X-ray structural investigations of the ferroelastic phase transition in halogenides of univalent mercury

Crystals of univalent mercury halogenides (UMH) Hg_Cl_2, Hg_2Br_2 and solid solutions on their base are considered in the paper aiming at the detection and complex, in the grand manner, X-ray structural, spectroscopic and thermal physical investigations of the ferroelastic phase transition in UMH and solid solutions. The application of the results obtained to the description of the PT mechanism in frameworks of Landau's theory and the soft water mechanism is also the aim of the paper. As a result the form of a thermodynamic potential has been suggested. Values of its coefficients have been obtained on the base of experimental data. The fact of the transition in the vicinity of a tricritical point (TP) has been substantiated. The symmetry of the low temperature phase of UMH crystals and solid solutions has been determined. The model of the non-own phase transition in the vicinity of TP has been suggested on this base. The representation about fundamental problems of solid-state physics and structural phase transitions has been extended. The paper results may find their field of application in the creation of optical, acoustooptical devicesAvailable from VNTIC / VNTIC - Scientific & Technical Information Centre of RussiaSIGLERURussian Federatio