Defects as a cause of continuity of normal-incommensurate phase transitions

Abstract

Almost all normal-incommensurate phase transitions observed experimentally are continuous. We show that there is not any theoretical reason for this general behaviour in perfect crystals. A normal-incommensurate phase transition that is not too far from the mean-field tricritical point should be discontinuous and it is highly improbable that so far reported normalincommensurate phase transitions lie very far from this point. To understand this behaviour we study influence of defects on a hypothetical first-order normal-incommensurate phase transition in a pure material. We have found that this influence is strikingly different from that on other kinds of first-order phase transitions. The change of the discontinuity of the order parameter at the transition is negative and formally diverges within our approximate theory. At the same time the diminishing of the phase transition temperature remains finite. We interpret these results as an indication that at least some of the observed seemingly second-order normal-incommensurate transitions would be first-order transitions in defectless crystals.

Shape memory ferromagnets

“…Defects in the crystal lattice play an important role in the nucleation of martensitic structures [224][225][226]. Detailed analysis of martensitic domain structures has been carried out in Refs [226][227][228].…”

Vacancy-tuned paramagnetic/ferromagnetic martensitic transformation in Mn-poor Mn 1-x CoGe alloys

“…In the MnCoGe crystal structure, the Co and Ge atoms form threedimensional chain networks via covalent bonds owing to the sp and sd orbital hybridization [9,25]. The existence of Co vacancies in the Co-Ge-Co chains inevitably destroys a part of the chemical bonds, which may result in destabilization of the M phase at lower temperature and in weakening of the elastic modulus of the A phase [26,27]. The phase transformation thus occurs at lower temperature and needs a smaller thermodynamic driving force to overcome the energy barrier of M nucleation [28,29].…”