Polar nanoregions of Ca0.28Ba0.72Nb2O6 probed by second harmonic generation and Raman spectroscopy at high pressure

摘要

The pressure-induced ferroelectric phase transition of calcium barium niobate Ca0.28Ba0.72Nb2O6 is studied by second harmonic generation (SHG) and Raman spectroscopy. We observe the ferroelectric to paraelectric phase transition at Pc = 4 GPa and evidence for two additional local changes at 8.5 and 11.5 GPa that are interpreted as the characteristic intermediate P* and the Burns pressure PB of a relaxor phase. The pressure dependence of the SHG intensity has a Landau critical exponent of γ = 1.3 between 4 and 8.5 GPa, indicating the presence of static polar nanoregions. At higher pressures, from 8.5 to 11.5 GPa, these nanoregions become dynamic, with γ = 2.4, and disappear at 11.5 GPa. The polar nanoregions appear when only one of the two non-equivalent octahedral positions of Nb is centrosymmetric. A Landau exponent of γ = 1 above 11.5 GPa together with a change in the compression mechanism indicates that the system completes a phase transformation to a centrosymmetric space group and the polar nanoregions vanish.