Li Flux: Void Metal

Article pubs.acs.org/IC

Low-Dimensional Nitridosilicates Grown from Ca/Li Flux: Void Metal Ca8In2SiN4 and Semiconductor Ca3SiN3H Matthew J. Dickman, Benjamin V. G. Schwartz, and Susan E. Latturner* Department of Chemistry and Biochemistry Florida State University, Tallahassee, Florida 32306, United States S Supporting Information *

ABSTRACT: Reactions of indium and silicon with lithium nitride in Ca/Li flux produce two new nitridosilicates: Ca8In2SiN4 (orthorhombic, Ibam; a = 12.904(1) Å, b = 9.688(1) Å, c = 10.899(1) Å, Z = 4) and Ca3SiN3H (monoclinic, C2/c; a = 5.236(1) Å, b = 10.461(3) Å, c = 16.389(4) Å, β = 91.182(4)°, Z = 8). Ca8In2SiN4 features isolated [SiN4]8− units and indium dimers surrounded by calcium atoms. Ca3SiN3H features infinite chains of cornersharing SiN4 tetrahedra and distorted edge-sharing H@Ca6 octahedra. Optical properties and band structure calculations indicate that Ca8In2SiN4 is a void metal with calcium and indium states at the Fermi level and Ca3SiN3H is a semiconductor with a band gap of 3.1 eV.

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focused on the use of Ca/Li flux. The successful growth of new carbide, hydride, and nitride phases such as Ca12InC13‑x, Ca54In13B4‑xH23+x, Ca11Sn3C8, and Ca6Te3N2 from this melt indicates that further exploration is merited.17−20 Calcium metal melts above 800 °C, but a 1:1 mixture of calcium with lithium metal melts at 300 °C. Such lowered temperatures enable incorporation of calcium into metastable nitride products; lithium more often acts as a spectator in the low temperature reaction, but will incorporate into products occasionally (as seen for Ca(LixFe1−x)Te2N3, LiCa3As2H, and Ca2LiC3H).20−22 Herein we report two new quaternary low-dimensional nitridosilicates, Ca8In2SiN4 and Ca3SiN3H, grown from molten mixtures of calcium and lithium. Ca3SiN3H is a semiconductor nitridosilicate-hydride; it is the only nitridosilicate-hydride reported thus far. The band gap evidenced by its orange color and by electronic structure calculations makes Ca3SiN3H a semiconductor, albeit with an indirect gap. Ca8In2SiN4 contains both ionic and metallic regions in its structure making it the first void metal nitridosilicate. The Ca8In2SiN4 metallic regions consists of indium dimers embedded in a calcium metal matrix, while the ionic regions of this compound are the rarely observed isolated [SiN4]8− units. These zero-dimensional (0D) anions, and the one-dimensional (1-D) nitridosilicate chains in Ca3SiN3H, are distinct from the far more ubiquitous twodimensional (2-D) and three-dimensional (3-D) networks typically observed in nitridosilicate structures.

INTRODUCTION Nitrides are a class of compounds that have useful optical, electrical, and mechanical properties. Binary nitrides such as Si3N4 are found in high performance ceramics, GaN and its III−V solid solutions are used in light emitting diodes (LEDs), and AlN is useful as a heat sink for electronics.1−6 Higher order nitrides such as the MAX phases (e.g., V2GaN and Ti4AlN3) are strong and highly machinable, TiAlN is used as a hard coating, and Ca2Si5N8 or CaAlSiN3 doped with Eu2+, Ce3+, Mn2+, or Tm3+ can be used as phosphors for white LED technologies.7−9 The latter examples represent a class of nitrides known as nitridosilicates. These materials are analogous to the ubiquitous oxosilicates, but feature more diverse connectivity of the [SiN4] units compared to the [SiO4] units. Unfortunately, traditional high temperature synthesis of novel nitride compounds is hindered by thermodynamics; there is a high-energy cost to breaking NN bonds and subsequently forming N3−.10 New synthetic methods that alter the thermodynamics and kinetics of reaction systems are needed to incorporate N3− into multinary compounds. One popular technique that has been used is the flux synthesis method. The flux method utilizes an excess of a low melting reagent (often a metal or salt) as a solvent for reactants.11−13 This enables reactions at low temperatures, which may circumvent thermodynamic sinks encountered with high temperature syntheses. Molten Li, Ca, or alloy melts of Na with alkaline earth metals are commonly used to synthesize nitride compounds.14−16 N2 is soluble and reactive in these melts, forming N3− due to the highly reducing nature of these metals. However, the use of a nitride salt such as Li3N or Ca3N2 as a reactant also provides N3− in solution and avoids this thermodynamic hurdle altogether. Recent work in our lab has © 2017 American Chemical Society

