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2009, 113, 11187–11189 Published on Web 06/10/2009
Role of Cation Size on the Structural Behavior of the Alkali-Metal Dodecahydro-closo-Dodecaborates Jae-Hyuk Her,*,†,‡ Wei Zhou,†,‡ Vitalie Stavila,§ Craig M. Brown,† and Terrence J. Udovic† NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, Department of Materials Science and Engineering, UniVersity of Maryland, College Park, Maryland 20742, and Sandia National Laboratories, 7011 East AVenue, LiVermore, California 94551 ReceiVed: May 27, 2009
The last unknown crystal structure among the alkali-metal dodecahydro-closo-dodecaborates (A2B12H12), that of Na2B12H12, was determined by powder X-ray diffraction. Compared to the structural symmetries of its neighboring, cubic, alkali-metal analogs, i.e., the lighter Li2B12H12 (Pa3j) and the heavier K2B12H12 (Fm3j), Na2B12H12 displays an intermediate monoclinic (P21/n) structural arrangement. This result allows us to understand more thoroughly the effect of cation size on the observed structural behavior of this technologically relevant series of compounds. The alkali-metal (A) and alkaline-earth-metal (Ae) dodecahydro-closo-dodecaborates (A2B12H12, A ) Li, Na, K, Rb, and Cs; AeB12H12, Ae ) Mg, Ca, Sr, and Ba) have gained notice recently, as there is evidence1-5 that they are typical intermediate compounds in the decomposition of the related borohydrides (ABH4 and Ae(BH4)2), an attractive class of hydrogen-storage materials. However, structural information for these dodecahydro-closo-dodecaborates has generally been lacking, in part, due to the frequent difficulty in synthesizing sufficiently crystalline materials from their solvated precursors. Thus, theoretical calculations have been carried out6,7 to predict needed structures to more fully understand the thermodynamic role of these materials during hydrogen cycling of borohydride materials. For A2B12H12, identical Fm3j-symmetric structures are already known for the three stable heavier elements (A ) K, Rb, and Cs; Fr is unstable),8,9 and we only very recently reported a Pa3j-symmetric structure for Li2B12H12.10 Hence, the Na2B12H12 structure is the last unknown A2B12H12 structure. In this paper, we report the crystal structure of Na2B12H12 as determined from powder X-ray diffraction (XRD). Moreover, we present first-principles calculations using density functional theory (DFT), which confirm the stability of this structure and are also in agreement with the phonon density of states (DOS) measured by neutron vibrational spectroscopy (NVS). This Na2B12H12 result completes the known set of structures for A2B12H12 compounds and provides a better understanding of their structural behavior in terms of the size of the A+ cation. In contrast to the cubic symmetries of all other A2B12H12 compounds, the XRD pattern of Na2B12H12 was indexed with a monoclinic lattice with half the volume of the typical cubic unit cell. The diffraction pattern was successfully explained by a structure model with P21/n symmetry (a ) 7.0306(3) Å, b ) * To whom correspondence should be addressed. E-mail:
[email protected]. † National Institute of Standards and Technology. ‡ University of Maryland. § Sandia National Laboratories.
10.1021/jp904980m CCC: $40.75
Figure 1. Structure of Na2B12H12 viewed along the (010) direction (Na: yellow; B: green; H: silver; corresponding K site: small blue ball). Blue dashed lines indicate Na-H distances (
