SIGN AND AMPLITUDE OF COMMENSURATE CHARGE-DENSITY WAVES IN 1T-TASE2

The three inequivalent Ta sites in the basis of 1T-TaSe2 predicted by the n(A)Ta:n(B)Ta:n(C)Ta = 1:6:6 charge-density-wave (CDW) models have been observed at 4.2 K in an unstrained single crystal with use of high-resolution Ta-181 Mossbauer effect. The data show that site A has only slightly higher isomer shift (IS) than site B and is different from site C, thereby determining the sign of the CDW. The IS results suggest that a simple sinusoidal model is insufficient to explain the data; harmonics have to be taken in consideration. The CDW amplitudes of the three sites are +0.48(5), +0.44(5), and -0.52(10) electron relative to Ta4+. The results are understood on the basis of a 13-Ta-atom Star-of-David-shaped-cluster model including the effect of three-dimensional ordering of the CDW.