Received: June 16, 2017 Published: July 27, 2017 9361

DOI: 10.1021/acs.inorgchem.7b01532 Inorg. Chem. 2017, 56, 9361−9368

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Inorganic Chemistry Table 1. Crystallographic Data Collection Parameters for the Title Phases formula weight (g/mol) crystal system space group a (Å) b (Å) c (Å) β (deg) Z volume (Å3) density, calc (g/cm3) index ranges reflections collected temperature (K) radiation unique data/parameters μ (mm−1) R1/wR2a R1/wR2 (all data) GOF a

Ca8In2SiN4

Ca3SiN3H

4 1362.5(3) 3.093 −16 ≤ h ≤ 16, −12 ≤ k ≤ 12, −14 ≤ l ≤ 14 7422 200 Mo Kα (λ = 0.71073 Å) 833/42 6.45 0.0175/0.0462 0.0177/0.0463 1.138

194.36 monoclinic C2/c (#15) 5.236(1) 10.461(3) 16.389(4) 91.182(4) 8 897.6(4) 2.877 −6 ≤ h ≤ 6, −13 ≤ k ≤ 13, −21 ≤ l ≤ 21 6475 127 Mo Kα (λ = 0.71073 Å) 1108/71 3.78 0.0458/0.1135 0.0470/0.1141 1.237

634.41 orthorhombic Ibam (#72) 12.904(1) 9.688(1) 10.899(1)

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

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The presence of hydrogen and nitrogen was inferred based on reactants used and bond lengths observed in the crystal structures. SEM-EDS data for both compounds are in agreement with single crystal structure refinements. Measurements on Ca8In2SiN4 indicated that calcium, indium, and silicon were present with average ratios of 68−72 atomic percent, 18−19 atomic percent, and 10−14 atomic percent, respectively. Data for Ca3SiN3H showed calcium content of 72−76 atomic percent, and silicon content of 24−28 atomic percent. Crystallographic Analysis. Samples of Ca8In2SiN4 and Ca3SiN3H were brought out of the glovebox under Parabar oil and examined under a microscope to select crystals for diffraction studies. Pieces of suitable size were cut from larger crystals and were mounted in cryoloops. Single-crystal X-ray diffraction data were collected at 200 K or below under a stream of nitrogen using a Bruker APEX 2 CCD diffractometer with a Mo Kα radiation source. Absorption corrections were applied to the data sets using the SADABS program.23 Refinements of the structures were performed using the SHELXTL package.24 Calcium, indium, and silicon sites were located using direct methods. Light element positions were located using difference Fourier calculations. Refinement of the nitrogen sites were straightforward, with occupancies refining at/near 100% and bond lengths confirming their identity as nitrogen atoms. In order to eliminate a nonpositive thermal parameter, the hydride site in Ca3SiN3H was refined isotropically. Refinement of this site as a hydrogen led to higher than 100% occupancy because it is instead a hydride (two electrons instead of one). It was therefore refined as a helium atom. Refinement as helium should yield site occupancy of 100%. Occupancies slightly below 100% were observed, but it is notable that accurate refinement of light atoms surrounded by calcium sites is difficult. We fixed the occupancy to 100% based on chargebalancing requirements (vide infra). Crystallographic data and collection parameters are shown in Table 1. Atomic positions and thermal parameters for both structures can be found in Supporting Information. Further details of the crystal structure investigations may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, on quoting the deposition number CSD-433003 for Ca8In2SiN4 and CSD-433004 for Ca3SiN3H. Electronic Structure Calculations. Band structure and density of states (DOS) calculations for both title compounds were carried out using the Stuttgart TB-LMTO-ASA software package, based on the unit cell dimensions and atomic coordinates derived from single crystal X-ray diffraction data.25 Empty spheres were added by the program

