Ca54In13B4–xH23+x: A Complex Metal Subhydride Featuring Ionic

Article pubs.acs.org/cm

Ca54In13B4−xH23+x: A Complex Metal Subhydride Featuring Ionic and Metallic Regions Trevor V. Blankenship, Banghao Chen, and Susan E. Latturner* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States S Supporting Information *

ABSTRACT: Reactions of CaH2 with group 13 metals in a 1:1 Ca/Li flux mixture produce Ca54In13B4−xH23+x (2.4 < x < 4). This compound has a complex new structure [Im3̅, a = 16.3608(6) Å, Z = 2] which can be viewed as a body-centered cubic array of Bergman-related clusters that are composed of a central indium atom surrounded by an icosahedron of 12 calcium atoms; hydride ions cap each face, forming a pentagonal dodecahedron that is further surrounded by a calcium shell. These In@Ca12@H20@Ca30 clusters are surrounded by a disordered calcium indium hydride network. Indium is not completely reduced by the flux; the structure features ionic hydride regions and metallic calcium indium regions, confirmed by electronic structure calculations and 1H and 115In solid-state NMR spectroscopy. This compound can therefore be viewed as a “subhydride”, akin to the alkali metal suboxides that feature ionic oxide clusters surrounded by metallic regions.

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INTRODUCTION Metal hydrides, of interest for potential use as battery and hydrogen storage materials, exhibit a broad range of bonding types and electronic characteristics.1 Ionic hydrides form with electropositive metals; these charge-balanced semiconductors or insulators include simple binaries like CaH2 as well as complex Zintl phase hydrides such as Ba21Ge2O5H24 and Yb3PbH2.2,3 Covalent metal hydrides contain hydrogen bonded to main group metals or metalloids, exemplified by LiBH4 and B2H6 as well as BaAlSiH and other polyanionic hydride Zintl phases.4,5 Transition metal hydrides such as PtHx and LaNi5Hx are metallic and can contain a variable amount of interstitial hydrogen. We report here the metal flux synthesis of a complex metal hydride that falls in between these classifications. Flux synthesis makes use of a low-melting element or compound present in large excess which acts as a solvent for the other reactants. This technique has been used to grow large crystals of known phases and to discover new compounds.6 Calcium metal (mp 845 °C) is too high melting to be useful as a flux, but a 1:1 mol ratio of Ca and Li melts at 300 °C. This Ca/Li flux mixture dissolves CaH2. It is also strongly reducing, and it will convert most main group metals and metalloids to their anionic state. As a result, reactions of CaH2 with metalloids in Ca/Li flux yield salt-like Zintl phase hydrides such as LiCa2C3H and LiCa7Ge3H3; these phases contain Li+ and Ca2+ cations surrounding the hydride anions and the tetrelide anions (C34− and Ge4−, respectively).7 Our exploration of Ca/Li/CaH2/M reactions with M = group 13 metals has led to the discovery of Ca54In13B4−xH23+x. While group 14 reactants are sufficiently electronegative to be reduced by Ca/Li flux to form anions, metals such as indium are not likely to be reduced to the −5 state. Instead, the title phase has ionic calcium hydride regions separated by a metallic calcium/indium network; conduction electrons are confined to © 2014 American Chemical Society

the latter regions of the structure, similar to the behavior seen for suboxides and subnitrides such as Cs 11 O 3 and Na16Ba6N.8−11 We have found solid-state NMR to be extremely useful in characterizing the Ca54In13B4−xH23+x “subhydride”, since both chemical shift and relaxation time are affected by the interaction of a nucleus with conduction electrons.12 The 1H and 115In NMR spectra, as well as electronic structure calculations, confirm the presence of conducting and insulating regions in this compound.

