The physics of high-temperature superconductors as an independent branch of solid state physics originates from the discovery in 1986 by Swiss scientists K. Muller and J. Bednorz of a compound based on copper, lanthanum and barium oxides with a superconducting transition temperature Tc = 28 K [1]. At present, several main families of high-temperature metal-oxide copper-containing superconductors are known, such as the family La2nMc0O4 (M = Sr, Ba or Ca) with 38 K for LawSro, 15Cu04 [2], a family of compounds with the general formula RBa2Cu307.5 (R -one of yttrium or rare-earth element, excluding Ce and Th) with Tc 90 K [3]; A family of superconducting phases whose composition can be represented by the homologous series Bi2 (Ca, Sr) n +] Cun02n + 4 + 5 (n = 1-3) with a maximum Tc of 117 K in the compound Bi2Sr2Ca2Cu30io + 5 [4], etc.
Compounds a2.lMlSi04 families and are RBa2Cu307.5 ferroelectric elastics [5], since they ferroelastic undergo phase transitions from high to low temperature tetragonal orthorhombic phase is accompanied by the appearance of spontaneous deformation twinning and samples in the low-symmetry phase. As representatives of ferroelectric compounds a2.hMlSi04 RBa2Cu3075 and have attractive physical properties for basic research and are the subject of a separate and promising field of modern solid state physics - Physics ferroelastics [6]. It is particularly important that the families and a2.lMlSi04 RBa2Cu307.5 are Ferroelastics-high-temperature superconductors, with the twin structure not only characterizes the ferroelastic properties of the compounds and a2.lMlSi04 RBa2Cu307.§, but can also affect their superconducting properties such as the superconducting transition temperature [7], the critical current density [8], etc. Therefore, you must take them into account in the study of nature ferroelastic features of the superconducting properties of ferroelectric-high-temperature superconductors. However, the fundamental problem of the interrelation between ferroelastic and superconducting properties still belongs to the category of poorly studied and debatable ones.
Another fundamental problem is connected with the search for and the establishment of a connection between structural instability and the appearance of high-temperature superconductivity. This problem occurs to the discovery of high temperature superconductors, when it has been experimentally established [9] that superconductors with structure A-15 transition into the superconducting state is preceded by the development of structural instability, often leading to a structural phase transition. This structural instability indicates the existence of a strong electron-phonon coupling and physically justifies the search for new superconducting compounds among structurally unstable substances.
Urgency studies ferroelastic properties and structural instability of high temperature superconductors is defined as a unique opportunity to "control" the superconducting parameters controlled by changing the state of the twinning structure of ferroelectric-temperature superconductors under the influence of mechanical stress and structural instability detection necessity of high temperature superconductors, enabling the occurrence of superconductivity.
This work was carried out under the programs of basic research conducted at the Department of FTT Voronezh State Technical University from 1987 to 2001 in priority areas of science and technology. The main results were obtained during the implementation of the state budget issue No. GB. 23, 2001. "Synthesis, structure and physical properties of promising materials for electronics," and studies supported the competitive center Fundamental Natural at St. Petersburg State University (Grant No. 95-0-7.3-31 «Structural instability in high temperature superconductors") and the Russian Fund of Fundamental Research (grant No. 98-02-16055 "Influence of defects on the properties of ferroelectrics of different classes").