Persistence up to $T_i$ of ferroelectric-phase-allowed modes in the incommensurate phase of ${\mathrm{K}}_2$${\mathrm{SeO}}_4$

Internal modes allowed by symmetry in the ferroelectric phase (T<$T_c$) but forbidden in the high-temperature paraelectric phase (T>$T_i$) of ${\mathrm{K}}_2$ ${\mathrm{SeO}}_4$ persist in the incommensurate phase, as observed by infrared and Raman spectroscopy. The observed scattering intensity from the modes in the incommensurate phase decreases on heating, reaching roughly zero at $T_i$. No significant discontinuity is observed in the behavior of the modes on either side of $T_c$, and no new additional mode is observed upon cooling just below $T_c$. The compatibility of the infrared and Raman spectra with the symmetry commonly accepted for the paraelectric phase is also discussed.