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EXPERIMENTAL SECTION

Synthesis. Boron powder (95−97%, Strem), calcium hydride powder (98%, Alfa Aesar), and indium powder (99.9%, Alfa Aesar) were used as received. Chunks of lithium (99.8%, Strem) were soaked in hexanes to remove their mineral oil coating and then stored and handled under argon. Calcium chunks (99%, Alfa Aesar) were purified by heating at 700 °C under a dynamic high vacuum of 10−5 Torr for 10 h to decompose any CaH2 and Ca(OH)2 (common contaminants in commercial calcium metal) and then stored and handled under argon. The procedure will not eliminate trace CaO contamination in the calcium metal, but XPS studies did not indicate any incorporation of oxygen into the products (see below). Reactants and flux metals were added to stainless steel crucibles (7.0 cm length/0.7 cm diameter) in a 10:10:1:1:1 mmol Ca/Li/In/B/CaH2 ratio in an argon-filled glovebox. The crucibles were sealed by arcwelding under argon and were placed in silica tubes that were flamesealed under vacuum. The ampules were heated from room temperature to 1050 °C in 4 h and held there for 2 h. The reactions were cooled stepwise to 800 °C over 36 h, 600 °C over 36 h, and 500 °C over 24 h. The reactions were held at 500 °C and then were removed from the furnace, inverted, and centrifuged for 2 min to separate the crystalline products from the Ca/Li melt. The steel Received: March 5, 2014 Revised: April 30, 2014 Published: May 1, 2014 3202

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crucibles were cut open in an argon-filled glovebox to isolate the highly air-sensitive product, which adheres to the sides of the crucible. It was subsequently determined that the best yield was found for reactions with a starting Ca/Li/In/B/CaH2 millimole ratio of 10:10:3:1:2. The reaction was repeated in a sealed niobium crucible to eliminate the possibility of contaminants leaching from the steel crucible. Elemental Analysis. Elemental analyses were performed using a JEOL 5900 scanning electron microscope with energy dispersive spectroscopy (SEM-EDS) capabilities. Samples of product crystals were affixed to an aluminum SEM stub using carbon tape and analyzed using a 30 kV accelerating voltage. The observed Ca:In atomic ratios ranged from 80:20 to 90:10. No incorporation of metals from the steel crucible was observed in any of the samples. This technique is not sensitive to the presence of light elements, so X-ray photoelectron spectroscopy measurements were carried out, using a Physical Electronics PHI 5100 series XPS with a non-monochromated dual anode (Al and Mg) source having a single channel hemispherical energy analyzer. A Mg Kα source was used. Sputtering of the sample was carried out to remove any surface oxidation. XPS spectra were taken after every 5 min of sputtering until the oxide peak disappeared, and no further changes in the spectra were observed. These measurements showed the clear presence of Ca and B in the sample, but the lithium 1s peak was very small (see Figure S1, Supporting Information). While the presence of a small amount of lithium was indicated by XPS spectra, 7Li MAS NMR studies did not show any lithium peaks. Lithium is likely present only in trace amounts on the surface of the crystals; the powder XRD pattern (Figure S2, Supporting Information) shows CaLi2 as a contaminant, formed from the freezing of Ca/Li flux residue on the product. Since the XPS samples were mounted on graphite tape, which physisorbs water and hydrocarbons, the presence of H was not confirmed with this method. Crystallographic Studies. Crystals of Ca54In13B4−xH23+x were brought out of the glovebox under Paratone oil and were mounted in a cryoloop. Single-crystal X-ray diffraction data were collected at 173 K in a stream of nitrogen using a Bruker APEX 2 CCD diffractometer with a Mo Kα radiation source. An absorption correction was applied to the data using the SADABS program.13 Refinement of the structure was performed using the SHELXTL package.14 The structure was solved in cubic space group Im3̅ (No. 204). Fully occupied calcium and indium sites were determined using direct methods. Partially occupied calcium sites and light element positions were located using difference Fourier calculations. Hydride ions were modeled as helium atoms to account for the extra electron on the atom; this yielded more stable thermal parameters on these sites and improved the fit.7b,15 Crystallographic data and collection parameters are shown in Table 1; atomic positions and thermal parameters can be found in Table S1 of the Supporting Information. Further details can be found in the CIF files in the Supporting Information and from the Fachinformationszentrum Karlsruhe [76344 Eggenstein-Leopoldshafen, Germany (fax, (+49) 7247−808−666; e-mail, crysdata@fiz-karlsruhe.de)] on quoting the depository number CSD-426758. Additional data sets were collected on several crystals from different synthetic batches to test the consistency of the calcium split sites and light element positions; they were found in the same locations in the unit cell (with similar occupancies) in all cases (see Tables S2 and S3, Supporting Information). Powder X-ray diffraction studies were carried out on reaction products to identify byproducts using a PANalytical X’Pert Pro X-ray powder diffractometer equipped with a Cu Kα Xray source. In a glovebox, samples of solid products from the optimized reaction (Ca/ Li/In/B/CaH2 millimole ratio of 10:10:3:1:2) were ground and placed in an airtight sample holder with a Kapton cover. The pattern (shown in Figure S2, Supporting Information) indicates that the product is predominantly Ca54In13B4−xH23+x; a few small extra peaks indicate the presence of CaLi2 (from solidified flux residue) and a small amount of Ca3In byproduct. NMR Spectroscopy. For solid-state NMR studies, crystals from several batches of Ca/Li/In/B/CaH2 reactions in a ratio of 10:10:3:1:2 mmol were ground together with NaCl (a 50:50 by volume mixture of Ca54In13B4−xH23+x and NaCl) to facilitate the spinning of the

