Temperature-dependent IR and Raman scattering studies of phase transitions in K2MgWO2(PO4)2

Temperature-dependent Raman scattering and IR experiments were performed on K2MgWO2(PO4)2, which is derived from KTP by the replacement of two Ti4+ ions with Mg2+ and W6+ cations. These studies revealed significant changes in phonon properties of these materials with increasing temperature due to the onset of phase transitions at 436, 537 and 637 K. Analysis of the obtained data showed that disordering processes in the sublattice of potassium ions play a significant role in the mechanism of these phase transitions. However, these transitions are also accompanied by weak tilts of the nearly rigid PO4, WO6 and MgO6 groups. Present results also provide strong arguments that the phase transition at 637 K can be classified as occurring from the normal high-temperature phase with D4 point symmetry into a modulated or incommensurate phase with the same D4 point symmetry. Our results also show that the order-disorder (displacive) contribution to the phase transition mechanism is significantly stronger (weaker) in K2MgWO2(PO4)2 when compared to KTP.