Abstract
The 49-K phase transition in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) is interpreted in terms of a phason instability of the charge-density-wave ground state. The order parameter couples linearly to an external strain, and the phase transition can be suppressed by clamping the crystal. Recent neutron scattering experiments show indications of this effect. Renormalization-group calculations yield pure classical critical behavior at this phase transition, and XY exponents for the 54-K transition.