Structural transitions of solid substances

A brief overview of the works in this article devoted to the phenomenon of polymorphism. Polymorphic transitions are observed in many substances including crystals of noble gases which are convenient objects for constructing a theory describing the phenomenon. Today crystals are studied in a wide range of pressures and temperatures including in the "exotic" sphere of negative pressures. Besides the use of modern technologies in the process of the experiments gives the posibility to explore various objects even in those conditions when a real experiment is not available.