Polar phonons and far-infrared amplitudon in Sr₂Nb₂O₇

Abstract

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 Tc = 1613 K was found in the E||c polarized infrared spectra. An amplitudon which softens to Ti = 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-C2S space group (Z = 4) for the monoclinic phase below T0.

Electronic and vibrational properties of the low-dimensional perovskitesSr1−yLayNbO3.5−Kuntscher1, Schuppler2, Haas3 et al. 2004Phys. Rev. B152190View full textAdd to dashboardCite

“…16 It was also observed in recent optical measurements and related to NbO 6 octahedra tiltings. 17 In agreement with Ref. 17, this soft mode is temperature independent between 300 and 100 K and shifts to higher frequencies below 100 K. The precise temperature dependence is, however, difficult to determine due to the overlap with another mode which appears around 50 cm −1 at ≈100 K during cooling down.…”

“…17, and at 5 K there are altogether 24 and 51 modes resolved for E a and E b, respectively. The number of observed modes at RT and 5 K is lower than expected from the factor group analysis;…”

Normal-incommensurate phase transition in (Sr1−xBax)2Nb2O7ferroelectric single crystals

“…Regarding the IC phase, a decrease in d and appearance of higher harmonics usually results in the deformation of the modulation wave to a domain-like picture in which IC regions are reduced to periodic domain walls between the quasi-commensurate domains [9]. For pure crystals (xZ0.0), volume fraction of the IC regions might be very small due to appearance of the domain pattern and the crystal may be considered almost as a commensurate one.…”

Raman scattering study on ferroelectric (Ba0.32Sr0.68)2Nb2O7 ceramics

“…It was known that the octahedra distort and tilt on cooling and SN undergoes three phase transitions at 1342, 215 and −156 • C, Cmcm → Cmc2 1 → Pbn2 1 → Pb11 [6,7]. The dynamics of phase transitions has been studied mainly by means of Raman scattering [8][9][10][11], ESR [12], and polarized far-infrared reflectivity and transmission measurements [13]. The ferroelectric soft mode was found in b(cc)b scattering geometry below T c = 1342 • C. Below 215 • C another soft mode appears showing the displacive character of this normal to incommensurate phase transition.…”