Dependence of the phonon spectrum of InP on hydrostatic pressure

The first- and second-order Raman spectra of InP have been measured in a diamond anvil cell as a function of hydrostatic pressure up to the phase transition (100 kbar). The Grüneisen parameters of the LO and TO phonons at $\Gamma$, and also those of several optical and acoustical phonons corresponding to critical points at the edge of the zone, have been measured. Particular emphasis is placed in the determination of the dependence of the dynamical charge as a function of lattice constant: a nonlinear decrease with decreasing lattice constant is found. The results are interpreted in terms of the bond orbital theory and of a pseudopotential calculation of the dynamical charge as a function of lattice constant.