Commensuration and discommensuration in 2H-TaSe2

Changes of satellite dark-field images of the incommensurate phase in 2H-${\mathrm{TaSe}}_2$ upon cooling are examined closely in order to investigate the behavior of the incommensurate-commensurate transition of this material. Moirélike fringes observed in the higher temperature range of the phase, which were confirmed to be due mostly to the interference of the first- and second-order diffraction beams from the incommensurate structure in our earlier, high-resolution study, are found to change, starting around 92 K, into the domain images of the commensurate structure which are due mostly to a dark-field diffraction contrast. The change from ‘‘interference fringes’’ to ‘‘diffraction contrast’’ is associated with the change of the modulation mode of the incommensurate phase from the ‘‘incommensurate modulation of the normal structure’’ in the upper temperature range to the ‘‘incommensurate modulation of the commensurate structure.’’ The upper temperature range above this transition corresponds to the hexagonal, triply incommensurate state while the lower temperature region is the transition region to the commensurate state.