Anomalous thermal expansion of the organic superconductor κ-(BEDT-TTF) 2Cu[N(CN) 2]Br

• Published in: Physica. B, Condensed matter Condensed matter, 1993-09, Vol.191 (3), p.274-280
• Main Authors: Kund, M., Müller, H., Biberacher, W., Andres, K., Saito, G.
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Heat capacities of solids; Organic superconductors; Physics; Superconducting materials (excluding high-tc compounds); Superconductivity; Thermal expansion; thermomechanical effects and density; Thermal properties of condensed matter; Thermal properties of crystalline solids
• ISSN: 0921-4526; 1873-2135
• DOI: 10.1016/0921-4526(93)90085-K

High-resolution thermal expansion measurements on single crystals of the organic superconductor κ-(BEDT-TTF) 2Cu[N(CN) 2]Br were performed along all three principal directions in the temperature range from 4 to 300 K by means of capacitive dilatometry. Large anisotropies were observed which originate in the layer-type structure and the orthorhombic symmetry of this compound. At 80 K an anomalous and pronounced increase of the orthorhombic distortion of the conducting a-c plane is seen which correlates, together with a small observed thermal hysteresis, with an anomaly in the electrical resistivity. Here, the volume expansion coefficient β shows two subsequent jumps of approximately the same size ( Δβ = (22±4) × 10 -6 1/ K), but of opposite sign. We assume that a phase transition occurs at this temperature. A possible explanation for this is the interaction of donor libration modes with the polymeric anion through their terminal ethylenes via hydrogen bonds, leading in effect to an ethylene order-disorder transition.