Structural phase transitions in RbBH4 under compression

In situ high pressure synchrotron powder X-ray diffraction and Raman techniques were used to study the structural stability of RbBH4 at room temperature up to 27 GPa. Two successive pressure induced structural transitions were observed in RbBH4 from the ambient cubic Fm3¯m phase to an orthorhombic Pnma phase around 2.5 GPa, and then to a monoclinic P2/C phase above 8 GPa. The ambient cubic phase is found to be reversible. The experiments show that the structural polymorphs of RbBH4 observed at high pressures are different from the similar cubic hydrogen storage compounds NaBH4 and KBH4.