Born effective charges and infrared response of LiBC

Abstract

Calculations of the zone-center optical-mode frequencies (including LO-TO splitting), Born effective charges ${Z}_{\ensuremath{\alpha}\ensuremath{\alpha}}^{*}$ for each atom, dielectric constants ${\ensuremath{\epsilon}}_{0}$ and ${\ensuremath{\epsilon}}_{\ensuremath{\infty}},$ and the dielectric response in the infrared, using density functional linear response theory, are reported. Calculated Raman modes are in excellent agreement with experimental values $(170{\mathrm{cm}}^{\ensuremath{-}1}$ and $1170{\mathrm{cm}}^{\ensuremath{-}1}),$ while it will require better experimental data to clarify the infrared-active mode frequencies. The Born effective charges ${Z}_{\ensuremath{\alpha}\ensuremath{\alpha}}^{*}$ (i) have surprisingly different values for B and C and (ii) show considerable anisotropy. Relationships between the effective charges and LO-TO splitting are discussed, and the predicted reflectivity in the range $0--1400{\mathrm{cm}}^{\ensuremath{-}1}$ is presented. These results hold possible implications for Li removal in LiBC and C substition for B in ${\mathrm{MgB}}_{2}.$