Optical transmittance and Raman scattering studies of (K, Na)(Nb, Ta)O3 single crystal

• Optical properties for KNN-based single crystal was studied for the first time.
• Temperature dependence of band gap energies and phonon energy was calculated.
• Structure of KNNT single crystal was characterized by Raman spectroscopy.

The optical properties of KNN-based single crystals were investigated firstly in this work. The optical transmittance of (K0.56Na0.44)(Nb0.77Ta0.23)O3 (KNNT) single crystal was measured at different temperatures. The band gap energies of direct transition Egd, indirect transition Egi, and the phonon energies Ep involved in the indirect transition were calculated based on the optical transmittance spectra. It was found that Egd and Egi show abrupt change at orthorhombic–tetragonal phase transition temperature (TO–T) at 119 °C due to the change of crystal lattice, while Ep varies rapidly at around 60 °C. In order to further understand the phase transition behavior and the assistant phonons in the indirect transition, the KNNT single crystal was characterized by using Raman spectroscopy. It is clarified that the υ1 vibration mode is involved in the indirect transition below 60 °C and υ5 mode is involved above 60 °C. The Raman scattering of KNNT crystal shows distinct behavior in different phases. The phase transitions at TO–T = 119°C and TC = 291 °C can be confirmed respectively as orthorhombic–tetragonal and tetragonal–cubic by the discontinuities of the Raman peaks’ intensities and positions.