Knowledge Bank

Near-normal incidence infrared reflectivity data in uniaxial crystals in which the principal plane is orthogonal to the plane of incidence can be simply decomposed into an extraordinary wave contribution and an ordinary wave contribution. The extraordinary wave contribution is shown to result from an oblique phonon resonance. The symmetry of oblique phonons Is examined by considering the direction of the phonon in the vicinity of the Brillouin zone center (B. Z. C.) $(k\simeq 0)$ rather than the formal symmetry precisely ac the B. Z. C. $(k\simeq 0)$. The apparent mixing of A and E modes which occurs in certain oblique phonons in these uniaxial systems is described in terms of a microscopic model rather than the usual macroscopic model.