Table 1. Crystallographic Data and Collection Parameters for Ca54In13B4−xH23+x parameter formula weight (g/mol) crystal system space group a (Å) Z vol (Å3) density (g/cm3, calc) index ranges

temp (K) wavelength (Å) reflns collected unique data/parameters μ (mm−1) R1/wR2a R1/wR2 (all data) residual peak/hole (e− A−3) a

Ca52.7In13B1.6H25.4 3643.3 cubic Im3̅ (No. 204) 16.3608(6) 2 4379.4(3) 2.763 −21< h < 21 −21 < k < 21 −20< l < 21 173 0.710 73 24 720 988/62 6.47 0.0165/0.0394 0.0171/0.0396 0.98/−0.55

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2.

conducting sample in the magnetic field. For magic angle spinning (MAS) 1H NMR data collections, the samples were loaded into a 1.3 mm zirconia rotor in a glovebox, for use in an Ultrafast spinning MAS probe with spinning speeds of 60 kHz on a Bruker AVIII HD 500 MHz WB spectrometer (B0 = 11.7 T). Data were collected after 32 scans with a recycle delay of 30s (T1 relaxation time was measured to be 5 s). An empty rotor was measured under the same conditions to subtract the background. Spectra were referenced to TMS at 0 ppm, with adamantane (1.6 ppm) used as a second reference. For 115In NMR experiments, the sample was packed in a 4 mm diameter zirconia rotor in a glovebox. For quadrupolar nuclei with large values of CQ, the highest practical applied magnetic field strength is usually advantageous. Therefore, a Bruker Avance II 800 MHz spectrometer with an 115In frequency of 175.4 MHz was used for the measurements. A 90° pulse width of 0.75 μs and relaxation delay of 1 s were used. Aqueous 0.1 M In(NO3)3 in 0.5 M HNO3 was used as a reference at 0 ppm. Due to the extreme broadness of the 115In spectrum, MAS was not effective. Instead, the QCPMG pulse sequence and stepped-frequency technique was used. The QCPMG (quadrupolar Carr−Purcell−Meiboom−Gill) experiment is a modern NMR technique that adapts the CPMG sequence to enhance the NMR signal of quadrupolar nuclei. This method is particularly useful for the acquisition of data for dilute or unreceptive quadrupolar nuclei. In the QCPMG pulse sequence, the magnetization is refocused repetitively, giving rise to a train of echoes upon Fourier transformation, which leads to a set of spikelets with manifold resembling the conventional powder pattern. Since all the intensity is allocated into sharp spikelets, a large gain in signal-to-noise ratio is obtained.16,17 Experimental conditions for QCPMG were set following literature procedures, with 4096 scans for each step.18 The individual steppedfrequency spectra were coded using the skyline projection method. 115 In NMR parameters were determined by visual comparison of experimental NMR spectra with those simulated using the DMFit software package.19 Electronic Structure Calculations. Density of states calculations were carried out using the TB-LMTO-ASA technique with the Stuttgart TB-LMTO 4.7 software package.20 Exchange and correlation effects were calculated within the local density approximation (LDA) using the von Barth−Hedin local exchange correlation potential.21−23 A self-consistent field calculation is performed by the program. Two structural models for the Ca54In13B4−xH23+x phase were used, based on the unit cell dimensions and atomic coordinates derived from single crystal diffraction data. For both models, instead of the three partially 3203

