Raman scattering from the filled tetrahedral semiconductor LiMgAs

Ordered LiMgAs (space group: F-43m), viewed theoretically as a zinc-blende-AlAs-like (MgAs)(-) lattice partially filled with He-like Li+ interstitials, is studied using a Raman-scattering method. The band gap of as-grown crystals is found to be 2.309 eV with a direct gap from the photoluminescence measurements at 20 K, in consistency with that estimated by the optical absorption. From Raman scattering, two LO and TO phonons at k similar to 0 for Li-As and Mg-As pair are found to be 656 and 638 cm(-1) and 329 and 307 cm(-1), respectively, indicating that LiMgAs crystallizes with the ordered Li and Mg atoms at their tetrahedral sites. These results confirm a prediction that the ordered LiMgAs is attributed to the low ionic character of both Li-As (Pauling's ionicity f(i)=0.22) and Mg-As (f(i)=0.15) bonds.