No abstract

closo-Borates, such as Na 2 B 12 H 12 , are an emerging class of ionic conductors that show promising chemical, electrochemical and mechanical properties as electrolytes in all-solid-state batteries. Motivated by theoretical predictions, high-pressure in situ powder X-ray diffraction on Na 2 B 12 H 12 was performed and two high-pressure phases are discovered. The first phase transition occurs at 0.5 GPa and it is persistent to ambient pressure, whereas the second transition takes place between 5.7 and 8.1 GPa and it is fully reversible. The mechanisms of the transitions by means of group theoretical analysis are unveiled. The primaryorder parameters are identified and the stability at ambient pressure of the first polymorph is explained by density functional theory calculations. Finally, the parameters relevant to engineer and build an all-solid-state battery, namely, the bulk modulus and the coefficient of the thermal expansion are reported. The relatively low value of the bulk modulus for the first polymorph (14 GPa) indicates a soft material which allows accommodation of the volume change of the cathode during cycling. Raman spectroscopyRaman spectra were collected in a diamond anvil cell on a Renishaw 1000 spectrometer with a 633 nm excitation laser. The pressure was monitored using an internal ruby standard. The spectra were recorded between 500 cm À1 and 2700 cm À1 . DFT calculationsDFT calculations were performed in order to resolve the structure of the -polymorph; to refine the crystal structure of the high-pressure "-polymorph and to determine their ther-research papers Acta Cryst. (2019). B75, 406-413 Romain Moury et al.

Magnesium octahydrotriborate (Mg(B3H8)2) is a crucial reaction intermediate in the thermal decomposition of the hydrogen storage material Mg(BH4)2 and is discussed as a potential solid-state Mg-ion conductor. So far, Mg(B3H8)2...