Phase transition, order parameter, and their manifestation in optical spectra of Hg2Cl2

This paper reports on a high-precision study of the effects induced by the phase transition in Raman spectra of Hg₂Cl₂ crystals. The odd (acoustic and IR-active) phonons, including the soft modes, from the Brillouin zone (BZ) X points of the paraphase of these crystals, which are induced by the phase transition, unit cell doubling, and the X ?? ?? BZ folding, have been investigated both theoretically and experimentally. The temperature dependences of the intensities of these phonons, complemented by a theoretical analysis, have yielded the first information on the behavior of the transition order parameter. The corresponding critical exponents have been derived, which correlate in magnitude with the data obtained from X-ray diffraction measurements and, thus, imply the closeness of the phase transition in these crystals to the tricritical point.