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

Almost all normal-incommensurate phase transitions observed experimentally are continuous. We show that there is not any theoretical reason for this g eneral behavior in perfect crystals. A normal-incommensurate phase transiti on that is not too far from the mean-field tricritical point should be disc ontinuous and it is highly improbable that normal-incommensurate phase tran sitions reported up to now lie very far from this point. To understand this behavior we study the influence of defects on a hypothetical first-order n ormal-incommensurate phase transition in a pure material. We have found tha t this influence is strikingly different from that of other kinds of first- order phase transitions. The change of the discontinuity of the order param eter at the transition is negative and formally diverges within our approxi mate theory. At the same time the diminishing of the phase-transition tempe rature remains finite. We interpret these results as an indication that at least some of the observed seemingly second-order normal-incommensurate tra nsitions would be first-order transitions in defectless crystals.