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Spectroscopy of metal hexaborides: Phonon dispersion models

Raman spectroscopy of many metal hexaborides is well understood for vibrations of Pm3m symmetry that can be ascribed to the breathing and deforming modes of boron octahedral clusters at the corners of the cubic unit cell. However, the significance of the lower energy modes has been subject to debate wherein interpretation is dependent on “rattling” of the metal ion within the boron cage or on alternative mechanisms such as the presence of defects or of isotope clusters. Furthermore, converged calculations of phonon dispersions (PDs) using Density Functional Theory (DFT) for YB6 with Pm3m symmetry in the published literature are limited and equivocal. Converged PDs across all reciprocal lattice orientations are an important indicator of thermodynamic stability for a phase. We use this criterion to evaluate symmetry conditions for hexaboride structures and compare DFT models of PDs with Raman spectra for the hexaborides BaB6, LaB6, and YB6. We demonstrate that models with lower symmetry using a P4/mmm superlattice along one primary axial direction allow the lower energy modes to be assigned to a point symmetry. This approach, which models displacements of the metal position along the superlattice direction, results in a convergent PD for YB6, an outcome difficult to achieve with Pm3m symmetry.