Internal rotation and phase transitions in lanthanum tetrahydroborates

Conclusion

The investigation carried out shows that in the low-symmetry compouds La(BH₄)₃ and La(BH₄)₃·3THF, as in the high-symmetry MBH₄, phase transitions occur. An unexpected result is the fact that the symmetry of the environment of the boron ions for these phase transitions practically does not change and remains close to cubic. The activation energy for reorientation of the BH ⁴₋ ion (1.2 kcal/mole) is somewhat overstimated, if we start from the difference in the electronegativity of the boron and hydrogen ions (2.0 and 2.15). This may mean that the nature of the barrier to reorientation of the BH ⁴₋ anion in lanthanum tetrahydroborates is connected with the van der Waals interaction of the tetrahedral BH ⁴₋ groups with the environment. From the¹³⁹La NMR data it follows that the coordination sphere of the triply-charged lanthanum is characterized by low-symmetry of the arrangement of the BH₄₋ and THF (OC₄H₈) ligands, and the La³⁺-(BH₄)⁻ bond has predominantly ionic character. Consequently, the potential energy of the BH ⁴₋ anions in the lattice is the sum of the strong isotropic ionic component and the substantially weaker anisotropic van der Waals component. In the low-temperature phases, for both compounds we observe structurally nonequivalent positions for the BH ⁴₋ anions, retained in the high-temperature phase. The phase transitions in the studied compounds are connected with a change in the nature of the hydrogen atom distribution. Due to the low height of the barriers, the reorientational mobility sets in even at low temperatures, and in the phase transition region the value of υ_c reaches 10¹⁰–10¹¹ Hz. For such a high reorientational frequency, there may be substantial effects from correlation rotation of the anions, leading to a change in the hydrogen atom distribution in the structure and an order-disorder type phase transition.