Phase transition and soft phonon modes in around by ultraviolet laser Raman spectroscopy

Raman spectra of (Formula presented) have been successfully obtained at temperatures from (Formula presented) to (Formula presented) by using a new continuous-wave ultraviolet Raman spectroscopic system. The phase transition between cubic and tetragonal phases is due to the collapse of the (Formula presented) mode at the R point of the high-temperature cubic Brillouin zone. Two phonon modes observed to soften (decrease) in frequency approach together and their peak intensities continuously decreased with increasing temperature. They disappeared between (Formula presented) and (Formula presented) The soft modes remain underdamped, the damping constant of soft modes increased with temperature toward phase-transition temperature satisfying the universal scaling law.