Published online by Cambridge University Press: 25 February 2011
Young’s modulus E measurements using the vibrating reed technique are reported for salts of the (TMTTF)2X series, where X = ReO4, SbF6, AsF6. All three exhibit softening of the modulus at the structureless transition, with the largest effect in the material with the non–centrosymmetric anion ReO4. The behavior is typical of quadratic coupling to an order parameter plus effects from fluctuations. There is a large stiffening of E below the anion ordering temperature of (TMTTF)2ReO4.