Microwave Measurements of the Thermal Expansion of Organic Metals

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Abstract
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 αb are measured along the organic chains in charge transfer complexes (TTF–TCNQ, HMTTF–TCNQ) and ion radical salts (TMTTF2–I, TMTSeF2–ClO4, TTT2–I3 and TSeT2–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 TMTSeF2–ClO4). These metals are found to exhibit strong anharmonic effects leading to a T2 law for the electrical resistivity (Weger's model). On the contrary metals with TTT or TSeT have weak coefficients αb and their electric properties are more similar to those of inorganic metals (their resistivity follows a Bloch‐Grüneisen law with ϱ ∝ T).