Quasiquartet CEF ground state with possible quadrupolar ordering in the tetragonal compound YbRu₂Ge₂

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e have investigated the magnetic properties of YbRu₂Ge₂ by means of magnetic susceptibility χ(T), specific heat C(T) and electrical resistivity ρ(T) measurements performed on flux grown single crystals. The Curie-Weiss behavior of χ(T) along the easy plane, the large magnetic entropy at low temperatures and the weak Kondo like increase in ρ(T) proves a stable trivalent Yb state. Anomalies in C(T), ρ(T) and χ(T) at T₀ = 10.2 K, T₁ = 6.5 K and T₂ = 5.7 K evidence complex ordering phenomena, T₀ being larger than the highest Yb magnetic ordering temperature found up to now. The magnetic entropy just above T₀ amounts to almost Rln4, indicating that the crystal electric field (CEF) ground state is a quasiquartet instead of the expected doublet. The behavior at T₀ is rather unusual and suggest that this transition is related to quadrupolar ordering, being a consequence of the CEF quasiquartet ground state. The combination of a quasiquartet CEF ground state, a high ordering temperature, and the relevance of quadrupolar interactions makes YbRu₂Ge₂ a rather unique system among Yb based compounds.