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

TL;DR

This paper challenges the common belief that normal-incommensurate phase transitions are always continuous, showing that defects can induce first-order behavior and that many observed transitions might be inherently discontinuous in perfect crystals.

Contribution

It demonstrates that defects can cause normally continuous phase transitions to appear continuous, suggesting many observed transitions could be fundamentally first-order in defect-free materials.

Findings

Defects significantly influence the order of phase transitions.

Discontinuity of the order parameter can diverge due to defects.

Many observed second-order transitions may be first-order in perfect crystals.

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 normal-incommensurate 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.