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occupied calcium split sites [Ca(4), Ca(5), and Ca(6)], it was assumed that the Ca(5) and Ca(6) sites were empty and the Ca(4) site was fully occupied. In one model, the B/H mixed occupancy 8c site was occupied only by boron, and in the other model, it was occupied by hydrogen. The stoichiometries of the resulting models are Ca54In13B4H23 and Ca54In13H27. The Wigner−Seitz radii were determined by an automatic procedure as follows: Ca = 1.703− 1.883 Å, In = 1.766−2.058 Å, H = 0.977−1.00 Å, B = 1.361 Å. Empty spheres were added by the program where appropriate to fill the unit cell volume. A 12 × 12 × 12 k-point mesh was used and integrated using the tetrahedron method. The basis sets consisted of 5s/5p/5d/4f for In, 4s/4p/3d for Ca, 2s/2p/3d for B, and 1s/2p/3d for H. The downfolded orbitals consist of 5d/4f for In, 4p for Ca, 3d for B, and 2p/3d for H.

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RESULTS AND DISCUSSION Ca54In13B4−xH23+x (2.4 < x < 4) forms from Ca/Li/In/B/CaH2 reactions as air-sensitive silver spheroid crystals up to 1 mm in diameter. It exhibits a new structure type in the cubic space group Im3̅ with a unit cell length of a = 16.289(1)−16.393(1) Å, as shown in Figure 1. The variation in unit cell parameter

Figure 2. Ordered crystallographic sites of Ca54In13B4−xH23+x. (a) The In(1) site is coordinated by an icosahedron of calcium ions. (b) Each face of the icosahedron is capped by a hydride anion. (c) The hydride anions are capped by calcium sites, to form the In(1)-centered cluster, In@Ca12@H20@Ca30. Octahedrally coordinated hydride sites are depicted as yellow polyhedra. (d) The In(2) atoms (red) and B(3)/ H(3) sites (black polyhedra) cap the clusters, with the B(3)/H(3) sites bridging two clusters.

2c) can be viewed as an indium atom coated with a calcium hydride shell. The calcium surface of the In@Ca12@H20@Ca30 cluster is capped by In(2) atoms (24g sites) and an additional light element in an 8c site. Twelve In(2) atoms form an icosahedron around the cluster; the resulting In@Ca12@H20@Ca30@In12 unit contains three of the four concentric shells of Bergman clusters seen in quasicrystals and approximants (see Figure S3, Supporting Information).26 The electron density at the light element 8c site is consistently higher than the H(1) and H(2) sites, but lower than expected for a boron atom. This position is assigned as a B/H mixture, denoted as B(3)/H(3). Refinements of single crystal XRD data for several crystals from different syntheses show boron ratios on this site ranging from 0% to 41% (yielding stoichiometries from Ca54In13H27 to Ca54In13B1.6H25.4). While it is inherently problematic to refine light element site occupancies in the presence of surrounding heavy elements, it was observed that the unit cell parameter increases with boron content. This 8c site links neighboring clusters together, as shown in Figure 2d, achieving an octahedral coordination from three calcium atoms on one cluster and three on a neighboring cluster, with B−Ca distances of 2.6026(5) Å. This is somewhat short compared to other Ca− B bond lengths; the observed range in CaB4 and CaB6 is

Figure 1. Structure of Ca54In13B4−xH23+x. Indium atoms are shown as red spheres. Ordered and disordered calcium sites are blue and gray spheres, respectively. Mixed B(3)/H(3) sites are shown as black polyhedra and hydride sites as yellow polyhedra.

stems from the presence of several calcium split sites and B/H mixing on another site. The structure is built upon a bodycentered cubic array of indium-centered calcium icosahedra; the In(1) atoms in the corners and center of the unit cell (2a Wyckoff sites) are coordinated by 12 Ca(1) ions at a distance of 3.3400(7) Å (Figure 2a). This is within the 3.2−3.7 Å range reported for Ca−In distances in Ca18Li5In25 and Ca2In.24,25 Each triangular face of the In@Ca12 icosahedron is capped by a hydride ion [H(1) and H(2) ions, in 24g and 16f Wyckoff sites, respectively], as shown in Figure 2b. The resulting pentagonal dodecahedron of hydride ions is in turn encapsulated by a sphere of 30 calcium ions [Ca(2) and Ca(3), in 12d and 48h Wyckoff sites, respectively]. Each hydride site thereby obtains an octahedral coordination of calcium cations, with Ca−H distances ranging from 2.47(2) to 2.63(2) Å. Similar bond lengths are seen in CaH2, LiCa2C3H, and LiCa7Ge3H3 (2.23− 2.67 Å).7 The resulting In@Ca12@H20@Ca30 cluster (Figure 3204

