SEARCH FOR INEQUIVALENT LATTICE SITES IN THE CHARGE-DENSITY WAVE DISTORTED PHASES OF 1T-TAS2

A perturbed angular correlation study of the nuclear quadrupole interaction at Ta in the commensurate charge density wave (CDW) state of 1T-TaS2 yields two inequivalent lattice sites with a population ratio of 0.86(9):1. The electric-field gradients (EFG's) for the low- and high-frequency site differ by 11.6(7)% and show different asymmetry parameters of eta =0.12(3) and eta =0.19(2), respectively. Both the EFG difference and the asymmetry allow the identification of sites within the star shaped cluster model of Brouwer and Jellinek (1976). In the quasicommensurate CDW state both the asymmetry parameters and the EFG difference remain constant whereas the population ratio and the EFG distribution vary, especially at high temperature. The disintegration of star-shaped clusters could be responsible for these observations. In the incommensurate CDW state a unique site with a 4.8(3)% EFG distribution is observed.