First-principles theory of the evolution of vibrational properties with long-range order in GaInP2

We predict the phonon spectra of zinc-blende GaP and InP, of CuPt-type ordered GaInP2, and of the random Ga0.5In0.5P alloy using first-principles density-functional linear-reponse theory. We find that (i) ordered GaInP2 exhibits two GaP-like and two InP-like phonon modes (a "two-mode" behavior), (ii) ordering reverses the order of frequencies of the TO-phonon modes of zinc-blende GaP and InP due to a frustrated bend geometry in the ordered phase, and (iii) each of the LO-phonon modes of the ternary GaInP2 phase represents a mixture of Gamma and L phonons of the corresponding binary compound (LO1 is GaP-like, LO2 is InP-like), yet the TO (LO) modes of the random alloy represent an amalgamation of the TO (LO) modes of GaP+InP. Consequently, the ordered phase is predicted to exhibit a two-mode behavior, while the random alloy is a pseudo-one-mode system. [S0163-1829(98)50112-9].