We report ultrasonic and thermal expansion measurements on quasi-one-dimensional (TaSe 4 ) 2 I. The elastic constant C 44 exhibits strong anomalies over a wide temperature range around the Peierls transition. Anisotropic behaviour of the slow shear mode C 44 propagating along and perpendicular to the chain direction is observed around T P . Elastic constants, like electrical resistance, are very sensitive to a small Nb content substituted for Ta. Anomalous behaviour of C 44 at low temperatures is reported. We also present measurements of the anisotropic thermal expansion coefficients.

Acoustic velocity of the shear mode C44 of quasi-one-dimensional (TaSe4)2I has been measured at 15 MHz frequency with the ultrasonic technique. It is found that the elastic constant C44 decreases when applied electric fields greater than the threshold ET are applied for charge-density wave (CDW) depinning. The decrease AC441C44 -10-4 observed at 15 MHz for E -10 ET is two orders of magnitude smaller than the 'decrease reported at low frequency (kHz). Present results are compared with the effects of CDW depinning on the elastic properties predicted by several models.

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