"NaB15": a new structural description based on X-ray and neutron diffraction, electron microscopy, and solid-state NMR spectroscopy

A boron-rich sodium boride, formerly known as NaB15, has been subjected to a comprehensive structural reinvestigation using X-ray single-crystal and powder diffraction, low-temperature neutron and electron diffraction, high-resolution transmission electron microscopy, and 23Na solid-state NMR spectroscopy. The results indicate that the previously published orthorhombic space group is incorrect. Consistent with all of the experimental results a modified structural description is developed in the monoclinic space group I1m1 (a=585.92(3), b=1039.92(6), c=833.17(5) pm, β=90.373(5)° from powder data). Because one of the interstitial boron atom positions remains unoccupied, the accurate compositional formula is NaB14.5 or Na2B29.