An empirical relationship between the long wavelength edge of the residual ray band in the IRreflectance
of partly ionic compounds and bulk hardness is demonstrated. The group of materials studied
includes alkali halides, semi-conductors and some hard compounds, mostly with cubic structure and a few
with hexagonal. The correlation is shown for the Young modulus, the melting temperature and also to
some extent the indentation hardness. The Young moduli cover a range from 10 to almost 500 GPa and the
melting temperatures vary from 600 to 3000°C.