Diffraction and imaging studies of the charge-density-wave-modulated phases of 1T-TaS2

New results are presented on 1T-TaS 2 . They were obtained by combining convergent-beam electron diffraction with high-quality satellite dark-field imaging. In the nearly commensurate 1T 2 state linear defects were analysed as displacement faults and the fault planes and vectors were determined. The visibility of these faults was found to be strongly temperature dependent. Temperature-dependent sequences were obtained in chosen areas to follow the dynamic changes which occurred on cooling. Direct dislocation interaction between crystal dislocations and the 1T 1 /1T 2 interface was observed. Radiation damage effects were detected and investigated using a 300 kV electron microscope with variable operating voltage. The first detailed TEM investigation has been carried out on the T phase often obtained on warming the material from its low-temperature locked-in (1T 3 ) state. Detailed comparison of the results obtained has been made with previously reported X-ray results and although there is considerable agreement some differences were detected. A simple Landau theory argument is developed to account for the out-of-plane components of the satellite vectors in the T phase. Some further investigation of the 1T 3 phase was carried out and the results discussed in relation to earlier TEM and X-ray investigations.