Local structural order in the disordered vanadium tetracyanoethylene room-temperature molecule-based magnet

Abstract

We determined the vanadium oxidation state and local coordination environment in disordered samples of magnetic $\mathrm{V}{[\mathrm{TCNE}]}_{x}(x\ensuremath{\approx}2)$ prepared by chemical vapor deposition (CVD). Systematic studies of the x-ray absorption near-edge structure (XANES) in this material and reference compounds show that V ions have a valence state near $2+$. Extended x-ray absorption fine structure (EXAFS) analysis shows that vanadium ions are coordinated by $6.04\ifmmode\pm\else\textpm\fi{}0.25$ nitrogen atoms at a room-temperature average distance of $2.084(5)\mathrm{\AA{}}$. The local environment is well defined with a distribution of V-N bond lengths comparable to that commonly found in ordered compounds. This distribution is mostly vibrational in origin, with static contributions being at least four fold smaller. The small disorder in V-N distances is a consequence of strong binding between $\mathrm{V}$ and TCNE, with an effective local force constant of $k=87\phantom{\rule{0.3em}{0ex}}\mathrm{N}∕\mathrm{m}$. This strong bonding leads to strong nearest neighbor coupling, which for the extended structure of $\mathrm{V}{[\mathrm{TCNE}]}_{x}$ with six $\mathrm{N}$ nearest neighbors results in magnetic ordering above room temperature. The strong V-N bonding explains in part the insoluble nature of this compared to other molecule-based magnets. The room-temperature XANES and EXAFS results for the CVD-prepared samples are compared to those for $\mathrm{V}{[\mathrm{TCNE}]}_{x}$ prepared as a powder from ${\mathrm{CH}}_{2}{\mathrm{Cl}}_{2}$ solvent, which has a similar magnetic ordering temperature but a magnetization that is more strongly temperature dependent. This comparison suggests that coordination of 6 nitrogens around each V(II) with little variation in the V-N distances is important for achieving the high magnetic ordering temperature of $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ associated with samples made by both the CVD and ${\mathrm{CH}}_{2}{\mathrm{Cl}}_{2}$ solution methods.