Alkali halide crystals must have a strong, optically active resonance due to
the oscillation of the two oppositely charged ions in the unit cell.
Nevertheless, these compounds are extensively used as window materials in
optical spectroscopy for frequencies above 400 cm 
(in units of 
). The bulk modulus of KBr is
dyn/cm 
, and the lattice spacing is a=6.59 Å.
Estimate the frequency of the optical mode at k=0.
Laszlo Mihaly
Thu Oct 31 13:23:11 EST 1996
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