Commensurate-incommensurate transition of charge density waves in o-TaS₃

We report a synchrotron X-ray study of charge density waves (CDWs) in an o-TaS₃ crystal. CDW of o-TaS₃ has been known to undergo a commensurate-incommensurate transition at 100 K, below which the wavevector locks in with the pristine lattice. However, the details of the commensurate-incommensurate transition remain open problems, the example being whether the discommensuration exists or not in the neighborhood of the transition. We exploited the beamline BL02B1 of SPring-8. A two-dimensional detector was placed at a distance of 1300 mm from the sample, providing an angle resolution of 0.0076°. Temperature dependence of the Bragg peak (0 0 2) and satellite peak (1 - 1 2) + q was measured from 7 to 180 K. We found that a new phase in the temperature range of 130-50 K, where two independent CDWs coexist. These waves are incommensurate and commensurate CDWs with longitudinal wave vectors q_c = 0.252 c^* and 0.250 c^*, respectively. By lowering the temperature, intensity of the incommensurate CDW was decreased, while that of the commensurate CDW was increased. Below 50 K, the incommensurate CDW was completely diminished. Based on the concept of discommensuration, we suggested the dislocation configuration from the intensity of the two CDWs.