The possibility of the mixed valence state in the uranium intermetallic compounds: UCoGa5, U2Ru2Sn and U2RuGa8

The mixed valence (MV) phenomenon has been observed so far in a large number of various compounds but containing only lanthanides. These properties are usually associated with the mixing of the localised f-state and the band states. The usual valence state for magnetic uranium intermetallics is the trivalent state 5f3 or hybridised 5f2 6d1, both are nearly degenerate in energy and can compete for a stability of the compound. In some cases a gain in an energy minimum may be achieved by very fast fluctuating between these two states with a time of 10-14 s, which does not allow to yield the ordered state even if the exchange interactions (favourite the U-U distances) would be able for that. The latter cases seem to concern the described here intermetallics: one ternary compound based on Co, UCoGa5, and the two uranium ternary compounds based on Ru, namely U2Ru2Sn and U2RuGa8 which all crystallize in a tetragonal unit cell. All these compounds show a maximum in their temperature dependences of the magnetic susceptibility measured along and perpendicular to the c-axis. Such a behaviour, which is reminiscent of a number of Ce (Sm, Eu) and Yb compounds for which χ(T) has in the past been considered by Sales and Wohlleben (SW) by applying their ICF model or by Lawrance et al. following their scaling procedure. It turned out that these phenomenological models can also be applied to the considered here two Ru-based uranium ternaries from which some reliable energy parameters could be found. In order to further support the mixing valence scenario for the first such cases in uranium compounds presented here, the transport and thermodynamic properties are also discussed. However, some of the most important results confirming the MV state, e.g., in U2RuGa8, has recently been achieved from the inelastic neutron scattering performed in the Rutherford Appleton Laboratory on the ISIS facility. From these measurements a characteristic gap of 60 meV has been estimated in the magnetic spectrum of this compound