Polar phonons and far-infrared amplitudon in Sr₂Nb₂O₇

Polarized far-infrared reflectivity and transmission measurements of an Sr₂Nb₂O₇ single crystal were performed in the temperature interval 10-550 K for the E||a and E||c polarizations. Modes at the centre of the Brillouin zone were analysed in the subsequent paraelectric-ferroelectric-incommensurate-monoclinic phases and the factor group analysis together with the correlation of the modes is presented. The ferroelectric A₁ soft mode revealed in Raman spectra below T_c = 1613 K was found in the E||c polarized infrared spectra. An amplitudon which softens to T_i = 488 K was observed in the incommensurate phase. On cooling the amplitudon anti-crosses a hard excitation near 33 cm^-1 located in discommensurations and then, at about 100 K, also the ferroelectric soft mode. No evidence was found for the earlier suggested multiplication of the unit cell below the phase transition at T₀ = 117 K. Neglecting the small volume of discommensurations, we suggest the Pb11-C²_S space group (Z = 4) for the monoclinic phase below T₀.