Displacement Parameter, Soft-Mode Frequency, and Fluctuations in Quartz Below Its α−β Phase Transition

The displacement parameter in α-quartz consists of a rotation of the SiO₄ tetrahedra around their two-fold symmetry axis. Its magnitude is given by x₀(ω,T)-x₀(ω,Tc)=A(ω)×(Tc-T)^β, with β=0.34±0.02 as determined independently by x rays, ultrasound, and Raman scattering.