Raman scattering of (K0.5Na0.5)NbO3-0.05LiNbO3-yMnO2 (y = 0% and 1.0%) single crystals have been reported in the temperature range from 300 to 800 K. The spectra exhibit a competition between a soft mode and a relaxation mode upon heating across the diverse transitions. The progressive change in the conflicting displacive mechanism (soft mode) and order-disorder (relaxation mode) can explain the origin of the successive orthorhombic-tetragonal-cubic phase transitions. Moreover, the polymorphic phase transition between orthorhombic and tetragonal structures can be confirmed through the observation of thermotropic phase boundaries for MnO2-doped (K0.5Na0.5)NbO3-0.05LiNbO3 single crystals.