Brillouin Scattering Study of Ferroelectric SrTi( 18 O X 16 O 1− X ) 3

The 90° Brillouin scattering experiment has been performed in two SrTi( 18 O x 16 O 1 m x ) 3 single crystal plates with x = 0.86 and x = 0.53. The c 44 transverse acoustic mode takes maximum intensity at T c (∼ 26 K for x = 0.86 and ∼ 19 K for x = 0.53) and splits into two modes below T c . Moreover, the central peak clearly appears showing maximum intensity around T c . These facts indicate that the crystal symmetry becomes lower than tetragonal one below T c , and that the ferroelectric phase transition really occurs at T c .