EXPERIMENTAL PROCEDURES

Synthesis. Ca8In2SiN4 was initially synthesized from reactions of indium and Li3N in Ca/Li flux. Calcium metal (99.5%, Alfa Aesar), chunks of lithium (99.8%, Strem), indium powder (99.9%, Alfa Aesar), and Li3N (99.5%, CERAC) were used as received. Reactants and flux metals were added to stainless steel crucibles (7.0 cm length/0.7 cm diameter) in a 7:7:1:1 mmol Ca/Li/In/Li3N ratio in an argon-filled glovebox. The crucibles were sealed by arc-welding under argon and were placed in silica tubes which were flame-sealed under a vacuum. The ampules were heated from room temperature to 1050 °C in 3 h and held there for 2 h. The reactions were cooled stepwise to 800 °C in 72 h, to 500 °C in 36 h, and then held at 500 °C for 24 h. The reactions were then removed from the furnace while at 500 °C, quickly inverted, and centrifuged for 2 min to decant the Ca/Li melt from the crystal products adhered to the sides of the crucible. The steel crucibles were cut open and products were stored in an argon-filled glovebox. Ca8In2SiN4 formed from leaching silicon from the stainless steel crucible (confirmed by EDS, vide infra); Ca3CrN3 and Ca2NH byproducts also formed. Subsequent reactions were prepared (similarly to those mentioned in the previous paragraph) using deliberately added silicon (99+%, Strem). After reactions using Li3N and Ca3N2 were compared, Ca3N2 (CERAC, 99%) was deemed best for optimizing subsequent reactions in order to minimize leaching from the steel crucible (see discussion). Optimal yields of Ca8In2SiN4 were obtained from reactions consisting of Ca/Li/In/Si/Ca3N2 in a 8/ 8/2/1/2 mmol ratio. Ca3SiN3H appeared as a byproduct in reactions aimed at optimizing Ca8In2SiN4. Hydride incorporation into Ca3SiN3H results from hydride and/or hydroxide contamination of calcium metal. Subsequent reactions were done using purified calcium, obtained by heating calcium metal to 750 °C under high vacuum (∼10−5 Torr) to remove any hydride contaminants. These reactions produced α-Ca5Si2N6 and an unidentified cubic phase. Optimal yields of Ca3SiN3H were obtained from reactions consisting of Ca/Li/Si/Ca3N2/CaH2 in a 7/ 8/1/1.5/0.5 mmol ratio. Elemental Analysis. Elemental analyses were carried out using a scanning electron microscope (SEM; FEI NOVA 400) with energy dispersive spectroscopy (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 EDS detector is not sensitive to the presence of light elements such as hydrogen and nitrogen, so only the relative ratios of calcium, indium, and silicon were observed. 9362


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Inorganic Chemistry where appropriate to fill the unit cell volume. A 16 × 16 × 16 k-point mesh was used for Ca8In2SiN4 and a 16 × 16 × 8 k-point mesh was used for Ca3SiN3H. Both models were integrated using the tetrahedron method. The basis sets for Ca8In2SiN4 consisted of Ca 4s/4p/3d, In 5s/5p/5d/4f, Si 3s/3p/3d orbitals, and N 2s/2p/3s orbitals. The Ca 4p, In 5d/4f, Si 3d, and N 3s orbitals were downfolded. The basis sets for Ca3SiN3H consisted of Ca 4s/4p/3d, Si 3s/3p/3d, N 2s/2p/3s, and H 1s/2p/3d orbitals. The Ca 4p, Si 3d, N 3s, and H 2p/3d orbitals were downfolded.

