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 (A 2 B 12 H 12 ), that of Na 2 B 12 H 12 , was determined by powder X-ray diffraction. Compared to the structural symmetries of its neighboring, cubic, alkali-metal analogs, i.e., the lighter Li 2 B 12 H 12 (Pa3 j ) and the heavier K 2 B 12 H 12 (Fm3 j ), Na 2 B 12 H 12 displays an intermediate monoclinic (P2 1 /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) dodecahy- dro-closo-dodecaborates (A 2 B 12 H 12 ,A ) Li, Na, K, Rb, and Cs; AeB 12 H 12 , Ae ) Mg, Ca, Sr, and Ba) have gained notice recently, as there is evidence 1-5 that they are typical intermediate compounds in the decomposition of the related borohydrides (ABH 4 and Ae(BH 4 ) 2 ), an attractive class of hydrogen-storage materials. However, structural information for these dodecahy- dro-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 out 6,7 to predict needed structures to more fully understand the thermodynamic role of these materials during hydrogen cycling of borohydride materials. For A 2 B 12 H 12 , identical Fm3 j -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 Pa3 j -symmetric structure for Li 2 B 12 H 12 . 10 Hence, the Na 2 B 12 H 12 structure is the last unknown A 2 B 12 H 12 structure. In this paper, we report the crystal structure of Na 2 B 12 H 12 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 Na 2 B 12 H 12 result completes the known set of structures for A 2 B 12 H 12 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 A 2 B 12 H 12 compounds, the XRD pattern of Na 2 B 12 H 12 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 P2 1 /n symmetry (a ) 7.0306(3) Å, b ) 10.6540(4) Å, c ) 7.0093(5) Å,  ) 94.676(4)°). The centroid of the ideal symmetry-constrained B 12 H 12 2- icosahedron is located on the inversion center at (0, 0, 1/2) so that only six B and H atoms are independent. The structure is shown in Figure 1, where the monoclinic unit cell is related to the original cubic cell by (1/2 × 1 × 1/2). Each B 12 H 12 2- icosahedron is surrounded by eight Na + cations, while each Na + cation interacts with four nearest neighbor B 12 H 12 2- icosahedra via nine H atoms. In order to confirm the stability of our refined Na 2 B 12 H 12 structure, we performed first-principles calculations within the plane-wave implementation of the generalized gradient ap- proximation to DFT. Structural optimizations were performed with respect to atomic positions with lattice parameters fixed at the experimental values. The room-temperature structure from XRD data was relaxed at 0 K. The optimized structure was close to that experimentally observed, suggesting that there is no * To whom correspondence should be addressed. E-mail: jhher@nist.gov. † National Institute of Standards and Technology. ‡ University of Maryland. § Sandia National Laboratories. Figure 1. Structure of Na 2 B 12 H 12 viewed along the (010) direction (Na: yellow; B: green; H: silver; corresponding K site: small blue ball). Blue dashed lines indicate Na-H distances (<3 Å); red dashed lines indicate the 1.037(3) Å deviation of the Na site from the tetrahedral K site; thin black lines define the unit cell boundary. 11187 10.1021/jp904980m CCC: $40.75  2009 American Chemical Society Published on Web 06/10/2009 2009, 113, 11187–11189 Downloaded by SANDIA NATL LAB on July 10, 2009 Published on June 10, 2009 on http://pubs.acs.org | doi: 10.1021/jp904980m