Justin
Mark
ab,
Kathleen
Lee‡
c,
Maxwell A. T.
Marple
d,
Shannon
Lee
ab,
Sabyasachi
Sen
d and
Kirill
Kovnir
*ab
aDepartment of Chemistry, Iowa State University, Ames, Iowa 50011, USA. E-mail: kovnir@iastate.edu
bAmes Laboratory, U.S. Department of Energy, Ames, Iowa 50011, USA
cDepartment of Chemistry, University of California, Davis, Davis, CA 95616, USA
dDepartment of Materials Science and Engineering, University of California, Davis, Davis, CA 95616, USA
First published on 9th January 2020
Two novel ternary phases, LiSi3As6 and Li2SiAs2, have been synthesized and characterized. Both phases have an identical Si:
As ratio of 1
:
2 providing insight on how layers of the parent phase SiAs2 accommodate excess electrons from Li cations to form Si–As anionic frameworks. LiSi3As6 exhibits a variety of bonding schemes involving Si–Si and As–As bonds, as well as corner-sharing SiAs4 tetrahedra, while Li2SiAs2 is isostructural to the previously reported Li2SiP2, with adamantane-like Si4As10 units connected into 3D framework. LiSi3As6 and Li2SiAs2 are predicted to be indirect semiconductors which was experimentally confirmed by optical properties characterization. Li2SiAs2 exhibits low thermal conductivity of 1.20 W m−1 K−1 at 300 K in combination with a room temperature ionic conductivity of 7 × 10−6 S cm−1, an order of magnitude greater than that of the phosphide and nitride analogues, indicating its potential as a solid-state Li-ion conductor.
Given the many similarities in phosphide and arsenide chemistry, we explored the Li–Si–As system for new phases to investigate further reduction of the ionicity of Li–X bonds and changes in Li conductivity. Upon our exploration of Li intercalation into the binary layered SiAs we reported a novel layered compound, Li3Si7As8, which was only the second phase ever reported in this ternary phase space.20 Based on the existence of another binary silicon arsenide, SiAs2, we kept the Si:
As ratio as 1
:
2 and studied how the Si–As anionic framework would react to different concentrations of Li cations (Fig. 1). This has resulted in the discovery of two new phases, LiSi3As6 (=Li0.33SiAs2) and Li2SiAs2, with the latter being isostructural to previously reported Li2SiP2.17,18 LiSi3As6 exhibits an intricate structure and bonding demonstrating that the complex bonding motifs frequently exhibited by silicon-phosphides are also available to the arsenide counterparts. Si–As frameworks in both reported crystal structures differ from that in the Li-free SiAs2 due to the extra electrons provided by Li cations causing Si–As bonding rearrangements. In this work we report on the syntheses and crystal structures of the title phases and detailed characterization of heat transport properties and Li-conductivity of Li2SiAs2.
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Fig. 1 Ternary phase diagram for the Li–Si–As system. SiAs2, LiSi3As6, and Li2SiAs2 are shown as stars connected with a red line, while Li5SiAs3 and Li3Si7As8 are shown as circles.19,20 The labels of the compounds correspond to the stoichiometric ratios of Li![]() ![]() ![]() ![]() |
A single crystal of LiSi3As6 was first obtained by loading elemental Li, Si, and As into a carbonized silica ampoule (9/11 mm inner/outer diameter) in a 0.8:
3
:
3 ratio, respectively. The ampoule was then flame sealed under vacuum. The initial ampoule was then placed into a larger silica ampoule and flame sealed under vacuum to create a protective outer jacket. The ampoule was placed in a muffle furnace and heated to 923 K over 48 h. The sample was annealed at this temperature for 240 h and then slowly cooled to room temperature over 48 h. It was subsequently found that LiSi3As6 could be synthesized by a stoichiometric mixture of the elements, under the same conditions, however the sample was still contaminated with significant amounts of SiAs2, Si, and As admixtures. Reducing the reaction temperature to 823 K prevents SiAs2 formation, however unreacted Si and As remain. Further attempts to synthesize single phase samples by varying the temperature profile were unsuccessful.
Li2SiAs2 was first synthesized by loading elemental Li, Si, and As in a 0.5:
1
:
2 ratio into a Nb ampoule, which resulted in a mixture of Li2SiAs2 as well as SiAs, Si, and As. Single phase samples of Li2SiAs2 were prepared using a stoichiometric ratio of the elements sealed in a Nb ampoule under Ar atmosphere. The Nb ampoule was then sealed inside a silica ampoule under vacuum to prevent Nb oxidation during annealing. The ampoule was placed in a muffle furnace and heated to 523 K over 24 h, annealed at this temperature for 24 h, and heated to 923 K over 24 h, and annealed at this temperature for 96 h. The furnace was then turned off and the furnace was cooled to room temperature. The product is a red powder that degrades after several minutes of exposure to ambient conditions.