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RESULTS AND DISCUSSION Synthesis. Reactions containing indium and lithium nitride in a molten flux mixture of calcium and lithium metals yielded Ca8In2SiN4. Silicon is present in the stainless steel crucibles in minor amounts (1−3 atomic%) and leached into the reaction mixture to give the aforementioned product. Leaching from the steel crucibles seems to be exacerbated by the presence of certain reactants that make the flux more corrosive; it often occurs when Li3N is used as the source of nitrogen in reactions. Our previously published work described the isolation of Ca6Li0.48Fe0.52Te2N3 that forms when iron is leached from the steel during reactions of Li3N and Te in Ca/Li flux.20 Additional elements in the steel (such as nickel, chromium, and carbon) have been observed to leach in flux reactions utilizing Li3N, producing byproducts such as CaNi2Si and Ca3CrN3. The use of Ca3N2 as the nitride source in subsequent reactions minimized the extent of leaching, and by deliberately adding silicon to the reaction mixture, Ca8In2SiN4 purity and yield were optimized with a 8/8/2/1/2 mmol ratio of Ca/Li/ In/Si/Ca3N2. The yield was 43% based on indium, with minor amounts of an unidentified cubic byproduct. Ca8In2SiN4 crystals grow as dark reflective rods with length ranging from about 0.3 mm to 3 mm (Figure 1). Widths of these crystals range from 0.1−0.6 mm. Crystals are air-sensitive with noticeable oxidation occurring around 4 h after exposure to air. Ca3SiN3H forms as light or dark orange crystals, with apparent color depending on crystal thickness; this phase was produced as an inconsistent byproduct in reactions aimed at optimizing Ca8In2SiN4. Further reactions without indium resulted in an increase in yield and purity of Ca3SiN3H, but the yield increased even more with the further addition of CaH2. The yield of Ca3SiN3H was optimized at 42% from Ca/ Li/Si/Ca3N2/CaH2 reactions in a 7/8/1/1.5/0.5 mmol ratio (matching the reactant ratio of Si, N, and H to that of the product compound). Only slight amounts of Ca2NH formed as a byproduct; additional byproducts might be lost during centrifugation at 500 °C. Formation of Ca3SiN3H results from hydride contaminants present in calcium metal. Previous compounds synthesized by this group including Ca54In13B4‑xH23+x, Ba12InC18H4, LiCa3As2H, and Ca2LiC3H also stem from hydride incorporation into products when calcium is used as purchased.17,18,21,22 Reactions similar to those which produced Ca3SiN3H were done with purified calcium to eliminate hydride contamination from the calcium metal and to explore what products form in the absence of a hydride source. Indeed, the use of purified calcium prevented formation of Ca3SiN3H and instead produced α-Ca5Si2N6 and an unidentified silvery metallic cubic phase. Structure of Ca8In2SiN4. Ca8In2SiN4 crystallizes with a new structure type in orthorhombic space group Ibam (Figure 2). The structure is comprised of isolated [SiN4]8− tetrahedra as well as indium dimers, both surrounded by calcium cations.

Figure 1. Microscope images of flux-grown products. (a) Ca8In2SiN4 rod-shaped crystals on 1 mm grid paper. Bottom crystal has a small octahedron-shaped crystal (unidentified minor byproduct) adhered to its surface. (b) Thin crystals of Ca3SiN3H and (c) thick crystals of Ca3SiN3H.

The isolated [SiN4]8− unit is extremely rare, with Ca4SiN4 being the only other known compound featuring this species.26 The Si−N bond lengths in the SiN4 tetrahedron are each 1.762(2) Å, which is within the ranges reported for Ca4SiN4 (1.767(4)−1.833(4) Å), α-Ca5Si2N6 (1.713(3)−1.831(3) Å), β-Ca5Si2N6 (1.734(5)−1.838(7) Å), and SrSiN2 (1.709(6)− 1.788(6) Å).26−28 The SiN4 units deviate slightly from ideal tetrahedral symmetry, with angles ranging from 105.26(15)− 111.68(15)°. Ca−N bond lengths vary from 2.322(2)− 2.620(2) Å, which are in the range of bond lengths reported for Ca4SiN4 (2.361(4)−3.078(4) Å), β-Ca5Si2N6 (2.368(5)− 2.927(5) Å), and CaSiN2 (2.406(3)−3.022(3) Å).26−28 The indium dimers in Ca8In2SiN4 (Figure 2d) are a unique structural feature. These dimers are slanted at a 29° angle to the ac-plane and alternate down the c-axis along a 21 screw-axis. In−In bonding is not uncommon in intermetallics and occurs as 1-D zigzag chains (Ca4In2N and Ca2InN), 3D nets (NaIn, CaIn2, SrIn2, and BaIn2), and ethane-like units Pn3In-InPn3, where Pn = pnictide, which may be eclipsed or anti conformation (CaIn2P2, SrIn2P2, EuIn2P2, Ba2In5P5, and Ba2In5As5).29−34 While indium does form In−In dimers within ethane-like units, or in Ge−In−In-Ge- zigzag chains within SrCa2In2Ge, isolated indium dimers surrounded by electropositive cations are unknown.35 Structures of binary intermetallics of indium and electropositive metals (such as CaIn2 and SrIn2) usually contain a 3-D network of In−In bonding.31 The presence of another p-block element such as a tetrelide or pnictide usually results in covalent bonding between the tetrelide or pnictide and indium. Ca8In2SiN4 is unusual in that it is a quaternary compound in which indium is in the presence of both a tetrelide and pnictide, but bound to neither. Rather than forming Si/In/N anionic building blocks surrounding calcium cations, tetrelide-pnictide anionic SiN4 units and 9363


