Evidence of a transition to reorientational disorder in the cubic alkali-metal dodecahydro-closo-dodecaborates

A neutron powder diffraction and differential scanning calorimetry (DSC) study indicates that Cs2B12H12 undergoes a second-order phase transition near 529 K that can be described as a reorientational disordering of the B12H122- icosahedral anions between two lowest-energy configurations within the cubic structure. Such a disordering requires the addition of another mirror plane to the low-temperature Fm3-bar structural symmetry to become Fm3-bar m. Differential scanning calorimetry measurements suggest the possible persistence of some short-range anion order at and above the transition. Additional DSC measurements of the lighter alkali-metal cubic isomorphs, Rb2B12H12 and K2B12H12, also indicate second-order transitions for these compounds near 742 K and 811 K, respectively. These results are suggestive of similar order-disorder phase changes as for Cs2B12H12, although confirmation of their existence requires analogous diffraction measurements. - Graphical Abstract: Cs2B12H12 undergoes an order-disorder phase transition near 529 K. Similar transitions are observed for K2B12H12 and Rb2B12H12. Highlights: → Diffraction and DSC reveal that Cs2B12H12 undergoes a 2nd-order phase transition near 529 K. → This phase transition is from the low-temperature Fm3-bar symmetry to Fm3-bar m at high temperatures. → DSC of Rb2B12H12 and K2B12H12 indicate 2nd-order transitions near 742 K and 811 K, respectively.