In this work, aluminium scandium nitride (Al1-xScxN) thin films are deposited by reactive DC magnetron co-sputtering on Pt(111) layers. The sputtering power of the Sc target is varied in a broad range up to 900 W to effectively vary the Sc content later assessed using energy-dispersive X-ray spectroscopy (EDX). We show that commonly used X-ray diffraction techniques may yield ambiguous results, and thus, additional verification is crucial to substantiate the actual composition of the sputtered thin films. Therefore, we employ optical spectroscopy techniques, such as Raman and Fourier transformed infra-red (FTIR) spectroscopies in order to study the vibrational properties of the alloys. Our Raman spectra show that low Sc content leads to a phase separation with the formation of cubic ScN and AlScN phases during the sputtering process. Nevertheless, small amounts of the wurtzite phase are detected in the rock-salt dominated samples owing to the enhancement of the A(1)(LO) phonon mode by the Berreman effect in the FTIR spectra. The Sc-induced shifts of the AlN phonon modes are determined in a broad range of Sc content.