MATTER AT EXTREMES

30 ESRF

EXPERIMENTAL EVIDENCE FOR A SUPERIONIC BCC FORM OF ICE

Superionic conductors are non-metallic solids that display a large electrical conductivity, typical of liquid electrolytes. Such materials have found applications in supercapacitors, batteries, fuel cells and various kinds of chemical sensors. The present work found evidence that water ice becomes a superionic conductor above 15 GPa and 850 K.

In 1999, Cavazzoni et al. [1] performed computer simulations of water ice and predicted that under extreme conditions of pressure (P~60 GPa) and temperature (T~2000 K), protons (H) would become highly mobile and diffuse through the oxygen sublattice, making ice a superionic conductor. This prediction gathered a large interest, mostly because ice is a major component of Neptune-like planets, and the presence of a superionic ice layer in these planets could be a key ingredient to understand their intriguing magnetic field. Experimental evidence for superionic ice has been claimed by several authors in the last 15 years, however, the specific conditions at which ice becomes superionic have remained controversial. The transition line between the molecular and superionic phases was expected to meet the melting curve at a triple point, inducing a significant change of slope of the latter. Unfortunately, previous experimental

reports of the melting curve of ice show large discrepancies above 15 GPa, preventing a clear identification of such a triple point.

Samples of water ice compressed in diamond- anvil cells were studied by X-ray diffraction at beamlines ID27 and ID09A (now ID15B) and at the PSICHE beamline at SOLEIL. A non- ambiguous detection of melting was obtained by monitoring the disappearance of the crystalline Bragg peaks and the simultaneous appearance of the liquid diffuse scattering. The new melting data in the range 8-44 GPa, 660-1500 K show a change in the melting slope at 14.6 GPa - 850 K, signalling a triple point between two solid phases and the liquid (Figure 19). The volume of ice was also measured as a function of pressure above the triple point temperature, which evidenced a first-order phase transition between two isostructural, body-centered cubic

Space Charge-Limited Current Transport Mechanism in Crossbar Junction Embedding Molecular Spin Crossovers, G. Cucinotta (a,c), L. Poggini (a,c), N. Giaconi (a,c), A. Cini (b,c), M. Gonidec (d), M. Atzori (a,c), E. Berretti (e), A. Lavacchi (e), M. Fittipaldi (b,c),

A.I. Chumakov (f), R. Rüffer (f), P. Rosa (d) and M. Mannini (a,c), ACS Appl. Mater. Interfaces 12, 31696-31705 (2020); https://doi.org/10.1021/acsami.0c07445. (a) Chemistry Department, University of Florence (Italy) (b) Physics and Astronomy Department,

University of Florence (Italy) (c) INSTM Research Unit, Florence (Italy) (d) CNRS, University of Bordeaux (France) (e) Institute for Chemistry of OrganoMetallic Compounds, ICCOM-CNR, Sesto Fiorentino (Italy) (f) ESRF

[1] M. Atzori et al., J. Mater. Chem. C 6, 8885-8889 (2018).

PRINCIPAL PUBLICATION AND AUTHORS

REFERENCES

On the basis of the SMS evidence, vertical multi-layered junctions embedding a 40 nm- thick SCO film were produced (Figure 18a). The final device was constituted by an Ag bottom electrode (200 nm), an [Fe(qnal)2] film (40 nm), a LiF decoupling layer (10 nm), and an Au top electrode (35 nm) (Figure 18b). Electrical characterisations of this multi-layered structure as a function of temperature allowed to determine the electronic transport properties of the SCO thin film sandwiched in between the two metal electrodes, and the results were supported by DFT calculations. In particular, the J-V characteristic (Figure 18c) showed that space charged limited current (SCLC) is the mechanism responsible for electron transport in this specific SCO device.

Moreover, the temperature dependence revealed a transition from a shallow traps regime (at low temperatures) to an exponential traps distribution (at high temperature), with a sharp transition from the first one to the second one that was consistent with the SCO temperature (Figure 18d). These findings suggest that the different molecular packing and orbital overlap associated with the LS and HS states can affect the electronic transport properties of this molecular thin-film device. They confirm the potential of SCO molecules, and particularly of [Fe(qnal)2], as suitable molecules for technological applications where the SCO triggers a change in the electronic behaviour of an electronic device.