Vacancy‐Induced and Two‐Phonon Raman Scattering in ZrC_x, NbC_x, HfC_x, and TaC_x

Abstract

Abstract Raman scattering is studied in the substoichiometric transition metal carbides. The carbon vacancies in these cubic solids induce a one‐phonon spectrum that essentially mirrors the phonon density of states. The superconducting group Vb carbides NbC 0.98 and TaC 0.99 have one‐phonon Raman spectra that show characteristic phonon softening. Strong A + A and O + A two‐phonon peaks are also seen. In NbC 0.76 , which has a low T c , the one‐phonon peaks harden, and the two‐phonon intensities weaken. Mechanical polishing of NbC 0.98 and TaC 0.99 reduces two‐phonon scattering and hardens and broadens the one‐phonon spectra. The non‐superconducting adjacent group IVb carbides ZrC 0.96 , ZrC 0.87 , and HfC 0.99 have higher‐frequency one‐phonon peaks and no two‐phonon features.