We have studied polycrystalline Yb 4 LiGe 4 , a ternary variant of the R 5 T 4 family of layered compounds characterized by a very strong coupling between the magnetic and crystallographic degrees of freedom. The system is mixed valent, with non-magnetic Yb 2+ and magnetic Yb 3+ present, and is characterized by coexisting ferromagnetic and antiferromagnetic correlations. We present measurements of resistivity, AC-susceptibility, specific heat, and muon spin relaxation (μSR), below 1 K. The low temperature measurements suggest a transition to a mesoscopically inhomogeneous magnetically ordered state below 2 K characterized by fluctuations well below the ordering temperature. This unusual state is believed to result from the enhanced twodimensionality produced by Li substitution and frustration effects inherent in the Yb sub-lattice geometry.2

a b s t r a c tSingle-phase samples of Yb 4 LiGe 4 and Yb 5 Ge 4 were synthesized using high frequency (HF) heat treatment. Yb 4 LiGe 4 crystallizes in orthorhombic space group Pnma with the Gd 5 Si 4 type of crystal structure and lattice parameters a = 7.0828(3)Å, b = 14.6415(7)Å, and c = 7.6279(4)Å. One Yb position in Yb 5 Ge 4 is substituted by the lithium atom and causes a distortion of the germanium tetragons in Yb 4 LiGe 4 . Investigation of the electronic state of Yb via magnetic susceptibility and X-ray absorption near-edge spectroscopy (XANES) revealed a presence of two electronic states of ytterbium, 4f 13 and 4f 14 (mixed valence), in Yb 5 Ge 4 and Yb 4 LiGe 4 . Studies of the temperature dependence of the electrical resistivity, magnetization, 7 Li spin-lattice relaxation rate and the specific heat indicate that strong electronic correlations are present in Yb 4 LiGe 4 , and below approximately 50 K there is a competition between ferromagnetic and antiferromagnetic correlations. Magnetic ordering in Yb 4 LiGe 4 , if present, occurs below the reported antiferromagnetic transition temperature of 1.7 K for Yb 5 Ge 4 .

Yb4LiGe4 and Yb5Ge4 are prepared from the elements in stoichiometric ratios (high frequency furnace, Ta ampules, 1070 K, 20 min).