Rattling mode and symmetry lowering resulting from the instability of the B12 molecule in LuB12

Abstract

The dodecaboride LuB12 with cage-glass state and rattling modes has been studied to clarify the nature of the large amplitude vibrations of Lu ions. Discovered anisotropy of charge transport in conjunction with distortions of the conventional fcc symmetry of the crystal lattice may be attributed to coherent motion of Lu ions along singular direction in the lattice. Arguments are presented in favor of cooperative dynamic Jahn-Teller effect in the boron sublattice to be the reason of the rattling mode, lattice distortion and formation of the filamentary structure of the conductive channels. PACS numbers: 61.66.Fn, 72.15.Gd:

Fine details of crystal structure and atomic vibrations in YbB12with a metal–insulator transition

“…All others undergo slight trigonal or tetragonal distortions depending on the charge n. The cooperative JT effect (Gehring & Gehring, 1975;Kaplan & Vekhter, 1995;Bersuker, 2006) distorts the cubic structure as a result. Small JT distortions (less than 0.005 Å in unit-cell edges and 0.05 in interaxial angles) were previously observed in the LuB 12 structure (Flachbart et al, 2008;Dudka et al, 2017;Sluchanko et al, 2018a;Bolotina et al, 2019). According to the type of distortion, they correspond to theoretical predictions and show a dependence on isotopic boron composition (Bolotina et al, 2019).…”

“…It is worth noting also that the collective JT effect develops on the B 12 complexes (Sluchanko et al, 2018a;Bolotina et al, 2018) and this effect is the source of both the collective dynamics of boron clusters and large-amplitude oscillations of the rare-earth ions embedded in the B 24 cavities (see Fig. 1).…”

“…Like the B 12 icosahedra in boron and boron-rich compounds (Franz & Werheit, 1989, the B 12 cuboctahedra with the orbitally degenerate ground state are distorted by the Jahn-Teller (JT) effect. According to quantum chemical calculations (Sluchanko et al, 2018a) carried out on model clusters (B 12 ) nÀ , 0 n 4, only (B 12 ) 4À is a regular cuboctahedron. All others undergo slight trigonal or tetragonal distortions depending on the charge n. The cooperative JT effect (Gehring & Gehring, 1975;Kaplan & Vekhter, 1995;Bersuker, 2006) distorts the cubic structure as a result.…”

Electronic phase separation: Recent progress in the old problem

“…There is a vast body of literature concerning this topic, see e.g., comprehensive reviews [395,396,397]. In addition to oxides, the existence of stripes has been also discussed recently for iron-based superconductors (where the stripe structures are closely related to the electron nematicity [398]) and in borides [399,400].…”

Observation of a crossover from nodal to gapped superconductivity in LuxZr1−xB12

“…In LuB 12 , vibrations of Lu 3+ ions have almost no contribution to the electron-phonon coupling, perhaps due to the 'volume filling factor' of the B cages, which tunes the hybridization between the Lu/Zr and B orbitals [9]. Another factor which may contribute to the lower T c in LuB 12 is the development of an electron instability due to the formation of dynamic charge stripes [13]. Though LuB 12 appears to display s-wave behavior [14], there has been considerable debate surrounding the nature of the superconducting gap function in ZrB 12 .…”