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2.738−3.141 Å.27 There are a few phases with shorter Ca−B bonds: A Ca−B bond of 2.554 Å is found in Ca[B(OH)4]2(H2O)2, 2.579 Å in Ca5Cl3C2(CBC), 2.459 Å in Ca9Cl8(BC2)2, and 2.458 Å in Ca2(BN2)F.28 The relatively short 2.6026 Å Ca−B/H bond length in Ca54In13B4−xH23+x may be stabilized by the high ratio of hydride ions mixed on this site. The In@Ca12@H20@Ca30 clusters and the boride/hydride sites that link them together form a 3-D network; the voids defined by this network (see Figure 2d) contain disordered calcium sites. This is similar to the encapsulation of disordered solvent molecules in the cages of framework compounds such as MOFs and zeolites grown solvothermally. In the synthesis of Ca54In13B4−xH23+x, the calcium-rich metal flux provides the disordered “solvent” atoms. The partially occupied Ca(4), Ca(5), and Ca(6) sites were consistently observed in structural refinements of several crystals. These sites are too close to each other to be fully occupied; their occupancies were therefore constrained to sum to 1. An additional hydride ion H(4), on the 6b Wyckoff site, is found in the midst of these disordered calcium cations, as seen in Figure 3b. Depending on the calcium site occupancy, this

Solid-State NMR Spectroscopy. The view of the structure of Ca54In13B4−xH23+x as interpenetrating lattices of linked In@ Ca12@H20@Ca30 clusters and a disordered calcium indide/ calcium hydride network may also extend to its electrical properties. 1H and 115In NMR spectra were collected to observe the effects of conduction electrons on the chemical shifts and relaxation times of the nuclei. There are four hydride sites in this compound. H(1) and H(2) form the ordered hydride shell around the In(1) site (Figure 2b). These two hydrides have very similar sites and local coordination environments and are expected to have very similar chemical shifts. H(3) and H(4) have much lower multiplicity, with H(3)/B(3) mixing further reducing the hydride content on this 8c site. The 1H MAS NMR spectrum is shown in Figure 4. The

Figure 4. 1H MAS NMR spectra collected on Ca54In13B4−xH23+x using a 1.3 mm rotor spinning at 60 kHz, referenced to TMS at 0 ppm.

dominant resonance at 7.7 ppm has a spin−lattice relaxation time T1 of 5 s and is likely due to the combination of H(1) and H(2) sites. This peak was broad at low spinning speeds, with a wide envelope of spinning side bands, but ultrafast MAS at 60 kHz narrowed the peak and collapsed the side bands. The observed chemical shift of 7.7 ppm is in the 3−9 ppm range observed for other ionic metal hydrides. For instance, CaH2, SrH2, and BaH2 have reported 1H NMR resonances at 4.5, 6.7, and 8.7 ppm, respectively.29,30 Hydride anions surrounded by Ca 2+ cations are also found in H − -doped mayenite [Ca24Al28O64]4+·4H−, yielding a 1H chemical shift of 5.1 ppm.31 1H resonances for more covalent hydrides such as LiH, MgH2, and LiCa2C3H are found around 3 ppm.7,32 The 7.7 ppm chemical shift for the hydride sites in Ca54In13B4−xH23+x is also similar to the 8.1 ppm shift reported for BaInGeH, although that compound is highly disordered and contains amorphous inclusions and the exact position of the (likely interstitial) hydride sites was not clear.33 No Knight shift is observed for the hydride resonance of Ca54In13B4−xH23+x, indicating that these hydrides are not interacting with conduction electrons. The presence of conduction electrons also typically facilitates fast relaxation of the nucleus; metallic hydrides such as LaNi5H6.8 and PdHx have T1 values below 500 ms.34 The 5 s T1 for this Ca54In13B4−xH23+x hydride resonance is similar to those reported for insulating ionic hydrides such as NaH and NaMgH3 (T1 = 5−50 s).35 Other features in the 1H spectrum include a small narrow peak at 4.4 ppm, and a very small broad peak at −6.2 ppm. The resonance at 4.4 ppm was also very narrow under slower spinning conditions (Figure S4, Supporting Information), and it is likely due to H2 gas. Metal hydrides may be in equilibrium with small amounts of hydrogen gas, even at room temperature;

