(Ga,In)P: A standard alloy in the classification of phonon mode behavior

Contrary to a broadly accepted assumption we show that random (Ga,In)P is not an exception with respect to the crude classification of the phonon mode behavior of random mixed crystals in terms of 1-bond->1-mode systems or 2-bond->1-mode systems, as established from the simple criterion derived by Elliott et al. [Rev.Mod.Phys. 46, 465 (1974)]. Consistent understanding of the puzzling Raman/infrared behavior of (Ga,In)P, that has been a subject of controversy, is achieved via a basic version of our 1-bond->2-mode model. The Raman/infrared features from (Ga,In)P are accordingly re-assigned, with considerable change with respect to the previous approaches. Besides, we show that the idea of two bond lengths per species in alloys, supported by our 1-bond->2-phonon picture, opens an attractive area for the discussion of spontaneous ordering in GaInP_2, and in mixed crystals in general. The whole discussion is supported by detailed re-examination of the (Ga,In)P Raman/infrared data in the literature, full contour modeling of the transverse and longitudinal optical Raman lineshapes via our phenomenological 1-bond->2-mode model, and first-principles bond length calculations.