Crystal and electronic structures and electrical, magnetic, and optical properties of two copper tetrahalide salts of bis(ethylenedithio)-tetrathiafulvalene

We report the crystal and electronic band structures at 295 K and measurements of temperature-dependent magnetic susceptibility, electron paramagnetic resonance, optical reflectivity, conductivity, and thermopower for two copper tetrahalide salts of bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF), (BEDT-TTF${)}_3$ ${\mathrm{CuBr}}_4$ and (BEDT-TTF${)}_3$ ${\mathrm{CuCl}}_2$ ${\mathrm{Br}}_2$. The two salts are isostructural with layers of BEDT-TTF having charges of 0 and +e separated by layers of pseudo-square-planar tetrahalides of copper (II). At ambient pressure these salts show conductivities near 1 S ${\mathrm{cm}}^{\mathrm{-}1}$, and the magnetic properties indicate coupled localized spins present on both BEDT-TTF and the ${\mathit{d}}^9$ copper layers. At 60 K, there is a discontinuous drop in susceptibility, a sharpening of the electron paramagnetic resonance linewidth, and an increase in g value that we attribute to the loss of the contribution from the BEDT-TTF sheets. This may be associated with a Jahn-Teller distortion of the planar copper complexes. Below the transition temperature the susceptibility can be fitted to a quadratic layer antiferromagnet with J=15 K (${\mathrm{CuBr}}_4$) and 8 K (${\mathrm{CuCl}}_2$ ${\mathrm{Br}}_2$) and one spin per formula unit. Under pressure there is a very rapid increase in conductivity, to 500 S/cm at 24 kbar, the largest increase of conductivity under pressure in any molecular solid yet studied. There is a sharp transition from metal to insulator at temperatures rising to 111 K near 5 kbar, and falling to 80 K at 20 kbar. We consider that these salts are nearly metallic at ambient pressure, with strongly enhanced susceptibilities, but are brought to a fully metallic state under pressure as a result of increased intermolecular contacts.