Growth and Inelastic Light Scattering Studies on Sr2Nb2O7 Single Crystal

A crack-free and high structural quality Sr₂Nb₂O₇ single crystals are grown by the optical float zone method using optimized growth parameters. The Laue pattern confirms the single crystalline nature of the grown crystal. Temperature-dependent Raman and Brillouin light scattering studies reveal a significant shift in phonon modes across normal to incommensurate phase transition (T_n-in) which occurs ≈488 K. In the temperature range from 900 down to 500 K, two optical phonon modes ≈63 (B1 mode) and 54 cm⁻¹ (A1 mode) are observed. The frequency of A1 mode strongly decreases with an increase in temperature above the T_n-in while the frequency of this mode almost remains constant below the T_n-in. In contrast, the frequency of B1 phonon mode is found to increase with temperature in the range of 500–900 K but it does not display a significant shift below the phase transition temperature. In addition, in the in-commensurate phase (T< 488 K), a new optical phonon mode (M1) at ≈35 cm⁻¹ also begins to appear and exhibits strong stiffening behavior with an increase in temperature in the range of 300–488 K. Moreover, the anomalous behavior of the acoustic phonon across T_n-in are further probed using Brilliouin scattering. Longitudinal acoustic phonon mode at 41 GHz exhibits a strong change in slope near T_n-in. In addition, the transverse acoustic modes at 28.6 and 22.4 GHz also exhibit strong anomalies with minimum frequency near T_n-in. The inelastic light scattering studies provide valuable information on the phase transition.