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Crystal Structure, Magnetic and Charge Transport Properties of a Neutral Radical Conductor

A. Mailman1, S.M. Winter1, X. Yu1, C.M. Robertson2,  W. Yong3, J.S. Tse4, R.A. Secco3, Z. Liu5, P.A. Dube6, J.A. Howard2, R.T. Oakley1.

Principal Investigator

Richard T. Oakley
Department of Chemistry
University of Waterloo

(1) University of Waterloo (2) University of Durham (3) University of Western Ontario (4) University of Saskatchewan (5) Carnegie Institution of Washington (6) McMaster University

Introduction

In principle the unpaired electrons provided by a solid state  array of neutral radicals should be capable of serving as charge carriers, giving rise to a half-filled (f = ½) energy band and a metallic ground state.1 The problem with the idea lies in the fact that there is an intrinsically high Coulombic barrier (U) to charge transfer in such a system.2 Overcoming the onsite charge repulsion requires radicals with extensive spin delocalization to lower U, and strong intermolecular resonance interactions to increase the electronic bandwidth W. When W > U, the Mott-Hubbard gap ΔE = U - W should vanish and a metallic state prevail.

Results

In the original work on which this summary is based, we described the preparation, spectroscopic and solid state characterization of the heterocyclic thiazyl radical FBBO. The highly delocalized spin distribution in this radical, manifest in its EPR spectrum (Figure 1), militates in favor of a low onsite Coulomb potential U. While charge correlation still dominates the low temperature transport properties of this material, a metallic state can be generated at a pressure of 3 GPa.

Figure 1. EPR spectrum of FBBO (in DCM). Reprinted with permission, J. Am. Chem. Soc. 134, 9886-9889 (2012). Copyright, 2012, American Chemical Society.

The crystal structure of FBBO, space group Cmc21 consists of ribbon-like arrays of radicals running parallel to the b-axis. Consecutive layers of these ribbons create a “brick wall” packing pattern (Figure 1) with an interplanar separation of a/2, or 3.151(1) Å.

Figure 2.  “Brick wall” π-stacking of molecular ribbons of FBBO radicals. Reprinted with permission, J. Am. Chem. Soc. 134, 9886-9889 (2012). Copyright 2012, American Chemical Society.

The results of variable temperature magnetic susceptibility (χ) measurements on FBBO, obtained using the SQUID facility at the BIMR, are shown  in Figure 3. The cooling curve plot of χT versus T (Figure 3a) indicates the effects of weak antiferromagnetic

(AFM) exchange coupling, but on cooling below 20 K there is a sudden increase in χT near 15 K. Subsequent zero-field cooled/field cooled (ZFC-FC) runs (Figure 3b) revealed a sharp bifurcation at 13 K which we interpreted in terms of a phase transition to a spin-canted AFM ordered state with a Néel temperature TN = 13 K. Field independent magnetization experiments (Figure 3c) supported this interpretation, and extrapolation of M to T = 0 K allowed an estimation of the spin canting angle φ = 0.11º. Measurements of M as a function of applied field were also performed, and these indicated a weak, quasi-linear M versus H dependence out to 5 T. Cycling of the field revealed a hysteretic response in M(H), giving rise (at 2 K) to a coercive field Hc = 290 Oe (Figure 3d), a surprisingly large value for a light heteroatom organic magnet.3

Figure 3. Magnetic measurements for FBBO. (a) Field-cooled χT versus T plot at H = 1 kOe. (b) ZFC-FC plot of χT versus T at H = 100 Oe. (c) Decay in spontaneous magnetization M with temperature, and (d) hysteresis in cycling of M versus H measurements at T = 2 K. Reprinted with permission, J. Am. Chem. Soc. 134, 9886-9889 (2012). Copyright, 2012, American Chemical Society.

The results of variable temperature conductivity (σ) measurements on FBBO, also performed at the BIMR, are presented in Figure 4, in the form of a plot of log σ versus 1/T. While the performance of other oxobenzene-bridged materials4 is uniformly superior relative to previously reported radicals, in terms of both enhanced conductivity and lowered thermal activation energy Eact, the value of σ(300 K) = 2 × 10–2 S cm–1 for FBBO is, to our knowledge, the highest ever observed for a neutral f = ½ radical. While its conductivity remains activated, indicative of a Mott insulating ground state, the value of Eact = 0.10 eV for T = 300-150 K constitutes the lowest ever found for a thiazyl radical.

Figure 4. Plot of log s versus 1/T for FBBO (left). Pressure dependence of the conductivity σ(300 K) and thermal activation energy Eact of FBBO (right). Reprinted with permission, J. Am. Chem. Soc. 134, 9886-9889 (2012). Copyright, 2012, American Chemical Society.

We also carried out high pressure variable temperature conductivity measurements on pressed pellets of FBBO over the range T = 298-368 K, using a cubic anvil press, to assess changes in conductivity and thermal activation energy with pressure. The results (Figure 4), recorded at the University of Western Ontario, revealed a steady increase in conductivity with pressurization, with σ(300K) reaching a plateau near 101 S cm-1 at just 3 GPa. Across the same pressure range the activation energy Eact drops sharply, reaching a value of 0 eV near 3 GPa. This finding is consistent with closure of the Mott-Hubbard gap and formation of a metallic state.

Conclusion

The use of bandwidth enhancement alone to improve conductivity in molecular materials that adhere to the f = ½ paradigm has presented more of a challenge, largely because the approach so often leads to dimerization and a consequent loss of charge carriers.

However, the unique sheet-like architecture of FBBO and the resulting interlayer π-overlap affords a high bandwidth, 2D electronic structure that resists spin-pairing. While the transport properties of FBBO indicate that the Mott insulator description still applies at ambient pressure, the insulator-to-metal transition is readily accomplished, with a critical pressure Pc = 3 GPa. In this light, FBBO may represent a benchmark example of a new class of strongly correlated materials based on neutral molecular radicals. To date, only a

limited section of the phase diagram of FBBO has been examined, but already it displays properties common to other classes of materials of current interest.5 That is, it possesses a highly 2D electronic structure, an ordered AFM phase at low T for P < Pc, and a metallic phase at high T for P > Pc. With further exploration of FBBO, as well as related neutral radical materials, there is little doubt that interesting physics will emerge.

Chair to J.S.T., and the EPSRC (UK) for support under Grant No: EP/C536436/1

Acknowledgements

We thank the NSERC (Canada) for partial financial support, a postdoctoral fellowship to C.M.R., and a graduate scholarship to S.M.W. We acknowledge the Government of Canada for a Tier I Canada Research

References

Principal publication:

A. Mailman, S. M. Winter, X. Yu, C. M. Robertson, W. Yong, J. S. Tse, R. A. Secco, Z. Liu, P. A. Dube, J. A. K. Howard and R. T. Oakley. “Crossing the Insulator-to-Metal Barrier with a Thiazyl Radical Conductor.” J. Am. Chem. Soc. 134, 9886-9889 (2012).

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