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From Magnets to Metals: The Response of Tetragonal Bisdiselenazolyl Radicals to Pressure

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journal contribution
posted on 20.04.2011, 00:00 by Alicea A. Leitch, Kristina Lekin, Stephen M. Winter, Laura E. Downie, Hideki Tsuruda, John S. Tse, Masaki Mito, Serge Desgreniers, Paul A. Dube, Sijia Zhang, Qingqing Liu, Changqing Jin, Yasuo Ohishi, Richard T. Oakley
The bromo-substituted bisdiselenazolyl radical 4b (R1 = Et, R2 = Br) is isostructural with the corresponding chloro-derivative 4a (R1 = Et, R2 = Cl), both belonging to the tetragonal space group P4̅21m 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 Eact (0.23 eV) than 4a (0.19 eV). Like 4a, radical 4b behaves as a bulk ferromagnet with an ordering temperature of TC = 17.5 K. The coercive field Hc (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 TC for both compounds, that of 4b reaching a maximum value of 24 K; further compression to 5 GPa leads to a decrease in TC and loss of magnetization. Variable temperature and pressure conductivity measurements indicate a decrease in Eact 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).