Faust-Henry coefficients are ratios describing the relative influence of lattice displacements and electric field onto the electric susceptibility. They are essential in order to access the charge carrier concentration as well as the mobility of polar semiconductors by Raman scattering from measured frequencies, bandwidths, and intensities of coupled phonon-plasmon modes. In the case of alpha-GaN only the Faust-Henry coefficient connected with the Raman tensor elements a(TO) and a(LO) of the axial modes has been reported with differing results and questionable sign. However, according to its wurtzite structure, in hexagonal GaN three Faust-Henry coefficients associated with phonon modes of different symmetry exist. In the present study, from Raman scattering efficiencies of corresponding transverse optical and longitudinal optical phonons which are accessible in different scattering configurations, Raman tensor elements, and respective Faust-Henry coefficients were deduced. It is shown that near-forward scattering of phonon-polaritons, depending on frequency, allows the unambiguous determination of the sign of Faust-Henry coefficients. In case of a-GaN the obtained Faust-Henry coefficients connected with the corresponding Raman tensor elements are C-a(FH) = 3.46; C-b(FH) = 3.81, and C-c(FH) = 2.31. (C) 2014 AIP Publishing LLC.