The spin-liquid candidate $\kappa \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_2{\mathrm{Cu}}_2(\mathrm{CN}{)}_3$ has been studied by measuring the uniaxial expansion coefficients ${\alpha }_i$, the specific heat, and magnetic susceptibility. Special emphasis was placed on the mysterious anomaly around 6 K—a potential spin-liquid instability. Distinct and strongly anisotropic lattice effects have been observed at 6 K, clearly identifying this feature as a second-order phase transition. Owing to the large anomalies in ${\alpha }_i$, the application of Grüneisen scaling has enabled us to determine the corresponding specific heat contribution and the entropy release. Comparison of the latter with available spin models suggests that spin degrees of freedom alone cannot account for the phase transition. Scenarios involving charge degrees of freedom are discussed.

• Received 3 September 2009

DOI:https://doi.org/10.1103/PhysRevLett.104.016403