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 Fm3̄ structural symmetry to become Fm3̄m. 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 AbstractCs2B12H12 undergoes an order–disorder phase transition near 529 K. Similar transitions are observed for K2B12H12 and Rb2B12H12.Figure optionsDownload as PowerPoint slideHighlights
► Diffraction and DSC reveal that Cs2B12H12 undergoes a 2nd-order phase transition near 529 K.
► This phase transition is from the low-temperature Fm3̄ symmetry to Fm3̄m at high temperatures.
► DSC of Rb2B12H12 and K2B12H12 indicate 2nd-order transitions near 742 K and 811 K, respectively.