Yb₅Rh₆Sn₁₈: a valence fluctuating system with ultra-low thermal conductivity

Yb₅Rh₆Sn₁₈ crystallizes with a unique structural arrangement [space group P4₂/nmc, a = 9.6997(4) Å, c = 13.7710(7) Å], which is related with primitive cubic Yb₃Rh₄Sn₁₃ and body-centered tetragonal (Sn_1-xTb_x)Tb₄Rh₆Sn₁₈ types. X-ray absorption spectroscopy showed that Yb atoms exhibit temperature-dependent valence fluctuations (VF) (i.e., intermediate valence state). Its complex mechanism is corroborated by the fact that the well-pronounced maximum in magnetic susceptibility can only be fairly described by the Bickers-Cox-Wilkins model developed for a J = 3/2 multiplet, atypical for Yb ions. Both Hall and Seebeck coefficients revealed a switch of the sign, indicating the change of charge carrier type from electrons to holes between 120 and 220 K. Both these effects together with electrical resistivity and theoretical DFT calculations confirm Yb₅Rh₆Sn₁₈ to be a metal, which disobeys the free electron gas theory. 'Rattling' motion of Sn1 atoms within the enlarged 16-vertices distorted Frank-Kasper polyhedra, concluded from the specific heat measurements, is argued to be the main reason for the appearance of a phonon resonance behavior, resulting in an ultra-low thermal conductivity in the studied stannide.