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Title: 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{submore » 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.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4] ; ; ;  [5] ;  [1] ;  [1] ;  [2]
  1. NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899-6102 (United States)
  2. (United States)
  3. GE Global Research, 1 Research Circle, Niskayuna, NY 12309 (United States)
  4. Sandia National Laboratories, Livermore, CA 94551 (United States)
  5. Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, S. Kovalevskoi 18, Ekaterinburg 620990 (Russian Federation)
Publication Date:
OSTI Identifier:
22334167
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Solid State Chemistry; Journal Volume: 212; Other Information: Copyright (c) 2014 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ANIONS; BCC LATTICES; CALORIMETRY; CATIONS; FCC LATTICES; ION MOBILITY; IONIC CONDUCTIVITY; MONOCLINIC LATTICES; NEUTRON DIFFRACTION; NUCLEAR MAGNETIC RESONANCE; PHASE TRANSFORMATIONS; X RADIATION