Study of the Temperature- and Pressure-Dependent Structural Properties of Alkali Hydrido closo-borate Compounds

In this work we report on the structural properties of alkali hydrido-closo-(car)borates, a promising class of solid-state electrolyte materials, using high-pressure and temperature dependent X-ray diffraction experiments combined with DFT calculations. The mechanical properties are determined from pressure dependent diffraction studies and DFT calculations; the shear moduli appear to be very low for all studied compounds revealing their high malleability (that can be beneficial for the manufacturing and stable cycling of all-solid-state batteries). The thermo-diffraction experiments also reveal a high coefficient of thermal expansion for these materials. We discover a pressure induced phase transition for K2B12H12 from Fm-3 to Pnnm symmetry around 2GPa. A temperature induced phase transition for Li2B10H10 was also observed for the first time by thermodiffraction and the crystal structure solved combining experimental data and DFT calculations. Interestingly, all phases of the studied compounds (including newly discovered high pressure and high temperature phases) may related via group-subgroup relationship, with the notable exception of the room temperature phase of Li2B10H10