Charge density waves and phase transitions in 1T- TaS2

Abstract

The integrated intensities of three Bragg reflections have been measured on an absolute basis between 5.0 and 300 K. The (00 16) varies smoothly over the whole temperature range, with Debye-like behavior above 100 K, with no discontinuities at the non commensurate-commensurate transition 1 T 2↔1 T 3 ( T ⋍ 190 K). The (00 16) is found to be 56% smaller than the expected value without CDW's, a discrepancy that cannot be attributed to extinction. On the contrary, reflections like the (0 4 0) and the (0 5 0) exhibit a temperature dependence entirely dominated by CDW's. Their intensities decrease as temperature is lowered, an opposite behavior to that predicted by standard Debye theory. Discontinuities are observed for the (0 5 0) data in the neighborhood of the hysteresis range associated with the 1 T 2↔1 T 3 transition. It is concluded that Ta atoms have a non-zero CDW amplitude component along the c-axis, which seems to be entirely temperature insensitive. In agreement with results obtained above 300 K, we conclude that all changes in CDW amplitudes at both polymorphic transitions only affect components along the basal planes.