Phonon spectra of TlTiOPO4 and KTiOPO4 have been measured in a few scattering geometries. A comparative analysis of the difference in the line positions has been performed. It has been shown that a strong relative shift of lines in the frequency range <100 cm-1 can be explained well by the mass ratio of the ions involved into vibration. At higher frequencies the mechanism of these shifts is more complicated and can be explained by the combined effect of a few factors.
Keywords: phonon spectrum, thallium titanyl phosphate, potassium titanyl phosphate