Local structure anomaly with the charge ordering transition of 1T−TaS2

The quasi-two-dimensional $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ displays a unique two-step charge density wave (CDW) transition, from incommensurate (ICDW) to nearly commensurate (NCDW), and from NCDW to commensurate (CCDW), which is reflected in the stepwise resistivity behavior. In this work, we show that the hysteresis observed in the resistivity across the NCDW-to-CCDW transition is coupled to a local structure anomaly, evident from the pair density function analysis of neutron and x-ray diffraction data. We find that upon cycling the system from high to low temperatures (through the NCDW-CCDW transition) and collecting data on warming, local distortions in the in-star Ta and out-of-plane S atoms become evident, disrupting the trigonal symmetry of the star of David structures, without breaking the lattice periodicity, and preserving the underlying $\mathrm{P}\overline{3}$ symmetry. When the system is warmed up from the NCDW to the ICDW state, the local distortions are absent and the stars are symmetric, indicating that it is the thermal cycling through the NCDW-CCDW that locks in the distortions. Furthermore, we verify the temperature dependence of the two types of stacking along the $c$ axis: at low temperatures, the layer stacking exhibits a $13{\mathrm{c}}_{o}$ order that disappears upon warming through the NCDW transition, while across the NCDW and ICDW phases, the layer stacking is $3{\mathrm{c}}_{o}$. The $3{\mathrm{c}}_{o}$ order gradually disappears in the ICDW state.