Figure 3. Disordered region of the Ca54In13B4−xH23+x structure. (a) The In(2) site is coordinated by an icosahedron of calcium ions (blue and gray spheres represent ordered and disordered Ca sites, respectively). (b) H(4) hydride site. (c) Network of disordered Ca sites, viewed along the [111] direction.

hydride site can be coordinated by four Ca2+ ions in a square planar configuration or by an octahedron of Ca2+ ions. These three partially occupied calcium sites also complete the coordination sphere of the In(2) atoms, producing a distorted icosahedral configuration around this site, with Ca−In bonds in the 3.089−3.820 Å range. The 3-D connectivity of this disordered calcium ion network, and the hydride and indium atoms coordinated by these ions, is highlighted in Figure 3c. The voids in this figure are along the body diagonal of the cubic unit cell and are filled by chains of ordered In@Ca12@H20@ Ca30 clusters linked by the 8c boride/hydride site. 3205

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ordered model compounds of the title phaseone with the B/ H mixed site fully occupied by boron, and one with it fully occupied by hydrogen. Since this site is richer in hydrogen than boron, the calculation on the Ca54In13H27 model compound is more reflective of the real phase; the resulting DOS data is shown in Figure 6. A pseudogap is found just below the Fermi

sharp peaks at 4.3−4.5 ppm have been reported in studies of other phases including AlH3 and MgH2.36 The small −6.2 ppm peak may be due to an impurity; it was not observed in 1H spectra collected of other samples of this compound. Indium solid-state NMR is complicated by the very large quadrupolar moment of the indium nucleus (I = 9/2); the interaction of this moment with the electric field gradient of the surrounding environment leads to broadening and shifting of the central transition resonance. This is quantified by the quadrupolar coupling constant CQ and the asymmetry parameter η. Of the two indium sites in Ca54In13B4−xH23+x, the In(1) atom in the low multiplicity 2a site has a highly symmetric coordination environment, in the center of the In@ Ca12@H20@Ca30 spherical cluster. The high symmetry should minimize the electric field gradient at the In(1) nucleus. The In(2) atoms have a lower overall symmetry, but will dominate the 115In spectrum due to their much higher multiplicity. The 115 In spectrum, shown in Figure 5, can be fitted to two sites: the

Figure 6. Density of states data calculated for model compound Ca54In13H27, with the B(3)/H(3) site fully occupied by hydrogen.

level, separating the states above EF (dominated by empty calcium bands) from the filled states below (dominated by indium and hydrogen bands). The small but nonzero DOS at EF indicates that this compound is metallic, which is supported by its silver coloration and luster. The presence of a pseudogap at EF is typical of polar intermetallic compounds.39 Incorporation of a small amount of boron on the 8c site may stabilize the phase by modifying the valence electron count (VEC) to position the Fermi level in the pseudogap. The DOS diagram of the completely boron substituted Ca54In13B4H23 is shown in Figure S5 of the Supporting Information. The boron states appear at EF, eliminating the pseudogap. The Ca54In13B4−xH23+x system may incorporate enough boron on the 8c site to position the Fermi level in the pseudogap, but not eliminate it. The Ca54In13H27 bands at the Fermi level are derived from indium and calcium orbitals, with no contribution from hydride states. The hydride states are found in narrow bands localized well below EF in the −4.5 to −7.5 eV range, in agreement with their anionic nature. This supports the hypothesis that the conduction electrons are interacting with the indium sites and causing the Knight shift observed in the 115In NMR data, while the hydride sites in the structure are largely ionic, with their 1H nuclei exhibiting chemical shifts and relaxation times typical of ionic insulators. The NMR spectra and DOS calculations indicate that Ca54In13B4−xH23+x is composed of ionic calcium hydride regions and metallic Ca/In regions. This is similar to the structural and electronic behavior observed by Simon et al. in many suboxides and subnitrides. Suboxide phases such as Cs7O can be described as ionic Cs11O3 clusters embedded in a metallic matrix of excess Cs atoms (Cs11O3 + Cs10 = Cs7O).11 Likewise, the subnitrides Na16Ba6N and Na5Ba3N can be viewed as ionic clusters of Ba6N in a metallic sodium matrix.10 Photoelectron spectroscopy studies and band structure calculations on these materials show anionic nitride states localized well below EF.40 These subvalent phases behave as “void metals”; the conduction electrons are repelled by the ionic clusters and are confined to the spaces between them. This confinement raises the energy of the conduction electrons and is

