Experimental and computational characterization of phase transitions in CsBH.

Metal hydroborates are versatile materials with interesting properties related to energy storage and cation conductivity. The hydrides containing BH (triborane, or octahydrotriborate) ions have been at the center of attention for some time as reversible intermediates in the decomposition of BH (3BH↔ BH + 2H), and as conducting media in electrolytes based on boron-hydride cage clusters. We report here the first observation of two phase transitions in CsBH prior to its decomposition above 230 °C. The previously reported orthorhombic room temperature phase (here named α-CsBH) with the space group Ama2 changes into a new phase with the space group Pnma at 73 °C (here named β-CsBH), and then into a face-centered cubic phase, here named γ-CsBH, at 88 °C. These phases are not stable at room temperature thus requiring in situ measurements for their characterization. The phase transitions and decomposition pathway of CsBH were studied with in situ synchrotron powder X-ray diffraction (SR-PXD), in situ and ex situ vibrational spectroscopies (Raman and FTIR), and differential-scanning calorimetry combined with thermo-gravimetric analysis (DSC-TGA). The structure determination was validated by vibrational spectroscopy analysis and modeling of the periodic structures by density functional methods. In γ-CsBH, a significant disorder in BH positions and orientations was found which can potentially benefit cation conducting properties through the paddle mechanism.