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 V͓TCNE͔ x ͑x Ϸ 2͒ 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± 0.25 nitrogen atoms at a room-temperature average distance of 2.084͑5͒Å. 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 V and TCNE, with an effective local force constant of k = 87 N / m. This strong bonding leads to strong nearest neighbor coupling, which for the extended structure of V͓TCNE͔ x with six 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 V͓TCNE͔ x prepared as a powder from CH 2 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 K associated with samples made by both the CVD and CH 2 Cl 2 solution methods.

Density functional study of the magnetic coupling inV(TCNE)2

“…Nevertheless its local coordination environment has been determined from extended x-ray absorption fine structure ͑EXAFS͒ measurements. 14 These data indicate V-N sixfold coordination with average V-N distance of 2.076͑4͒ Å ͑10 K͒ in a slightly distorted octahedral ge-ometry. Further insight into the structure of V͑TCNE͒ 2 can be provided by x-ray-determined structural features of ordered M-TCNE analogs such as ͓MnTPP͔ + ͓TCNE͔ ·− ͑TPP = meso-tetraphenylporphinato͒ ͑Refs.…”

“…These constraints include ͑i͒ the observed stoichiometry V͑TCNE͒ 2 , 3,8,10 ͑ii͒ the suggested octahedral coordination for the metal ion, 14 ͑iii͒ the presence of uncoordinated cyano groups, 7,10,31 and ͑iv͒ the dimensionality of the material forming a periodic crystalline network of V atoms and TCNE molecules. The resulting structure consists of a rectangular planar array of V atoms and TCNE molecules which form bidimensional planes linked through parallel chains of trans--bonded TCNEs.…”

“…12,13 Studies based on x-ray absorption near-edge spectroscopy ͑XANES͒ and XPS measurements have suggested that the oxidation state of V is near +2. 7,14 From saturation magnetization and x-ray magnetic circular dichroism ͑XMCD͒ results, it is found that V 2+ ions are in the high spin state ͑S =3/ 2͒ and that ͓TCNE͔ ·− has net spin S =1/ 2 with one unpaired electron in the ‫ء‬ orbital. 7,10 The coupling between the 3/2 spin on the V ion and the 1/2 spin on each ͓TCNE͔ ·− is proposed to be antiparallel, yielding a saturation moment of ϳ1 B per formula unit.…”

The unoccupied electronic structure of the semi-conducting room temperature molecular magnet V(TCNE)2

“…From previous XPS and XAS studies the vanadium in V(TCNE) x is assigned to be in the V 2+ oxidation state [17,18]. In contrast to the C(1s) and N(1s) core levels, the V(2p) core level remains unchanged upon Na-doping, no energy shifts, new peaks or changes in intensity are visible.…”

“…Development of chemical vapor deposition (CVD) fabrication techniques has however improved stability and reproducibility of the material [13][14][15][16][17]. An extended x-ray absorption fine structure (EXAFS) study has probed the local structure of the compound, showing that the vanadium ion coordinates to about six nitrogen atoms, most likely in a slightly distorted octahedral environment, each with an average distance in room temperature of 2.084(5) Å [18]. The small variation of V-N bond lengths in the local structure is also consistent with strong bonding, as required for magnetic ordering at relatively high temperatures.…”