Acoustic behavior of the incommensurate state of Rb2Zncl4 by brillouin scattering?

Temperature dependence of elastic stiffness constant C₁₁ was studied from 60 to -100°C by Brillouin scattering and refractive index measurements. It is found that C₁₁ decreases with decreasing temperature over the incommensurate phase and its temperature gradient becomes more or less flat at several degrees above the incommensurate-commensurate transition point T_c, where the dis-commensurate structure appears. This unusual temperature dependence of C₁₁ seems to be attributable to the incommensurately modulated structure of Rb₂ZnC1₄.