An ultrasonic and thermal expansion study of the quasi-one-dimensional compound (TaSe4)2I

We report ultrasonic and thermal expansion measurements on quasi-one-dimensional (TaSe4)2I. The elastic constant C44 exhibits strong anomalies over a wide temperature range around the Peierls transition. Anisotropic behaviour of the slow shear mode C44 propagating along and perpendicular to the chain direction is observed around TP. Elastic constants, like electrical resistance, are very sensitive to a small Nb content substituted for Ta. Anomalous behaviour of C44 at low temperatures is reported. We also present measurements of the anisotropic thermal expansion coefficients. (author)