High borides: determining the features and details of lattice dynamics from neutron spectroscopy

We review wide-ranging research that combines inelastic neutron scattering spectroscopy with phenomenological and ab initio calculations to study the lattice dynamics and specifics of the electron–phonon interaction in three-dimensional boron cluster network systems $M\mathrm B_6$ and $M\mathrm B_{12}$ ($M= \mathrm{La}$, $\mathrm{Sm}$, and $\mathrm{Yb}$, $\mathrm{Lu}$, $\mathrm{Zr}$). A close similarity is found between the atomic vibration spectra of these systems, which is fundamentally due to a strong hierarchy of interatomic interaction in these systems and which manifests itself both in the shape of the low-energy phonon dispersion and in the position of the high-energy edge of the spectrum. Manifestations of strong electron–phonon interactions in the lattice vibration spectra of borides are studied in detail and their relation to the nature and features of the valence-unstable state of rare-earth ions is examined. Resonance nonadiabaticity and magnetovibration interaction effects in spin- and valence-fluctuating systems are given special attention.