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while the nitrogen bonds to silicon to form an anionic nitridosilicate unit. Each indium dimer is surrounded by nine calcium atoms with In−Ca bond lengths ranging from 3.2545(9)−3.8793(6) Å. Ca−In bonds within 3.2545(9)− 3.4339(6) Å are comparable to CaIn2 (3.24−3.64 Å), Ca4In2N (3.34−3.59 Å), and Ca2InN (3.46−3.66 Å).29−31 Larger Ca−In bonds, 3.8793(6) Å, are a result of a calcium site (Wyckoff site 8g) bridging both indium atoms in the dimer. Ca8In2SiN4 may be viewed as a void metal, a classification in between ionic and intermetallic compounds. Void metals contain both anionic and metallic regions within the same compound. Classic examples are the suboxides and subnitrides studied by the Simon group, including Cs11O3 and NaBa3N which feature O2− and N3− anions surrounded by a sea of excess alkali or alkaline earth atoms.36 The presence of the negatively charged anionic regions within the structure repels the conduction electrons, which are confined to the metallic regions. This confinement may cause a low work function, which makes void metals potential candidates as IR sensors. The class of “void metals” may be expanded to include subvalent hydrides and carbides, as exemplified by Ca54In13B4‑xH23+x and Ca12InC13‑x.17,18 Ca8In2SiN4 offers the first example of a void metal with a complex nitridosilicate anion; DOS calculations also support this (vide infra). Structure of Ca3SiN3H. Ca3SiN3H crystallizes in monoclinic space group C2/c (Figure 3). While Ca8In2SiN4 can be viewed as an “orthonitridosilicate”, analogous to the orthosilicate class of oxides (structures with isolated SiO44− groups surrounded by cations, exemplified by olivine, Mg2SiO4), Ca3SiN3H is an “inonitridosilicate”, being comprised of SiN4 tetrahedra corner-sharing to form chains. This is analogous to the inosilicates (such as enstatite, MgSiO3) and is highlighted by the 1:3 ratio of Si/N in the formula. It is comparable to another inonitridosilicate, Eu2SiN3.37 Both structures contain 1D infinite unbranched chains with two polyhedra per repeat unit of the chain. The nitridosilicate chains of Ca3SiN3H propagate along the a-axis with the SiN4 tetrahedra exhibiting a stretching factor of fs = 0.974, similar to that of Li5La5Si4N12 ( fs = 0.977) and slightly smaller than that of Eu2SiN3 (fs = 1.00).37,38 The stretching factor of a chain of tetrahedral units is indicative of its deviation from a maximum stretched chain (fs = 1.00). In between the chains are divalent cations (calcium cations in the case of Ca3SiN3H) balancing the charge. Ca3SiN3H differs from Eu2SiN3 in that the c-axis of Ca3SiN3H has been elongated to make room for layers of distorted edgesharing H@Ca6 octahedra, which wedge between the nitridosilicate chains. Hydride sites surrounded by six calcium cations (H@Ca6) are frequently observed in complex metal hydrides, including Ca54In13H27, Ca6[Cr2N6]H, and LiCa11Ge3OH4.18,39,40 The Ca−H bond lengths in these compounds span the range from 2.30(3) to 2.75(5) Å. The H@Ca6 octahedra in Ca3SiN3H are distorted, with five typical Ca−H bonds (2.49(3)−2.73(3) Å) and one longer Ca−H bond (2.87(3)Å), see Figure 3c. Within the nitridosilicate chain, Si−N bond lengths range from 1.713(2)−1.759(5) Å. These bond lengths are within the ranges of 1.734(5)−1.838(7) Å found in β-Ca5Si2N6 and 1.713(3)−1.831(3) Å found in αCa5Si2N6.26,27 Ca−N bond lengths in Ca3SiN3H range from 2.336(4)−2.861(5) Å. These bond lengths are within the ranges of Ca 4 SiN 4 (2.361(4)−3.078(4) Å), β-Ca 5 Si 2 N 6 (2.368(5)−2.927(5) Å), and CaSiN2 (2.406(3)−3.022(3) Å).26,28

