In contrast to the usual structural "lock-in" incommensurate-commensurate transitions—where the amplitude of the modulation wave continues to increase at $T_c$ but the incommensurate part of the average wave vector of the modulation wave vanishes—in ${\left({\mathrm{C}}_3{\mathrm{H}}_7\mathrm{N}{\mathrm{H}}_3\right)}_2$Mn${\mathrm{Cl}}_4$ the amplitude of the incommensurate modulation wave vanishes outside $T_{c1\mathrm{}}$ and $T_{c2\mathrm{}}$ whereas the wave vector is not critical. This reentrant behavior results from a coupling of the incommensurate order parameter to the temperature-dependent interlayer distance.

• Received 5 April 1982

DOI:https://doi.org/10.1103/PhysRevLett.49.1019