Phase Transitions in Incipient Ferroelectrics of Perovskite Structure With Impurities

摘要

The name perovskite has been dated back to 1839 when Rose, a Russian mineralogist, discovered a new mineral CaTiO_3, which he baptized perovskite after the then Russian Minister of Lands, count L.A.Perovsky. This name has been later used to designate a large ABO_3 perovskite family. A lot of well-known classical ferroelectrics belong to this family. Two members of the family, SrTiO_3 and KTaO_3, have a polar soft mode, but do not exhibit a ferroelectric phase transition similar to ~4He quantum liquid in which a crystalline state has never been realized at normal pressure. This is explained by the stabilization of their paraelectric phase by quantum fluctuations [1,2]. So SrTiO_3 and KTaO_3 are quantum paraelectrics or incipient ferroelectrics. They may be, in a certain sense, classified as marginal systems which are at the limit of their paraelectric phase stability. Small perturbations can destroy the stability and induce a ferroelectric phase. For example, SrTiO_3 becomes a ferroelectric at low temperatures under action of an uniaxial stress of only 1 kbar [3,4] or under application of a static electric fields as low as 1 kV/cm [5-7]. This means that elastic lattice strains about 10~-4 and electric fields of the order of 10~-5 of atomic fields are sufficient to induce a ferroelectric phase. It appeares [8,9] that impurities with concentration around 10~-3 can also destroy the stability of the paraelectric phase and induce a ferroelectric phase. Just the effect of various impurities on phase transitions in the incipient ferroelectrics will be discussed in the present paper. The main attention will be paid to SrTiO_3 but KTaO_3 and a new incipient ferroelectric CaTiO_3 will be also shortly discussed.