The crystal distortions commonly observed in the layered transition-metal dichalcogenides have been attributed to a Fermi-surface instability, i.e., a charge-density wave. I find that the temperatures of these distortions correlate with the ionicity of the metal-chalcogen bond. This results suggests that these distortion temperatures can be specified without knowing the details of the Fermi surface.

• Received 7 November 1974

DOI:https://doi.org/10.1103/PhysRevLett.34.520