2.1. Fabrication of ferroelectric materials

The ferroelectric single crystals have been grown by a high temperature solution method (flux method) [13, 14]. High-purity powders were selected as starting materials. The raw material powders were stoichiometrically weighed, mixed by milling with zirconia media in the ethanol as a solvent. After drying. The powders were calcined at a certain temperature for hours to form the desired perovskite phase. Details of the fabrication process for the single crystals can be found elsewhere [13].

The bulk ceramics were fabricated by a conventional solid state reaction sintering, using the appropriate amount of reagent grade raw materials [15, 16]. The samples were sintered at different temperature for several hours in air atmosphere, and then remilled for several hours to reduce the particle size for sintering. The calcined powders were added with 8 wt.% polyvinyl alcohol (PVA) as a binder. Before Raman measurements, the ceramics with the diameter of 15 mm and the thickness of 1 mm were rigorously single-side polished and cleaned in pure ethanol with an ultrasonic bath and rinsed several times by deionized water.

#### 2.2. Raman experiment details

Raman scattering experiments were carried out using a Jobin-Yvon LabRAM HR 800 UV micro-Raman spectrometer, excited by 632.8 nm He-Ne laser or 488 nm Ar laser and recorded in the frequency range of 10–1000 cm<sup>1</sup> with a spectral resolution of 0.5 cm<sup>1</sup> . For the different temperature Raman spectra, we choose a 50 microscope with a long working distance of 18 mm. The spectrometer grating can be choosed by 600, 1800 or 2400 grooves/mm grating which is depending on the different excitation wavelength. In order to learn more about the variation trend of vibration modes, all of the experimental spectra were fitted with independent damped harmonic oscillators. The polarized Raman spectra were recorded in backscattering geometry in parallel <x|zx|y > (VH) and perpendicular <x|zz|y > (VV) polarization configurations. Temperature dependent Raman spectra were collected with a THMSE 600 heating/cooling stage (Linkam Scientific Instruments) in the temperature range from 77 to 800 K with a resolution of 0.1 K.
