Anomalous rattling and single crystalline properties of the caged compound URu2Al10

We report on an experimental single-crystal study of URu2Al10, crystallizing in the YbFe2Al10 type orthorhombic structure, supplemented by the results of crystal field and band structure calculations. We investigated the magnetic, thermal and transport properties of this caged-type compound. Based on the local character of the 5f2−electron configuration of the U4+ ion in URu2Al10, the effective crystal field (CF) potential in the intermediate coupling form was estimated using the CF level scheme, composed only of singlets. This was carried out in a similar manner to that reported for UFe2Al10 [Phys. Rev. B 92 (2015) 104427]. The obtained scheme satisfactorily reproduces both the magnetic susceptibility (measured along the three main crystallographic directions) and the Schottky-type anomaly of the specific heat. The latter was estimated using the specific heat data of ThRu2Al10 as a phonon reference. In addition, the strong anisotropic behavior of the Seebeck coefficient measured along the three principal directions, and its low-temperature pronounced maxima, have been approximately explained by the CF effect. The latter dominates in the S-shaped temperature dependencies of the electrical resistivity, measured using the current flowing along the three main axes. However, the magnetoresistivity reveals an anisotropic electronic structure that could originate from a c-f hybridization effect in an orthorhombic unit cell. This gives rise to the typical metallic character of URu2Al10, as is also the case for UFe2Al10. This behavior underlines the dual character of the 5f-electrons in these ternaries. In turn, the presence of low-frequency Einstein modes reflects the presence of regular rattling of the U4+ ion located in the [Ru4Al16] cage. This rattling is, however, disturbed at low temperatures by applying an external magnetic field which causes strong scattering of the experimental electrical resistivity points. This effect is also anisotropic, as proved by a comparison of the resistivity results determined at zero and 9 T for a single-crystalline sample of URu2Al10. The above effect also exists for isostructural UFe2Al10, but its anisotropy is less apparent.