A ferroelastic phase transition in Hg2(ClxBr1-x)2 crystals

The paper presents the results of an optical study of mixed mercurous halide crystals, Hg2(Cl, Br)2. The Raman spectra (RS) obtained reveal single-, double-, and three-mode behavior of various fundamental vibrations of the Hg2(Cl, Br)2 lattice. A structural phase transition to the ferroelastic state was observed to occur in the mixed crystals throughout the concentration range studied (0 > x > 1). The concentration dependence of the transition temperature, Tc(x), exhibits a nonmonotonic behavior with a clearly pronounced minimum at X = 0.3-0.4. The RS of the Hg2(Cl, Br)2 crystals reveal in the low-temperature phase (T > Tc) additional lines, their flare-up being due to a transfer of the X-point of the paraphase Brillouin zone (BZ) to the BZ center of the ferroelastic phase. In the paraphase (T > Tc), first-order Raman scattering from the acoustic soft mode from the BZ boundary (X-point) was discovered in the low-frequency region of the spectrum. This scattering is induced by long-range static distortions ...