Single crystal XRD was performed using a Bruker AXS SMART diffractometer and Bruker D8 Venture diffractometer, both utilizing Mo-Kα radiation. Datasets were collected at 90 K under a N2 stream and under ambient conditions with ω-scans recorded with a 0.3° step width and integrated with the Bruker SAINT software. Structure determination and refinement of the crystal structures were carried out using the SHELX suite of programs.21 Further details of the crystal structure determination may be found through Cambridge Crystallographic Data Centre by using CCDC-1954349 (Li2SiAs2) and CCDC-1954350 (LiSi3As6).†
Determination of the ionic transference number was accomplished by using the dc polarization technique on a cell of Ag/Li2SiAs2/Li. A Keithley 230 voltage source in series with a Keithley 617 electrometer was used to simultaneously apply 5 mV and measure the current response with time. The polarity of the cell (+)Ag/Li2SiAs2/Li(−) results in depletion of Li ions at the Ag electrode/Li2SiAs2 interface allowing only electronic charge carriers as the source of electrical current. Comparing the initial current when the voltage was activated to the current after reaching steady state provides the electronic contribution to conductivity and consequently the ionic transference number through, tion + telec = 1.
Differential scanning calorimetry (DSC) was used to investigate the thermal stabilities of the title phases. DSC of LiSi3As6 exhibits two peaks upon heating, the first at 801 K is attributed to the sublimation of elemental arsenic impurities in the sample, while the peak at 995 K is the melting/decomposition of LiSi3As6; however no sharp peaks corresponding to recrystallization were observed upon cooling (Fig. S1† top). PXRD of the sample after the DSC measurement shows the major phase to be SiAs2, with a Si impurity which was in the initial sample (Fig. S2†). DSC experiment indicates that upon heating a peritectic decomposition of LiSi3As6 into SiAs2 took place. The DSC experiment was performed in a silica container and the released Li may have reacted with silica at high temperature. The melting point of SiAs2 is 1250 K which is higher than the maximum temperature used in the DSC experiment. Thus, SiAs2 was not melted and no sharp crystallization peaks were observed upon cooling. For Li2SiAs2, DSC shows the onset of melting at 1167 K with a sharp crystallization peak beginning at 740 K (Fig. S1† bottom). The melting temperature was higher than the synthesis temperature of 923 K. PXRD of the products after the DSC measurement shows that the sample contains mainly Li2SiAs2 with SiAs impurities, most likely due to the Li reacting with the silica ampoule during heating.
LiSi3As6 crystallizes in the orthorhombic Cmce space group (No. 64), Pearson symbol oS-80. Similar to a number of complex ternary silicon phosphides,33–40 LiSi3As6 exhibits several different bonding schemes including corner sharing SiAs4 tetrahedra, Si–Si dumbbells, and As–As bonds. The structure is composed of alternating planes of SiAs4 tetrahedra and Si2As6 antiprisms along the b-direction (Fig. 3A and B). The SiAs4 tetrahedra have Si–As distances of 2.316(2)–2.405(2) Å, similar to distances reported in compounds such as Cs5SiAs3, Li3Si7As8, and CeSiAs3, among others.20,32,41–47 These tetrahedra are interconnected through three different bonding arrangements to form a slab, shown in Fig. 3D. Along [100] direction, chains of corner sharing tetrahedra are further linked through alternating As–As bonds which are 2.512(2) Å in length. These chains are combined into a slab by perpendicular As–As bonds of 2.445(1) Å along the [001] direction between neighboring chains (Fig. 3D). Both As–As distances are within reported lengths observed for As–As bonds.41,42,48–51
The plane of Si2As6 antiprisms is formed through isolated Si2As6 units, which are surrounded by 6 Li atoms in the ab-plane, shown in Fig. 3C. The Si–Si distance in these antiprisms is 2.341(6) Å, while the Si–As distances measure 2.367(2)–2.387(3) Å, all within typical reported distances taking into account covalent radii for Si (1.17 Å) and As (1.22 Å).20,32,36,41–47,52–54,58 The Li atoms within this plane have distorted octahedral coordination with Li–As distances of 2.83(2)–2.95(2) Å, which fall within reported values.19,20,55–57 All As atoms in the Si2As6 units are corner shared with SiAs4 tetrahedra from neighboring slabs along the b-direction, creating the complex 3D structural arrangement.