Figure 5. 115In NMR spectrum for Ca54In13B4−xH23+x, collected using the stepped frequency technique (top), compared to calculated total spectrum and individual contributions from In(1) and In(2) sites.

In(1) resonance at 1071 ppm, with a CQ of 1.2 ± 0.2 MHz and a very small asymmetry parameter η = 0.1, and the In(2) resonance at 1207 ppm, with a CQ of 6.5 ± 0.2 MHz and η = 0.88.19 It is notable that both indium resonances exhibit a significant paramagnetic shift to values far higher than the negative shifts or small positive shifts seen for indium nuclei in ionic species (such as InCl4− ions in solution) or coordination compounds such as In(acac)3, which typically have 115In chemical shifts of less than 200 ppm with respect to the In(NO3)3(aq).37 Semiconducting indium phases such as Ba2In5P5 and InAs have resonances in the 300−800 ppm range.38 Heavy doping into the metallic regime can shift the resonances of indium III−V phases above 900 ppm. The conduction electrons in metallic indium-containing alloys are polarized by an applied magnetic field, causing a large Knight shift of the 115In resonance to the 3000−9000 ppm range, as observed for intermetallics such as Ni2In3, as well as indium metal itself, which has a resonance at 8000 ppm.12 The indium sites in Ca54In13B4−xH23+x have shifts in the metallic regime, indicating that these nuclei are interacting with conduction electrons at the Fermi level. Electronic Structure Calculations. To further investigate this, density of states (DOS) calculations were carried out on 3206

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demonstrated by the lower than expected work function observed for suboxides.8,9

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CONCLUSIONS Analogous to the suboxides and subnitrides, Ca53In13B4−xH23+x can be viewed as a main group metal “subhydride”, a metallic compound in which the conduction electrons avoid the hydridic regions in the structure. It is notable that no main group subhydride has been reported until now, although recent computational work predicts the stability of several lithium subhydrides (LimH, 4 < m < 9) at pressures above 50 GPa.41 Ba9In4H may also be classed as a subhydride; although the authors did not identify it as such or compare it to the suboxide phases, they carried out DOS calculations and saw similar anionic hydride states, and metallic indium states at the Fermi level.42 Further AE/Li (AE = Ca, Sr, Ba) flux reactions of AEH2 with other group 13 metals are being explored as a potential source of new subhydrides; reactions of aluminum have already shown promising results. In addition to having complex structures, unique electronic properties, and potential use as hydrogen storage materials, the subhydrides are particularly amenable to solid-state NMR studies. While the known alkali metal suboxides do not contain suitable nuclei, it would be of interest to collect 23Na and 15N MAS spectra on subnitrides such as Na5Ba3N; the sodium resonances would be expected to show Knight shifts, while the 15N resonances should not.

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ASSOCIATED CONTENT

S Supporting Information *

Tables of atomic positions and displacement factors, X-ray photoelectron spectra, powder diffraction data, 1H MAS NMR spectra, and calculated density of states data for Ca54In13B4−xH23+x, comparison of structural motifs in the title phase to Bergman clusters in NaAuSn, and additional crystallographic data for the title phase as a CIF file. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation, Division of Materials Research under award number DMR-11-06150. This work made use of the NMR facilities of the Department of Chemistry and Biochemistry, Florida State University, and the SEM and XPS facilities of the Department of Physics. We thank Dr. Zhehong Gan and Dr. Eric Ye for useful discussion about NMR experiments and data interpretation.

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