Unified theory of the dielectric properties and lattice vibrational intensities of the HCN crystal

The effective polarizability of the HCN molecule in its two crystal phases is calculated from the mean crystal dielectric constant and molecular- orbital dipole moment calculations, using lattice dipole sums for molecules treated as one, two and three points. The axial polarizability agrees with the gas-phase value, but the transverse polarizability from each calculation is roughly twice the gas-phase value. The lattice infrared absorption intensities and LO-TO splitting are calculated; the three-point treatment with the crystal-phase polarizability yields agreement with experiment. The lattice Raman intensities are also calculated; they depend markedly on the choice of polarizability. The results illustrate how several properties can be calculated within one model. Some inconsistencies for HCN may be attributable to the effects of hydrogen bonding, but could be resolved by dielectric tensor, refractive index, or lattice Raman intensity measurements.