Phonon, defect and magnetic contributions to heat capacity of EuxYb1-xB6 solid solutions

Abstract

Zero-field heat capacity of EuxYb1-xB6 (x = 0, 0.127, 1) family was investigated at temperatures 1.9 − 300 K. The applied procedure of C(T) decomposition allowed to identify the Debye component resulted from the rigid cage of boron atoms (ΘD ≈ 1160 K), as well as the contribution from the quasilocal vibrational modes of rare-earth (RE) ions with Einstein temperatures to be different in end-point compounds: ΘE(YbB6) ≈ 91.6 K and ΘE(EuB6) ≈ 125 K. Our results also suggest the existence of additional low-temperature defect mode for the Eu-low systems (x ≤ 0.127) which is related to ≈ 1.15–1.3% vacancies at boron position. The magnetic contribution for EuB6 was analyzed in the framework of mean-field theory. The estimates show that 97% of spin entropy, associated with the 8S7/2-state of Eu2+ ions, is frozen out at TC. The finding that short-range magnetic ordering in the paramagnetic (PM) phase is limited by no more than 3% of EuB6 spin entropy should be taken into account in modern approaches explaining electronic and magnetic phase separation in this extraordinary compound.