Structural changes in borohydride hydrogen storage materials

Angle dispersive powder x-ray diffraction and Raman experiments were performed on ABH$_{4}$ (A = K, Rb) at high pressures up to 27 GPa. We demonstrate that KBH$_{4}$ exhibits structural phase transitions from the ambient $\alpha $-KBH4 phase (cubic Fm-3m) to $\beta $-KBH$_{4}$ (tetragonal-P421c) at 3.8 GPa and to $\gamma $-KBH$_{4}$ phase (orthorhombic-Pnma) at 6.8 GPa which is similar to the phase transition sequence observed for NaBH4 earlier [1]. However, RbBH$_{4}$ undergoes two successive pressure induced structural transitions from the ambient cubic Fm-3m phase to an orthorhombic Pnma phase around 2.9 GPa and then to a monoclinic phase above 8 GPa. The high pressure monoclinic phase is found to be stable up to 27 GPa. The experimental results reveal the phase transition sequence exhibited by RbBH$_{4}$ is different from the pressure induced changes observed in similar cubic compounds NaBH$_{4}$ and KBH$_{4}$. The results further show that both the transition pressure and the structural ordering at high pressures are influenced by the atomic size of the alkali cation in these compounds. [1]. R.S. Kumar and Andrew L. Cornelius, App.Phys.Lett., 2005, 87, 261916.