FERROELASTIC PHASE-TRANSITION IN K3NA(SEO4)2 - BRILLOUIN-SCATTERING STUDIES AND THEORETICAL MODELING

High-resolution Brillouin spectroscopy was used to study the elastic properties of ferroelastic K3Na(SeO4)2. The temperature dependence of nine Brillouin modes of the acoustic phonons propagating in the [100], [010], [001], and [01 1] directions were measured in the temperature range 290-380 K. These results indicate a 3mBAR --> 2/m transition at T(c) = 334 K followed, by approximately 12 K, by a transition involving an unknown mode Q. Mode softening in the c66 and c44 elastic constants results from the main first-order transition while an additional softening in c33 and c44 is due to coupling with Q. A Landau-type free-energy expansion has been postulated and discussed in order to explain the qualitative features of the system.