Abstract
The low-frequency vibrations of micro- and nanograin ceramics of strontium titanate are investigated by hyper-Raman spectroscopy. The combination of a confocal optical microscope with an ultranarrow Notch filter enabled an accurate spectroscopy on both the Stokes and anti-Stokes side down to about 8 cm−1. A splitting of the soft mode into E-u- and A-2u-symmetry vibrations is clearly observed in the micrograin sample. The peculiar temperature dependence of ω^2_A2u−ω^2_Eu emphasizes a nonlinear coupling between the square of the TiO6 tilting angle and the spontaneous strain c/a − 1. In addition, the very different temperature behavior of the linewidth of the two soft components is assigned to the growth of a disorder anisotropy on cooling, probably on the titanium site.