Figure 2. Structure of Ca8In2SiN4: calcium represented by blue spheres, indium by red, silicon by green, and nitrogen by black spheres. (a) Orthorhombic structure of Ca8In2SiN4 viewed down the c-axis. (b) Structure viewed down the b-axis. “Layers” of alternating metallic and ionic regions are evident. (c) Coordination environment of SiN4 tetrahedra. (d) Coordination environment of indium dimer.

isolated indium dimers form instead, separated from each other by a sea of calcium cations. The indium−indium bond length is 3.0654(7) Å. This is contrary to shorter bond lengths found in compounds such as Ba2In5Pn5 (Pn = P, As; In−In = 2.750(1)−2.776(1) Å and 2.757(1)−2.777(1) Å respectively) and AIn2Pn2 (A = Eu, Ca, Sr, or Ba); In−In = 2.7608(10), 2.7625(19) Å, 2.7620(13) Å, and 2.7460(10)−2.8161(10) Å respectively), where indium atoms are in a more covalent environment bonded to pnictides.32−34 Zintl phases such as NaIn, CaIn2, and BaIn2 where indium forms 3-D nets have comparatively longer In−In bond lengths (3.175 Å, 2.92−3.13 Å, and 2.97−3.12 Å respectively).31 These bond lengths are comparable to the indium dimers within Ca8In2SiN4. Indium will form bonds with nitrogen to form the binary InN, but generally does not bond to nitrogen in ternaries or higher order compounds.29 This is evidenced herein where indium bonds to itself and participates in metallic bonding, 9364


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Figure 3. Structure of (a) monoclinic Ca3SiN3H compared to (b) orthorhombic Eu2SiN3, both viewed down the a-axis. Calcium/europium cations are blue, silicon is green, nitrogen is black, and hydrogen is orange. (c) H@Ca6 distorted octahedron with bond lengths. (d) View of 1D Si−N chain perpendicular to the propagation direction.

Electronic Structure of Ca8In2SiN4. The presence of recognizable cations and nitridosilicate anions (as well as the charge-balanced nature of Ca3SiN3H, vide infra) invites an attempt to charge-balance Ca8In2SiN4. However, the eight Ca2+ and a [SiN4]8− in the formula unit mandates a −8 charge on the indium dimer, requiring each indium to have a net valence charge of −4. DiSalvo et al. reported examples of nitrides containing indium with a negative charge: Ca2InN (In1−) and Ca4In2N (In2.5‑) both feature indium 1-D zigzag chains surrounded by calcium atoms; the indium−indium bond lengths increase from 2.93 to 3.16 Å as the negative charge on indium increases.29 These bond lengths are comparable to the In−In bond lengths found within Ca8In2SiN4. It would therefore make sense that the indium dimers accumulate negative charge from calcium electron donation. However, due to the lack of effective core potential required of indium to achieve a −4 charge, it is highly unlikely that the indium dimer can be described as an [In2]8− Zintl polyanion. Similarly, the compound Mg2Ga contains gallium dimers (which could ostensibly be viewed as [Ga2]8− to balance the charge) but it is clearly metallic.41 Further evidence of the lack of charge-balancing in Ca8In2SiN4 are the metallic luster of the crystals and the DOS diagram shown in Figure 4, both of which indicate the compound is metallic. Zintl phases typically exhibit either a true semiconducting gap or a pseudogap at EF resulting from their charge-balanced nature; this is not seen for Ca8In2SiN4,

