Evolution of thermoelectric properties in Eu_xYb_1-xB₆family

Seebeck effect in the crystalline samples of Eu_xYb_1-xB₆(x= 0, 0.082, 0.127, 0.9, 1) was investigated at temperatures 2-300 K. For all the compounds thermopower is shown to be well described by the sum of diffusion (S_d=AT) and phonon drag components. The latter contribution is induced by quasilocal (Einstein) modes of ytterbium and europium ions with characteristic temperatures Θ_E(YbB₆) ≈ 91 K and Θ_E(EuB₆) ≈ 122 K. The estimation of effective massm^*of the charge carriers proves that increasing of Eu content induces crossover from 'heavy' holes withm_h^*(x⩽ 0.127) ≈ 0.3-0.36m₀to 'light' electrons withm_e^*(x⩾ 0.9) ≈ 0.12-0.13m₀(m₀-free electron mass). For the Eu-rich compounds we propose the existence of additional point on the phase diagram, which corresponds to short-range magnetic order with enhanced spin fluctuations preceding the stabilization of magnetic polarons.