Experimental and computational characterization of phase transitions in CsB₃H₈

Metal hydroborates are versatile materials with interesting properties related to energy storage and cation conductivity. The hydrides containing B₃H₈^- (triborane, or octahydrotriborate) ions have been at the center of attention for some time as reversible intermediates in the decomposition of BH₄^- (3BH₄^-↔ B₃H₈^- + 2H₂), and as conducting media in electrolytes based on boron-hydride cage clusters. We report here the first observation of two phase transitions in CsB₃H₈ prior to its decomposition above 230 °C. The previously reported orthorhombic room temperature phase (here named α-CsB₃H₈) with the space group Ama2 changes into a new phase with the space group Pnma at 73 °C (here named β-CsB₃H₈), and then into a face-centered cubic phase, here named γ-CsB₃H₈, 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 CsB₃H₈ 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-CsB₃H₈, a significant disorder in B₃H₈^- positions and orientations was found which can potentially benefit cation conducting properties through the paddle mechanism.