M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

6 4 H I G H L I G H T S 2 0 2 3 I

In-situ X-ray diffraction reveals formation and polymorphism of hydrogen-rich hypervalent hydridosilicates

Hydrogenation reactions at gigapascal pressures have opened up for novel, hydrogen-rich materials with properties relating to superconductivity, ion conductivity and hydrogen storage. In-situ diffraction experiments enable the study of intricate phase relations of such materials at high pressure and temperature conditions.

The discovery of high-temperature superconductivity in highly compressed H2S in 2015 [1] spurred immense research into hydrogen-rich hydrides, also called superhydrides, in which H/M ratios exceed conventional valences. The past years have seen numerous theoretical predictions of such exotic compounds with spectacularly high critical temperatures TC, and superconductivity has been measured at around 260 K for LaH10 when pressurised to about 170 GPa [2]. At the same time, superconducting superhydrides are only observable in situ at highly extreme conditions, typically beyond 100 GPa (1 Mbar), and as minute samples, which makes their characterisation extremely challenging.

Recently, efforts shifted to identifying ternary or generally multinary hydrogen-rich hydrides for which synthesis and stabilisation pressures are closer to ambient and in the range of large volume press (LVP) methodology [3]. Attainable pressures with LVPs are limited to about 25 GPa. In exchange, sample volumes are drastically increased (to tens of mm3) and reaction environments at high pressures and temperatures (P-T) can be stably maintained and well controlled over prolonged periods of time. This can be exploited for in-situ mapping of hydride formation and the development of recovery strategies for new multinary hydrogen-rich hydrides. The addition of a third and more elements other than hydrogen enormously expands the phase space. Thus, the exploration of multinary systems bears a high propensity for tapping into new realms of hydrogen-rich hydrides, potentially adjoining to superconducting superhydrides, within the comparatively lower pressures of LVPs. In LVP hydrogenations, H2 has to be delivered by an internal, solid source and will correspond to a supercritical fluid at the targeted P-T conditions. Ammonia borane, BH3NH3, has been recognised as an ideal H-source as it possesses a high H content and decomposes neatly to inert BN and H2 at high pressures [4].

In the search for ternary hydrogen-rich Si hydrides, reaction mixtures of NaH-Si-H2(BH3NH3) were sealed in NaCl capsules and investigated in the pressure range 5 10 GPa and at temperatures up to 850°C. In-situ synchrotron diffraction high-pressure experiments were

Fig. 46: Evolution of X-ray diffraction patterns (λ = 0.233933 Å) at ID06-LVP upon heating of a

reaction mixture NaH-Si-H2(BH3NH3) at ~9 GPa and formation of tetragonal Na3SiH7 above

560°C, which transforms into an orthorhombic polymorph below 130°C.