Evidence for structural and electronic instabilities at intermediate temperatures in κ-(BEDT-TTF)₂X for X=Cu[N(CN)₂]Cl, Cu[N(CN)₂]Br and Cu(NCS)₂: Implications for the phase diagram of these quasi-2D organic superconductors

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We present high-resolution measurements of the coefficient of thermal expansion α(T)=∂ l(T)/∂ T of the quasi-twodimensional (quasi-2D) salts κ-(BEDT-TTF)₂X with X = Cu(NCS)₂, Cu[N(CN)₂]Br and Cu[N(CN)₂]Cl. At intermediate temperatures (B), distinct anomalies reminiscent of second-order phase transitions have been found at T = 38 K and 45 K for the superconducting X = Cu(NCS)₂ and Cu[N(CN)₂]Br salts, respectively. Most interestingly, we find that the signs of the uniaxial pressure coefficients of T are strictly anticorrelated with those of Tc. We propose that T marks the transition to a spin-density-wave (SDW) state forming on minor, quasi-1D parts of the Fermi surface. Our results are compatible with two competing order parameters that form on disjunct portions of the Fermi surface. At elevated temperatures (C), all compounds show α(T) anomalies that can be identified with a kinetic, glass-like transition where, below a characteristic temperature Tg, disorder in the orientational degrees of freedom of the terminal ethylene groups becomes frozen in. We argue that the degree of disorder increases on going from the X = Cu(NCS)₂ to Cu[N(CN)₂]Br and the Cu[N(CN)₂]Cl salt. Our results provide a natural explanation for the unusual time- and cooling-rate dependencies of the ground-state properties in the hydrogenated and deuterated Cu[N(CN)₂]Br salts reported in the literature.