New Phase of Ca(BH4)2 at Near Ambient Conditions

Calcium borohydride, as a promising hydrogen storage material, has been investigated by synchrotron X-ray diffraction (XRD) and Raman spectroscopy upon compression in a diamond anvil cell. A pressure-induced phase transition is confirmed by XRD patterns and Raman spectra. The new phase matches the predicted isoenergetic structure with C2/c space group. In the process of phase transition, there is a wide region from 2.36 to 7.97 GPa in our XRD patterns, and the volume collapse is 3.86%. We have also carried out Raman spectroscopy up to 43.5 GPa, and the same phase transition is found at 3.59 GPa. Another initial phase γ-Ca(BH4)2 is stable, and the pressure-induced amorphization occurs above 13.8 GPa.