Elastic properties of molecular spin-crossover solids investigated by synchrotron techniques and by MEMS technology

24-07-2018

Combining nuclear inelastic scattering, high pressure X-ray diffraction and micromechanical measurements, the macroscopic elastic properties of the spin-crossover complex [FeII(HB(tz)3)2] were assessed. The results highlight the potential of this material for applications in microelectromechanical systems (MEMS) technology.

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Spin-crossover (SCO) complexes are a prominent example of smart, multifunctional molecular materials that exhibit a reversible change of their molecular spin state from a low-spin (LS) to a high-spin (HS) electronic configuration under the application of a variety of external stimuli such as temperature or pressure variations, light irradiation, etc. This spin-state conversion is accompanied by a dramatic change of optical, magnetic, electrical and mechanical properties of the material providing scope for various applications [1]. In the solid state, the strong elastic coupling can lead to the emergence of cooperative effects such as first-order phase transitions and associated hysteresis phenomena (bistability) (Figure 1a). This cooperativity has been extensively investigated since the early stages of SCO research and today it is generally agreed that it can be attributed to a combination of short- and long-range interactions, primarily of elastic origin, which arise from the significant volume change of the coordination octahedron upon the SCO (ca. 25% for FeIIN6 coordination cores). A detailed knowledge of the crystal structure and elastic properties of SCO materials is therefore essential for rationalising their cooperativity. It is thus rather surprising that, while numerous structural studies have been reported on SCO compounds, their elastic properties remain largely unknown. The few studies performed so far provided incomplete information and were mainly limited to only one spin state.

Accurate determination of the elastic/mechanical properties of SCO materials is crucial for the fundamental understanding of the SCO phenomenon and for engineering purposes. Indeed, recently it was proposed that the lattice volume change (typically 1–10% in most SCO materials) that accompanies the spin transition can be readily exploited for actuating purposes. Various actuating devices, mainly consisting of bilayer structures where a SCO thin film is coated on a freestanding cantilever, have been constructed and investigated for their actuating properties in response to the spin-state change of the thin film [1-2]. Obviously, the actuating performance of these devices is directly related to the elastic properties (Young’s modulus, Poisson’s ratio …) of the SCO material, which need to be accurately characterised.

Thermal-induced spin transition curves and partial density of vibrational states.

Figure 1. (a) Thermal-induced spin transition curves of [FeII(HB(tz)3)2] (1), FeII(btz)2(NCS)2 (2) and [FeII(pyrazine)Ni(CN)4] (3) (χ stands for the molar magnetic susceptibility) showing the variability of spin transitions. (b) Partial density of vibrational states of [FeII(HB(tz)3)2] in the two spin states extracted from NIS data at ID18. The Young’s modulus is estimated from the low energy part of the spectra.

In this study, three experiments were carried out to investigate the elastic and structural properties of the anisotropic spin-crossover complex [FeII(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl). This neutral compound is promising for application due its possibility to be deposited by thermal evaporation [2]. At beamline ID18, nuclear inelastic scattering was used to investigate the density of vibrational states of the powder (Figure 1b). From the microscopic vibrational properties, the macroscopic mechanical properties such as the Young’s modulus (Y = 10.9 ± 1.0 GPa) were obtained. This experiment was complemented with single-crystal X-ray diffraction under high pressure at beamline ID15B. From this, important information was deduced about the pressure dependence of the lattice volume and the unit cell anisotropy (Figure 2a). In particular, the bulk modulus was determined (B0 = 11.5 ± 1.5 GPa) and the elastic anisotropy was quantified. Combining the two moduli, the Poisson’s ratio was calculated (ν = 0.34 ± 0.04). All these results were compared with micromechanical measurements carried on thin films from which a comparable value of the Young’s modulus was extracted (Y = 12.0 ± 1.4 GPa). Crystal structure analysis at different pressures revealed that the pronounced anisotropy of the lattice compressibility is correlated with the difference in spacing between the molecules and the distribution of the stiffest C-H···N interactions in different crystallographic directions (Figure 2b). Remarkably, switching the molecules from the low-spin to the high-spin state leads to a substantial drop of the Young’s modulus to 7.1 ± 0.5 GPa in bulk and to 9.9 ± 1.4 GPa in thin film samples. The difference was attributed to the anisotropy of the thin film that presents favoured crystallographic orientations during the deposition while the powder is randomly oriented. These results allowed the actuating properties of the films to be assessed in terms of strain (ε = -0.17 ± 0.05%), recoverable stress (σ = -21 ± 1 MPa) and work density (W/V = 15 ± 6 mJ/cm3), highlighting their potential for applications in MEMS/NEMS technology.

Pressure dependence of the lattice parameters and packing of a sheet of molecules.

Figure 2. (a) Pressure dependence of the lattice parameters of [FeII(HB(tz)3)2] extracted from single-crystal X-ray diffraction data at ID15B. (b-c) Packing of a sheet of molecules (blue) that propagates infinitely in the ab-plane connected by short C-H···N interactions (red dotted lines). Views in the (b) bc-plane and (c) ab-plane.

 

Principal publication and authors
A complete set of elastic moduli of a spin-crossover solid: spin-state dependence and mechanical actuation, M. Mikolasek (a), M.D. Manrique-Juarez (b,c), H.J. Shepherd (d), K. Ridier (b), S. Rat (b), V. Shalabaeva (b), A.-C. Bas (b), I.E. Collings (a), F. Mathieu (c), J. Cacheux (c), T. Leichle (c), L. Nicu (c), W. Nicolazzi (b), L. Salmon (b), G. Molnar (b) and A. Bousseksou (b), Journal of the American Chemistry Society, accepted (2018); doi: 10.1021/jacs.8b05347.
(a) ESRF
(b) LCC-CNRS, Universite de Toulouse, CNRS, Toulouse (France)
(c) LAAS-CNRS, Universite de Toulouse, CNRS, Toulouse (France)
(d) School of Physical Sciences, University of Kent, Canterbury (UK)

 

References
[1] G. Molnár, S. Rat, L. Salmon, W. Nicolazzi, A. Bousseksou, Adv. Mater. 30, 17003862 (2018).
[2] M.D. Manrique-Juárez, F. Mathieu, V. Shalabaeva, J. Cacheux, S. Rat, L. Nicu, T. Leïchlé, L. Salmon, G. Molnár, A. Bousseksou, Angew. Chem., Int. Ed. 129, 8186 (2017).