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 (NaCB 9 H 10 ) 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 CB 9 H 10 − anions prevails at the lowest temperatures, but a first-order transformation to a slightly modified orthorhombic packing is largely complete by 240 K. The CB 9 H 10 − 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. [ABSTRACT FROM AUTHOR]