Spectroscopy of metal hexaborides: Phonon dispersion models

Abstract

Raman spectroscopy of many metal hexaborides is well understood for vibrations of italicPmtrue3¯italicm 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 italicPmtrue3¯italicm 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 italicPmtrue3¯italicm symmetry.

Insights from Systematic DFT Calculations on Superconductors

“…Thus, we advocate an alternative approach for superconductivity prediction that complements the McMillan formalism. In this alternative approach, appropriate values for Δk and the cut-off energy enable ab initio DFT calculations to estimate values for T δ that correlate with experimentally determined values for the T c of MgB 2 [23,27], for compounds of the form (Mg 1-x M x )B 2 (where M = Al, Sc, Ti) [23][24][25], and for disilicides [23] and metal hexaborides [74].…”

“…The predictive value of the approaches we advocate to estimate T c that utilizes calculation of a value for T δ using a phonon anomaly [23][24][25]74] is evident for Ba-substitution into MgB 2 [56]. Our estimates for (Mg 1-x Ba x )B 2 at three levels of Ba substitution (x = 0.33, 0.5 and 0.66) and using both LDA and GGA approximations suggest that 62.1 K < T δ < 64.4 K with an error of ±4.9 K. These estimates are higher by ~15 K than the experimentally determined value of ~45 K by Palnichenko et al [56].…”

“…Fully converged PDs are a useful indicator of phase stability [26,74]. The sensitivity of PDs to changes in stoichiometry, composition or Δk is significantly higher than typically encountered in an EBS [26].…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XIII

“…Alarco and co‐workers have proposed phonon dispersion models for the Raman spectroscopy of metal hexaborides. They used a proposed criterion to evaluate symmetry conditions for hexaboride structures and compare DFT models of phonon dispersions with Raman spectra for the hexaborides BaB 6 , LaB 6 , and YB 6 . Dias and Moreira identified the polarization‐resolved Raman modes of monoclinic SrAl 2 O 4 ceramics.…”

“…They used a proposed criterion to evaluate symmetry conditions for hexaboride structures and compare DFT models of phonon dispersions with Raman spectra for the hexaborides BaB 6 , LaB 6 , and YB 6 . [140] Dias and Moreira identified the polarization-resolved Raman modes of monoclinic SrAl 2 O 4 ceramics. Knowledge of the optical phonon features can help to understand the exceptional luminescence properties of SrAl 2 O 4 in its monoclinic phase.…”