We have investigated the lattice dynamics for a number of rare-earth hexaborides based on the superatom model within which the boron octahedron is substituted by one superatom with a mass equal to the mass of six boron atoms. Phenomenological models have been constructed for the acoustic and lowenergy optical phonon modes in RB₆ (R = La, Gd, Tb, Dy) compounds. Using DyB₆ as an example, we have studied the anomalous softening of longitudinal acoustic phonons in several crystallographic directions, an effect that is also typical of GdB₆ and TbB₆. The softening of the acoustic branches is shown to be achieved through the introduction of negative interatomic force constants between rare-earth ions. We discuss the structural instability of hexaborides based on 4f elements, the role of valence instability in the lattice dynamics, and the influence of the number of f electrons on the degree of softening of phonon modes.

Original Russian Text © D.A. Serebrennikov, E.S. Clementyev, P.A. Alekseev, 2016, published in Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 2016, Vol. 150, No. 3, pp. 526–535.

https://doi.org/10.1134/S1063776116070220

Citation:

Serebrennikov, D.A., Clementyev, E.S. & Alekseev, P.A. J. Exp. Theor. Phys. (2016) 123: 452. https://doi.org/10.1134/S1063776116070220