From magnets to metals: the response of tetragonal bisdiselenazolyl radicals to pressure.

The bromo-substituted bisdiselenazolyl radical 4b (R(1) = Et, R(2) = Br) is isostructural with the corresponding chloro-derivative 4a (R(1) = Et, R(2) = Cl), both belonging to the tetragonal space group P(4)2(1)m and consisting of slipped π-stack arrays of undimerized radicals. Variable temperature, ambient pressure conductivity measurements indicate a similar room temperature conductivity near 10(-4) S cm(-1) for the two compounds, but 4b displays a slightly higher thermal activation energy E(act) (0.23 eV) than 4a (0.19 eV). Like 4a, radical 4b behaves as a bulk ferromagnet with an ordering temperature of T(C) = 17.5 K. The coercive field H(c) (at 2 K) of 1600 Oe for 4b is, however, significantly greater than that observed for 4a (1370 Oe). High pressure (0-15 GPa) structural studies on both compounds have shown that compression reduces the degree of slippage of the π-stacks, which gives rise to changes in the magnetic and conductive properties of the radicals. Relatively mild loadings (<2 GPa) cause an increase in T(C) for both compounds, that of 4b reaching a maximum value of 24 K; further compression to 5 GPa leads to a decrease in T(C) and loss of magnetization. Variable temperature and pressure conductivity measurements indicate a decrease in E(act) with increasing pressure, with eventual conversion of both compounds from a Mott insulating state to one displaying weakly metallic behavior in the region of 7 GPa (for 4a) and 9 GPa (for 4b).