Temperature dependence of the Raman dispersion of Sr2Nb2O7: Influence of an electric field during the synthesis

We report on the preparation and structural characterization of powder samples of the perovskite‐slab layered polycrystalline Sr2Nb2O7 (hereafter named SNO) ferroelectric compound. Comparison is performed on samples grown without (SNO) and with an applied electric field of 3.34 kV/cm (SNOE). The Raman effect produced by these samples as a function of temperature, from 27°C (room temperature) to 400°C is presented and discussed. Significant differences are observed and discussed among SNO and SNOE. The electric field promotes the growth of platelets, preferentially oriented along the (0b0) planes. The temperature coefficients of 35 phonon wavenumbers were determined for both samples. A majority of the phonons exhibit monotonic wavenumber softening's with increasing temperatures, but a significant number of them show discontinuities on the wavenumber temperature slopes at two temperatures 215°C and 307 ± 5°C. The first temperature corresponds to the well‐known incommensurate phase to the ferroelectric phase transition (Phases III to II). The second temperature is indicative of an unreported phase transition for both the SNO and SNOE samples. For the sample SNOE, a phonon f7 (at 121 cm−1 at room temperature) displays a behavior that may be indicative of the existence of another ordering parameter that vanishes at 488 ± 5°C in the SNOE sample, induced by the electric field applied during the growth. The phonon broadenings with temperature are explained in terms of the Klemens model, which considers that the broadenings are due to the thermal expansion of the lattice, with a substantial contribution in magnitude from anharmonic phonon–phonon interactions. Two phase transitions of Sr2Nb2O7 perovskites were identified by Raman spectroscopy performing measurements from 27 to 450°C. The behavior with temperature of the phonon wavenumber reveals two phase transitions at 215 and 307°C. The former corresponds to the incommensurate phase to ferroelectric phase transition and the later to an additional unreported phase transition.