LiSi3As6 has a Wyckoff sequence of g3f2ed. Other compounds with this Wyckoff sequence include Ta6Br14, Ta6I14, CuP4Se4I, and MnTl2As2S5.59–62 Ta6Br14 and Ta6I14 contain isolated clusters forming 1D structures, while CuP4Se4I crystallizes in a 2D layered structure.59–61 Previously MnTl2As2S5 was the only 3D phase reported with this Wyckoff sequence, however this structure lacks homoatomic bonds like the Si–Si bonds found in LiSi3As6.62
Li2SiAs2 crystallizes in the tetragonal space group I41/acd (No. 142) in the same structure type as Na2SnAs2 and Li2SiP2.17,18,63 Adamantane-like Si4As10 units are connected to each other through shared As atoms (Fig. 4). In the structure, each Si atom is coordinated by four As atoms forming a tetrahedron while each As is coordinated by two Si atoms (Fig. 4C). Li atoms are tetrahedrally coordinated by four As atoms (Fig. 4D). The Si–As bond distances found in Li2SiAs2 are in a much narrower range compared to LiSi3As6, falling between 2.3424(8)–2.3778(7) Å. These bond lengths fall in the mid-range of those found in LiSi3As6, matching common lengths in other compounds.20,32,41–47 The Li–As distances range from 2.516(3)–2.853(1) Å and are shorter than those found in LiSi3As6, however they span distances found for other reported Li–As containing compounds and are not nearly as short as distances found in Li3Si7As8.19,20,55–57 Larger alkali metals, A = K, Rb, and Cs, form 2–1–2 compounds, A2SiAs2.32,44,64 In the crystal structure of these compounds one-dimensional anionic [SiAs2]2− chains composed of edge-sharing SiAs4 tetrahedra are present. A reduction of the cation size leads to rearrangements of the same building units, SiAs4 tetrahedra, into a three-dimensional framework.
We have previously shown that the parent SiAs2 binary compound is able to accommodate ∼0.16 cations as large as Cs per formula unit in the interlayer space forming Cs0.16SiAs2−x without significant modification of the 2D SiAs2 layers, given that the extra electrons are compensated either by As vacancy formation or by aliovalent M/Si substitutions (M = Ga, Zn, Cu).30 Further increase of the cation content leads to the structural reconstruction in the Si–As framework. Doubling the cation content in the case of LiSi3As6 (=Li0.33SiAs2) results in a collapse of the layered structure of SiAs2 into a 3D framework with the formation of Si–Si bonds. However, LiSi3As6 maintains the SiAs4 corner-sharing and As–As bonding within a “layer,” similar to the Si–As layers found in SiAs2.42 The Li2SiAs2 stoichiometry results in more drastic structural rearrangements due to the excess electrons added to the Si–As network. This results in the removal of homoatomic bonds, as the Li2SiAs2 no longer contains any Si–Si or As–As bonds. Partial removal of homoatomic bonds was also observed for the Li incorporation into SiAs to form Li3Si7As8.20
Considering a polar bonding scheme for LiSi3As6, each tetrahedral Si can be considered as Si4+, while each Si in the antiprism can be considered as Si3+. Similarly, As coordinated to three or two Si atoms is As3−, while As atoms forming one or two bonds to As atoms are As2− and As−, respectively. This bonding scheme results in the electron balanced composition of (Li+)(Si4+)2(Si3+)(As3−)2(As2−)2(As−)2. Alternatively, the Si–As bonds can be considered as covalent bonds where the electron pair is shared between As and Si. In such a description four-coordinated Si atoms and three-coordinated As atoms have oxidation states of 0, while two-coordinated As atoms have formal oxidation states of −1, leading to the electron-balanced formula (Li+)(Si0)3(As0)5(As−). For Li2SiAs2, both polar and covalent assignments of the oxidation states result in charge balanced compositions of (Li+)2(Si4+)(As3−)2 and (Li+)2(Si0)(As−)2, respectively.
To gain a better understanding of the bonding in these phases electronic structure calculations were performed using the TB-LMTO-ASA program.22 From the band structures and DOS, LiSi3As6 and Li2SiAs2 were found to be indirect semiconductors with bandgaps of 0.97 eV and 1.43 eV, respectively (Fig. 5). For Li2SiAs2 both direct transitions at the Z and Γ points have larger energy of 1.67 eV, while for LiSi3As6 the direct transition at the Γ point is 1.31 eV. The density of states for both phases exhibit significant contributions from Si and As in the conduction and valence bands, suggesting strong covalent bonding in the Si–As frameworks. For Li2SiAs2 electron localization function analysis shows the expected covalent bonding between Si and As with two electron lone pairs on each As atom (Fig. 6). This is expected because each As is coordinated to two silicon atoms. This result favors the covalent description of the chemical bonding in Li2SiAs2 as (Li+)2(Si0)(As−)2.