indicating it is not a Zintl phase. The states at the Fermi level (from 0 to −2 eV) are dominated by the indium dimers and surrounding calcium ions. The states contributed by the silicon and nitrogen orbitals of the SiN48− units are significantly lower in energy (−3 to −5 eV), supporting the isolated and largely anionic nature of these species. This data supports the “void metal” model of this compound; charge transport will occur within the layers of indium dimers and surrounding calcium ions, and the conduction electrons will avoid the anionic nitridosilicate species. Similar electronic characteristics are found in our previously reported work on Ca54In13B 4‑xH 23+x, Ba12InC18H 4, and Ca12InC13‑x.17,18 Bailey and DiSalvo also reported DOS data for Ca2InN, which showed similar behavior.29 In each of the aforementioned compounds, Ca−In interactions make up the metallic regions, while Ca−X interactions make up the ionic regions (X = H, C, or N). Crystal orbital Hamiltonian population calculations (COHP) were carried out to investigate the nature of the bonding within and around the indium dimer. In Figure 5 it is evident that all Ca−In interactions are bonding at the Fermi level, signifying stable interactions. In−In interactions are also bonding below EF and antibonding above EF, further contributing to the stability of this unit. Similarly, the indium zigzag chain found in Ca2InN has In−In bonding interactions at the Fermi level.29 Electronic Structure of Ca3SiN3H. The stoichiometry of Ca3SiN3H is charge-balanced (3Ca2+ + Si4+ + 3N3− + H−), making it a valence-precise semiconductor. This is supported by its yellowish-orange color which indicates a bandgap of 2.5−3.0 eV (the material is too air-sensitive to collect UV−vis absorbance data). Calculations indicate a band gap of 3.1 eV (Figure 6). A band gap of 3.1 eV is at the edge of the visible part of the electromagnetic spectrum, which correlates well with the appearance of these crystals. It should be noted that the TB-LMTO-ASA software has a tendency to underestimate the bandgap, so Eg for Ca3SiN3H would be expected to be slightly higher than 3.1 eV.42 The orange color of the crystals may therefore indicate the presence of point defects such as dopants or substitutions. Most states above the Ef are from empty valence states formed from calcium donating valence electrons to nitrogen and hydrogen. Most states just below Ef are from hydride and

Figure 4. Density of states data for Ca8In2SiN4. The Fermi level is marked with the dashed line. 9365


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nitridosilicate compounds may require an excess of highly reducing media to surround and stabilize SiN4 monomers or 1D chains and prevent their polymerization into 2-D or 3-D networks.15,26 Electronic structure calculations reveal the void metal nature of Ca8In2SiN4. The segregation of metallic and ionic regions may lead to unusual electronic properties; XPS/UPS studies are planned to measure the work function of this compound. Low work function materials are potential candidates for infrared sensors. The synthesis of isovalent analogues of Ca8In2SiN4 (i.e., Sr8In2SiN4, Ca8Ga2SiN4, or Ca8Tl2SiN4) have been unsuccessful so far, but further explorations are needed. Ca3SiN3H is charge-balanced and electronic structure calculations reveal an indirect band gap of 3.1 eV, rendering its use as a semiconductor material unlikely. However, nitridosilicates with large band gaps are useful as host lattices for phosphors such as Eu2+ or Ce3+, which may be used in white LEDs.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 5. Crystal orbital Hamiltonian population (COHP) data for the indium dimer within Ca8In2SiN4. Fermi level is set at 0 eV; bonding states are plotted as positive COHP values. Top: In−Ca interactions and bottom: In−In interactions. Interactions are color coordinated with the coordination environment of the indium dimer shown in the inset figure.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01532.

nitride valence states as well as associated states from surrounding calcium atoms. Hydride anion states are found from 0 to −2 eV, and the nitridosilicate states become dominant at lower energies (−3 to −5 eV), similar to the energy range of the SiN4 states in Ca8InSiN4. The band structure shows an indirect gap; the highest occupied states are located between the V and Z vector, and the lowest unoccupied states are along the Z vector.

Accession Codes

Tables of atomic positions and thermal displacement factors for Ca8In2SiN4 and Ca3SiN3H (PDF) CCDC 1558214−1558215 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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CONCLUSIONS Ca8In2SiN4 and Ca3SiN3H are new low-dimensional nitridosilicates grown in metal flux. The isolated SiN4 units found in Ca8In2SiN4 are extremely unusual, in comparison to the large number of examples of network nitridosilicates that feature highly stable 2-D and 3-D linkages of corner-sharing SiN4 groups. On the basis of our work and the reported sodium flux synthesis of Ca4SiN4, the formation of low dimensional

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Susan E. Latturner: 0000-0002-6146-5333 Notes

The authors declare no competing financial interest.

Figure 6. Band structure and density of states diagram of Ca3SiN3H. The Fermi level is set at zero. 9366


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Inorganic Chemistry

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ACKNOWLEDGMENTS This research was supported by the Division of Materials Research of the National Science Foundation (Grant Number DMR-14-10214). This work utilized the scanning electron microscope facilities of the Biological Sciences Imaging Resource (BSIR) in the FSU Biology Department; we thank Dr. Eric Lochner for his assistance with this equipment.

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Li Flux: Void Metal

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