Complex high-temperature phase transitions in Li{sub 2}B{sub 12}H{sub 12} and Na{sub 2}B{sub 12}H{sub 12}
Differential scanning calorimetry measurements of Li{sub 2}B{sub 12}H{sub 12} and Na{sub 2}B{sub 12}H{sub 12} indicate hysteretic transformations to high-temperature phases at ≈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 Li{sub 2}B{sub 12}H{sub 12}, the diffraction data are consistent with a previous study suggesting that the overall face-centered-cubic arrangement of icosahedral B{sub 12}H{sub 12}{sup 2−} anions is maintained upon transformation to the high-temperature polymorph, although the anions are now orientationally disordered and the Li{sup +} cations crystallographically disordered within an enlarged lattice. For Na{sub 2}B{sub 12}H{sub 12}, 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 Im3{sup -}m symmetry and exhibits a body-centered-cubic arrangement of orientationally disordered anions. The interstitial, disordered Na{sup +} cations appear to favor off-center positions within the distorted tetrahedral sites formed by the anions in this structure. An intermediate Pm3{sup -}n-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. {sup 1}H NMR measurements of Li{sub 2}B{sub 12}H{sub 12} and Na{sub 2}B{sub 12}H{sub 12} 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. - Graphical abstract: Na{sub 2}B{sub 12}H{sub 12} undergoes a phase transformation from an ordered monoclinic structure at low temperature to a partially disordered body-centered-cubic (bcc) structure at ≈529 K, and finally to a more-fully disordered bcc structure by ≈545 K. - Highlights: • Li{sub 2}B{sub 12}H{sub 12} undergoes a high-temperature hysteretic phase transformation at ≈615 K. • Na{sub 2}B{sub 12}H{sub 12} undergoes a high-temperature hysteretic phase transformation at ≈529 K. • Increased anion mobility is observed with NMR upon these H-T phase transformations. • Three high temperature phases are observed for Na{sub 2}B{sub 12}H{sub 12}, with increasing disorder.