  In quasi-two-dimensional 1T-TaS2, a charge density wave (CDW) prototype, the
transition occurs in two steps, from incommensurate (ICDW) and nearly
commensurate (NCCDW) and from NCDW to commensurate (CCDW), locked in the
resistivity step-wise behavior. The hysteresis observed in the resistivity
across the NCDW-to-CCDW transition has roots to a structural hysteresis, where
local distortions of the sqrt(13)a x sqrt(13)a superstructure are revealed by
neutron and X-ray diffraction. The structural hysteresis is due to faulty stars
of David (SODs) because of Ta displacements away from the perfect trigonal
geometry as well as out of plane S distortions. Furthermore, the superstructure
exhibits a 3co layer stacking order that weakens on warming and fully
disappears in the ICDW state.