Analysis of the chemical bonding in LiSi3As6 also supports the covalent nature of the Si–As interactions. Si1 atoms are engaged in the three covalent Si–As bonds in addition to the Si1–Si1 covalent bond, while Si2 atoms form four Si–As bonds similar to Si atoms in Li2SiAs2 (Fig. 7A). As4 forms three As–Si bonds and has an electron lone pair (Fig. 7B). Every As atom in the SiAs4 pentagonal ring fragment has one electron lone pair in addition to two As–Si + one As–As bonds (for As1) or two As–As + one As–Si bonds (for As3) (Fig. 7C). Finally, two coordinated As2 is an analogue of the As atoms in Li2SiAs2, possessing two-electron lone pairs in addition to two As–Si bonds, which supports its description as As− (Fig. 7B). Li atoms are not engaged in the covalent bonding and have spherical distribution of ELF corresponding to core 2s electrons (Fig. 7D).
To confirm the theoretical bandgap of Li2SiAs2 diffuse reflectance spectroscopy was utilized to determine the experimental bandgap. Measurements were performed on pellets sealed under argon inside polypropylene bags to prevent sample oxidation during data collection. Tauc plots of Li2SiAs2 show that it has an indirect transition of less than the instrument limit of 1.1 eV and a direct transition of 1.40 eV (Fig. 8). The large bandgaps are supported by quantum chemical calculations and by the red-brown color of the crystals. Due to the black color of LiSi3As6 and Si impurities in samples the experimental band gap was not measured for LiSi3As6.
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Fig. 8 Solid-state UV-vis Kubelka–Munk diffuse reflectance spectrum of Li2SiAs2 (left). Tauc plots for allowed direct and indirect transitions (right). |
Presence of significant amounts of Si and As impurities in the samples of LiSi3As6 prevented property characterization for this silicon-arsenide. However, Li2SiAs2 was synthesized near single-phase and its thermal, charge, and ion conductivities were studied in detail. Li2SiAs2 is expected to have a low thermal conductivity due to the large number of atoms in the unit cell and overall complexity of the crystal structure which should result in complex phonon structure. The measured temperature-dependent thermal conductivity has a typical trend for crystalline compounds with a peak at 50 K and decreasing thermal conductivity as a function of increasing temperature at higher temperatures due to Umklapp phonon–phonon scattering (Fig. 9). At 300 K the thermal conductivity reaches a value of 1.20 W m−1 K−1. The electrical resistivity of Li2SiAs2 was too high to be measured using the Physical Property Measurement System. We estimated the electrical resistivity to be above 10 kΩ m at room temperature based on our previous experience with highly resistive compounds and PPMS.65,66 The impedance measurements confirmed high dc electrical resistivity of the Li2SiAs2 to be over 70 kΩ m.
Given the isostructural nature of Li2SiAs2 to Li2SiP2 and the promising Li-ion conductivity observed in the latter,17,18 the Li-conductivity of a pressed pellet of Li2SiAs2 was characterized. Fig. 10 shows the temperature-dependence of the bulk electrical conductivity for Li2SiAs2 with a room temperature conductivity of 7 × 10−6 S cm−1. The impedance curves are provided in the Fig. S4 and S5.† This conductivity is over an order of magnitude greater than that of the analogous phosphide (4 × 10−7 S cm−1) and nitride (8 × 10−8 S cm−1) phases,17,67 indicating the potential of Li2SiAs2 as a solid-state Li-ion conductor. This conductivity may be further enhanced through trivalent doping, as predicted for Li2SiP2 by Yeandel et al.68 or even by increasing the Li content as seen by enhanced performance when going from Li8SiP4 to the related Li14SiP6.15 The activation energy for conduction is calculated from the slope of conductivity using an Arrhenius expression of σ(T) = σ0exp(−Ea/kT) and found to be Ea = 0.53 eV. DC polarization experiments reveal the ionic transference number is 0.98, indicating the conductivity is almost entirely ionic in character, which is a prerequisite for solid ionic conductor applications in Li-ion batteries. A combination of high Li-ionic conductivity with low electrical conductivity suggests Li2SiAs2 is a promising solid-state Li-ion conductor.
Footnotes |
† Electronic supplementary information (ESI) available: Experimental details, figures, and tables pertinent to powder and single crystal X-ray diffraction, SEM/EDXS, differential scanning calorimetry results, impedance curves for Li2SiAs2. CCDC 1954349 and 1954350. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9ta11150f |
‡ Current Address: Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena CA 91109, USA. |
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