Raman microprobe and diffuse-reflectance Fourier transform IR spectra
of a series of seven metal hexaborides show that the basic cubic symme
try of these systems is lowered owing to lattice vacancies. The result
ing wave vector is oriented along the C4 axis of the first Brillouin z
one. Studies of enriched boron samples indicate that the vacancies in
the B-6 structure are located at boron sites.