The equation-of-motion technique has been used to study the optic response functions of a computer-generated periodic random network containing 1536 ions. Long-range Coulomb effects were included for the first time. Our results show unambiguously the existence of an LO-TO splitting. The long-wavelength LO response function peaks at the band edge in the density of states and is very sharp, whereas the log-wavelength TO response function peaks at a lower frequency and is much broader.

• Received 30 August 1985

DOI:https://doi.org/10.1103/PhysRevLett.55.2879