Microwave Measurements of the Thermal Expansion of Organic Metals

The thermal expansion of organic metals is measured by the Buravov‐Shchegolev method (microwave cavity at 9.5 GHz). A relation is found between the frequency shift of the cavity and the length of the sample, independent of the skin depth in the sample. The linear thermal expansion coefficients α b are measured along the organic chains in charge transfer complexes (TTF–TCNQ, HMTTF–TCNQ) and ion radical salts (TMTTF 2 –I, TMTSeF 2 –ClO 4 , TTT 2 –I 3 and TSeT 2 –I). Organic metals with TTF and its selenium analog TSeF show large coefficients for bi‐chain compounds (like TTF–TCNQ) as well as for monochain compounds (like TMTSeF 2 –ClO 4 ). These metals are found to exhibit strong anharmonic effects leading to a T 2 law for the electrical resistivity (Weger's model). On the contrary metals with TTT or TSeT have weak coefficients α b and their electric properties are more similar to those of inorganic metals (their resistivity follows a Bloch‐Grüneisen law with ϱ ∝ T ).