Polar phonons and far-infrared amplitudon in Sr2Nb2O7

Polarized far-infrared reflectivity and transmission measurements of an Sr2 Nb2O7 single crystal were performed in the temperature interval 10-550 K fo r tile E \ \a and E \ \c polarizations. Modes at the centre of the Brilloui n zone were analysed in the subsequent paraelectric-ferroelectric-incommens urate-monoclinic phases and the factor group analysis together with the cor relation of the modes is presented. The ferroelectric A(1) soft mode reveal ed in Raman spectra below T-c = 1613 K was found in the E \ \c polarized in frared spectra. An amplitudon which softens to T-i = 488 K was observed in the incommensurate phase. On cooling the amplitudon anti-crosses a hard exc itation 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 T- 0 = 117 K. Neglecting the small volume of discommensurations, we suggest th e Pb11-C-s(2) space group (Z = 4) for the monoclinic phase below T-0.