Imperfection of the Sm sublattice and valence instability in compounds based on SmB6

The magnetic susceptibility, electrical resistance, specific heat, and ther mal expansion coefficient of SmB6, Sm0.8B6, and Sm1-xLaxB6 (x = 0.1 and 0.2 ) are measured in the temperature range T = 4 - 300 K. The dispersion curve s of the acoustic phonon branches in lanthanum-doped samples are studied. A combined analysis of the results confirms the existence of an activation g ap in the electron density of states in both nonstoichiometric and lanthanu m-doped compounds. The anomalies in the electronic component of the thermal expansion coefficient are associated to a considerable degree with the tem perature variation of the valence and, like the magnetic susceptibility, re flect features of the f-electron excitation spectrum. It is found that lant hanum doping does not lead to significant changes in the anomalies in the p honon spectrum of SmB6. It is established that the homogeneous intermediate -valent state of the samarium ion is fairly stable and is maintained when t he perfection of the Sm sublattice is violated. (C) 1999 American Institute of Physics. [S1063-7761(99)01903-4].