap below 200 K. The crystal structure of YbPtGe2 is refined from single-crystal and powder high-resolution synchrotron x-ray diffraction data at different temperatures. Analysis of the structural features of YbPtGe2, together with a combined study of Yb L-III x-ray absorption spectroscopy, magnetic susceptibility chi(T), thermopower S(T), and Yb-171 and Pt-195 NMR indicate half of the Yb atoms to be in an intermediate valence state with an electronic configuration close to 4f(13) (Yb3+), while for the remaining Yb atoms the 4f(14) (Yb2+) configuration with almost no valence fluctuations is most likely. A drastic drop of the magnetic susceptibility and a decrease of the isotropic shift K-195(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(B) similar to 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. DOI: 10.1103/PhysRevB.86.235138