Electron microscopy of phase transformations in 1T-${\mathrm{TaS}}_2$

Phase transformations in 1T-${\mathrm{TaS}}_2$, both on cooling from the incommensurate (IC) phase and on warming from the commensurate (C) phase are investigated by transmission electron microscopy. The transformations are strongly hysteretic, and so proper care is required in the temperature cycle of observation in order to obtain consistent results. Our results revealed that, in the intermediate region between the IC phase and the C phase [or the noncommensurate (NC) phase], the character of the transformation is different on cooling from the IC phase and on warming. On cooling from the IC phase, the structure of the NC phase essentially remains incommensurate in the basal plane and only the rotation of modulation direction occurs. However, on warming from the C phase, a discommensuration structure composed of small domains with the structure of the C phase appears. These domains are separated by a discommensuration network whose density changes with temperature. As a result, both the rotation of diffraction spots around the c axis and the decrease of the averaged period with increasing temperature occur by this mechanism. High-resolution images for both the C phase and the NC phase are taken in a direction parallel to the c axis as well as perpendicular to the c axis. Details of the change in the stacking order during the transformation process are thus revealed.