Brillouin Scattering Study on Lithium Ammonium Tartrate Monohydrate in Paraelectric Phase

Abstract

The Brillouin scattering studies on lithium ammonium tartrate monohydrate (LAT) have been made in paraelectric phase. The elastic stiffness constants C₅₅^p and C₅₅^E vanish at T₀=94.5 K and T_c=97.5 K, respectively. Some Raman scattering spectroscopies of this crystal have been also made. From the phenomenological theory and the discussion of the elastic dispersion, LAT has been concluded to undergo the proper ferroelastic transition, or, the soft mode in LAT is an acoustic phonon of which energy is determined by C₅₅^p. Measurements of refractive indices at room temperature are reported.