Brillouin-scattering study of the broad doublet in isotopically exchanged SrTiO3

A comparative Brillouin-scattering study of the broad doublet and acoustic phonons in isotopically substituted SrTi( 1 8 O 1 1 6 O) 3 and "pure" SrTi 1 6 O 3 crystals is presented. The acoustic-phonon frequency is found todecrease as a result of the substitution, but the doublet spectra above 20 K show no obvious differences. The results are examined in connection with the second-sound interpretation of the doublet. Influence of the phase transition on the doublet spectra is discussed.