A Theoretical Study on the Rotational Motion and Interactions in the Disordered Phase of MBH4 (M = Li, Na, K, Rb, Cs)

The rotational motion in the high-temperature disordered phase of MBH4 (M = Li, Na, K, Rb, Cs) is investigated utilizing two complementary theoretical approaches. The first one consists of high-level periodic DFT calculations which systematically consider several instantaneous representations of the structural disorder. The second approach is based on a series of in vacuo calculations on molecular complexes suitably extracted from the crystal and chosen as to possibly disentangle the energetic factors leading to the observed rotational barriers. The results of the first part demonstrate that the motion of the BH4- anion is dominated by 90° reorientations around the 4-fold symmetry axes of the cubic crystal, and depending on the instantaneous structural disorder activation energies are found to be between 0.00 and 0.31 eV for LiBH4, 0.05 and 0.26 eV for NaBH4, 0.16 and 0.27 eV for KBH4, 0.22 and 0.31 eV for RbBH4, and 0.21 and 0.32 eV for CsBH4. The increasing rotational barriers as well as the movement of the transition state from 7° to 44° observed along the series of alkaline metals, M = Li-Rb, appear to be simply accounted for by an analysis of the energy profiles for the C2 rotation of a BH4- group in M+-BH4- and BH4--BH4- in vacuo complexes. The energy gained from the introduction of disorder shows a trend opposite to that of the rotational barriers as it decreases along the Li-Rb series. Similar considerations apply to the C3 rotational motion of the BH4- anion, which likewise has been studied in the crystal and in the in vacuo molecular complexes. CsBH4 deviates from the systematic trends observed for LiBH4-RbBH4. Depending on the structural starting point of the rotation, its C2 rotational barriers are found to be slightly higher or slightly lower than for RbBH4, whereas its energy gain due to the introduction of disorder is found to be positioned between that of KBH4 and RbBH4. The C3 rotational barriers of CsBH4 are instead significantly smaller compared to those of RbBH4 and even marginally below those of KBH4.