ABSTRACT
This chapter presents a review of the physical properties of rareearth (RE) dodecaborides RB12 that are characterized by a cageglass crystal structure with loosely bound RE ions. The analysis of available literature leads to a conclusion that the RE dodecaborides are strongly correlated electron systems with simultaneously active charge, spin, orbital, and lattice degrees of freedom. This explains the complexity of all RB12 compounds including antiferromagnetic (TbB12 –TmB12) and nonmagnetic (LuB12) metals, on one hand, and the so-called Kondo insulator compound YbB12 and Yb-based Yb x R 1−x B12 solid solutions, on the other. It is shown that the reason for the complexity is based on the development of the cooperative dynamic Jahn–Teller instability of the covalent boron network, which produces trigonal and tetragonal distortions of the rigid cage and results in the symmetry lowering of the fcc lattice in the dodecaborides. The ferrodistortive dynamics in the boron sublattice generates both the collective modes and quasilocal vibrations (rattling modes) of the heavy RE ions, causing a modulation in the density of conduction electrons and the emergence of dynamic charge stripes in these strongly correlated compounds.We consider the manifestation of the charge stripes both in the properties of the nonmagnetic reference compound LuB12 and in the phase diagrams of the RB12 antiferromagnets that exhibit multiple magnetic phases with anisotropic field-angular phase diagrams in the form of the Maltese cross. We will also discuss the metal–insulator transitions in YbB12 and in Yb-based dodecaborides in terms of the instability of the Yb 4f -electron configuration, which appears in addition to the Jahn–Teller instability of the boron cage, providing one more mechanism of the charge and spin fluctuations. The experimental results lead to conclusions that challenge the established Kondoinsulator scenario in YbB12 , providing arguments in favor of the appearance of Yb–Yb vibrationally coupled pairs which should be considered as the main factor responsible for the charge- and spin-gap formation.
