The Low-Temperature Structural Behavior of Sodium 1-Carba-Closo-Decaborate: NaCB9H10

Two ordered phases of the novel solid superionic conductor sodium 1-carba-closo-decaborate (NaCB9H10) were identified via synchrotron x-ray powder diffraction in combination with first-principles calculations and neutron vibrational spectroscopy. A monoclinic packing of the large ellipsoidal CB9H10- anions prevails at the lowest temperatures, but a first-order transformation to a slightly modified orthorhombic packing is largely complete by 240 K. The CB9H10- anion orientational alignments and Na+ cation interstitial sitings in both phases are arranged so as to minimize the cation proximities to the uniquely more positive C-bonded H atoms of the anions. These results provide valuable structural information pertinent to understanding the relatively low-temperature, entropy-driven, order-disorder phase transition for this compound.