Complex high-temperature phase transitions in Li2B12H12 and Na2B12H12

Differential scanning calorimety measurements of Li2B12H12 and Na2B12H12 indicate hysteretic transformations to high-temperature phases at approximate to 615 K and 529 K, respectively, upon heating (1 K/min) from room temperature. X-ray and neutron powder diffraction measurements corroborate the phase-change behavior. For Li2B12H12, the diffraction data are consistent with a previous study suggesting that the overall face-centered-cubic arrangement of icosahedral B12H12- anions is maintained upon transformation to the high-temperature polymorph, although the anions are now orientationally disordered and the Li+ cations crystallographically disordered within an enlarged lattice. For Na2B12H12, the diffraction data indicate the existence of three different high-temperature phases in addition to the known low-temperature monoclinic phase. The highest-temperature structure possesses Im (3) over barm symmetry and exhibits a body-centered-cubic arrangement of orientationally disordered anions. The interstitial, disordered Na+ cations appear to favor off-center positions within the distorted tetrahedral sites formed by the anions in this structure. An intermediate Pm (3) over barn-symmetric phase at lower temperature is the result of a partial ordering of this higher-temperature structure. A third, minor, face-centered-cubic phase coexists with these high-temperature polymorphs. H-1 NMR measurements of Li2B12H12 and Na2B12H12 reveal an approximately two-orders-of-magnitude increase in the reorientational jump rate of the anions in both cases upon transformation to their high-temperature structures. The enhanced anion mobilities were corroborated by neutron scattering fixed-window scans across the respective phase boundaries. The inherent cation disorder associated with these high-temperature polymorphs suggests their potential use as superionic conductors. (C) 2014 Elsevier Inc. All rights reserved.