Lattice dynamics and high-pressure Raman scattering studies of ferroelectric K2MgWO2(PO4)2

K(2)MgWO(2)(PO(4))(2) ferroelectric single crystal, derived from KTiOPO(4) (KTP) by replacement of two Ti(4+) ions with Mg(2+) and W(6+) cations, was investigated at ambient pressure by micro-Raman scattering and infrared spectroscopies with a focus on polarization properties of vibrational modes. These results were analyzed based on classical lattice dynamics calculations which allowed to propose the normal-mode symmetries and assignments. In addition to the ambient pressure studies, high-pressure Raman scattering studies were performed. These studies showed the onset of a reversible first-order phase transition near 2.0 GPa which is associated with marked softening of the low wave number mode corresponding to motions of the K(+) ions. The structural changes at the phase transition are relatively weak and either the crystal exhibits transformation from the ambient pressure P1 to a high-pressure monoclinic structure or there is no symmetry change at the phase transition, i.e., this transition is isosymmetric.