TEMPERATURE EFFECTS ON THE CHARGE TRANSPORT PROPERTIES OF A SPIN CROSSOVER-BASED DEVICE

MATTER AT EXTREMES

28 ESRF

An in-depth characterisation of spin crossover (SCO) complex thin films, based on synchrotron Mössbauer source (SMS) spectroscopy, has revealed the retention of SCO properties and paved the way for a multi-layered device in which the molecular transport properties have been evaluated, evidencing different electric transport mechanisms associated with the spin conversion.

SCO complexes are molecular compounds able to switch their physical properties upon application of external stimuli (temperature, light-irradiation, applied magnetic and electric

fields, pressure, etc.). This is associated with a switch between two spin states (low-spin, LS, and high-spin, HS) of a coordinated metal ion. As these transitions are reversible, these

High-pressure synthesis of metal-inorganic frameworks Hf4N20·N2, WN8·N2, and Os5N28·3N2 with polymeric nitrogen linkers, M. Bykov (a), S. Chariton (b), E. Bykova (c), S. Khandarkhaeva (a), T. Fedotenko (d), A. V. Ponomareva (e), J. Tidholm (f), F. Tasnádi (f), I.A. Abrikosov (f), P. Sedmak (g), V. Prakapenka (b), M. Hanfland (g), H.-P. Liermann (h), M. Mahmood (i), A. Goncharov (c), N. Dubrovinskaia (d) and L. Dubrovinsky (a),

Angew. Chem. Int. Ed. 59, 10321 (2020); https://doi.org/10.1002/anie.202002487. (a) Bayerisches Geoinstitut, University of Bayreuth (Germany) (b) Center for Advanced Radiation Sources, University of Chicago, Argonne (USA) (c) Geophysical Laboratory, Carnegie Institution of Washington (USA) (d) Material Physics and Technology at Extreme Conditions, Laboratory of Crystallography, University of Bayreuth

(Germany) (e) Materials Modeling and Development Laboratory, National University of Science and Technology MISIS , Moscow (Russia) (f) Department of Physics, Chemistry and Biology, Linköping University (Sweden) (g) ESRF (h) Photon Science, Deutsches Elektronen- Synchrotron, Hamburg (Germany) (i) Department of Mathematics, Howard University, Washington DC (USA)

[1] M. Bykov et al., Nat. Commun. 9, 2756 (2018). [2] M. Bykov et al., Angew. Chem. Int. Ed. 57, 9048-9053 (2018).

PRINCIPAL PUBLICATION AND AUTHORS

REFERENCES

anvil cells loaded with molecular nitrogen, then the samples were compressed to pressures of ~1 Mbar and laser-heated to 2000-2700 K. The reaction products were studied by means of single-crystal X-ray diffraction at beamlines ID15 (W sample) and ID11 (Os sample). Although the starting reagents were in the form of powders, numerous single-crystalline grains belonging to previously unknown phases were observed, which produced high-quality diffraction patterns of a surprisingly complex nature. Despite a common logic that ultrahigh pressure would lead to simple and densely packed structures, all three studied metals revealed the formation of porous framework structures of different topologies and compositions: Os5N28, WN8, Hf4N20 (Figure 15). The frameworks are built of metal atoms that are linked by planar polymeric nitrogen chains and, in addition, by the dinitrogen units [N=N]2- in the case of the Os and Hf compounds. The pores are filled with triply bonded dinitrogen molecules N2, leading to the overall composition of the synthesised compounds Os5N28·3N2, WN8·N2 and Hf4N20·N2. There are no strong covalent or ionic interactions between the guest molecules and the framework atoms, which suggests the host guest nature of these compounds.

The crystallites that form in the high-pressure diamond-anvil cell synthesis are often of a submicron size. The ID11 materials science beamline, which allows submicron focusing of the X-ray beam, provides extremely high spatial resolution (Figure 16a). This allows the collection of extremely high-quality single- crystal X-ray diffraction data from a submicron- sized grain and reduces the contribution of parasitic diffraction from other phases present in the sample, which can complicate the analysis of the high-pressure diffraction patterns. Crystal chemical analysis and theoretical calculations demonstrate that the polymeric nitrogen chains possess conjugated π-systems, which define the metallic properties of the compounds (Figures 16b,c).

This study demonstrates the elegant one-step synthetic route from elements to unexpectedly complex compounds. The common assumption that structural and chemical behaviour becomes simpler at ultrahigh pressures was shown to be invalid for the series of metal-inorganic frameworks synthesised at pressures over 100 GPa, and opens new horizons in both experimental and theoretical high-pressure science.