The cubic structure of Li₃As stabilized by substitution – Li₈TtAs₄ (Tt = Si, Ge) and Li₁₄TtAs₆ (Tt = Si, Ge, Sn) and their lithium ion conductivity

Abstract

The new lithium arsenidotetrelates Li₈SiAs₄, Li₈GeAs₄, Li₁₄SiAs₆, Li₁₄GeAs₆ and Li₁₄SnAs₆ were synthesized via ball milling and structurally characterized by Rietveld analysis of X-ray powder diffraction data. The aliovalent substitution of lithium in hexagonal Li₃As by introducing a tetravalent tetrel cation stabilizes cubic structures for Li₈TtAs₄ (Tt = Si, Ge) in the space group Pa3̄ and for the lithium richer compound Li₁₄TtAs₆ (Tt = Si, Ge, Sn) in the higher symmetrical space group Fm3̄m (no. 225). Thermal properties of the arsenidotetrelates were investigated via high temperature powder diffraction and differential thermal analysis revealing a decomposition process of the lithium richer arsenidotetrelate (Li₁₄TtAs₆ → Li₈TtAs₄ + 2Li₃As) into the lithium poorer arsenidotetrelates and lithium arsenide at moderate temperatures. Impedance spectroscopy shows moderate to good lithium ion conductivity for the lithium arsenidotetrelates.

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Introduction

Lithium phosphidotetrelates have become an interesting and intensely discussed family of compounds over the last years, because of their interesting structural diversity and high ionic conductivity.^1–3 First mentioned in the 1950s by Juza, the nitridosilicates, phosphidosilicates and arsenidosilicates received little attention.^4,5 In 2016, the compounds Li₂SiP₂ and LiSi₂P₃ were synthesized and characterized independently by Johrendt et al. and by Fässler et al. who also discovered Li₈SiP₄ in the same year.^3,6 Exchanging silicon by germanium leads to Li₈GeP₄ which crystallizes in two modifications, one of them is isostructural to Li₈SiP₄.⁷ In 2019 the group of Fässler reported the hitherto most lithium-rich phosphidosilicate Li₁₄SiP₆ and its high lithium conductivity up to 1 mS cm⁻¹ at room temperature.¹ Replacing silicon for germanium or tin subsequently led to the discovery of Li₁₄GeP₆ and Li₁₄SnP₆ in 2020.⁸ Arsenidosilicates, the corresponding heavier homologues to the phosphidosilicates, are known since the 1950s, but were only scarcely mentioned since then. Kovnir et al. synthesized the ternary compound Li_1−xSn_2+xAs₂ (0.2 < x < 0.4) via solid-state reaction of the elements and Li_0.9Ge_2.9As_3.1 and Li₃Si₇As₈ by electrochemical lithiation of the tetrel arsenides GeAs and SiAs.^9,10 One year later the same group reported on the silicon compounds Li₂SiAs₂ featuring adamantane-like Si₄As₁₀ units and LiSi₃As₆ build up by corner-sharing SiAs₄-tetrahedra.¹¹ In 2022 we could proof ion conductivity for Li₃As.¹² DFT modelling and conductivity experiments on Li₃P and Li₃As show significant similarities between the two compounds. Recently our group presented the lithium arsenidotrielates Li₃AlAs₂, Li₃GaAs₂ and Li₃InAs₂ showing, that we can exchange phosphorus by arsenic not just in binary, but also in ternary compounds.¹³ With the new compounds Li₈TtAs₄ (Tt = Si, Ge) and Li₁₄TtAs₆ (Tt = Si, Ge, Sn) discussed here we expand the family of arsenidotetrelates to higher lithium content showing similar structural features and properties like the phosphide-based compounds. These ternary phases can be understood as an aliovalent substitution of four lithium cations in Li₃As by one tetrel cation (Li₁₂As₄ → Li₈Tt⁴⁺As₄, Tt = Si, Ge; Li₁₈As₆ → Li₁₄Tt⁴⁺As₆, Tt = Si, Ge, Sn). The formal aliovalent substitution of a tetrel cation in hexagonal Li₃As forces the ternary phase into a cubic crystal system. Both crystal structures of Li₈TtAs₄ and Li₁₄TtAs₆ can be derived from the antifluorite structure type or alternatively can be described as a deficient Li₃Bi structure type. The formal substitution of Li⁺ ions in Li₃As by a highly charged cation resulting in a cubic structure was first described for transition metals. The systems Li–M–Pn (M = Ti, Zr, Hf; Pn = P, As, Sb, Bi) investigated by Adam and Schuster show the formation of ternary cubic phases stabilized by the substitution of lithium with four valent titanium, zirconium or hafnium. Monconduit et al. describe the same stabilizing effect for the Nb⁴⁺-cation substituting four lithium cations in Li₃As leading to a cubic antifluorite-type ternary phase (Li₁₂As₄ → Li₇M⁵⁺As₄).^14,15 The new phases in the systems Li–Tt–As (Tt = Si, Ge, Sn) show that this substitution also works for tetrels, i.e., main group elements. The crystal structures of the new arsenidotetrelates were determined by Rietveld refinement of X-ray powder diffraction (XRPD) data. High temperature XRPD and differential thermal analysis (DTA) reveal structural changes for different thermal conditions. Further, impedance spectroscopy proves good lithium ion mobility.

Results and discussion

Synthesis and crystal structure determination

The synthesis of the compounds under discussion was performed via ball milling of stoichiometric amounts of the elements and subsequent annealing of the pre-reacted mixture at elevated temperatures. Samples with the composition Li₈TtAs₄ (Tt = Si, Ge) were taken out of the furnace and cooled in air, while ampoules containing the samples Li₁₄TtAs₆ (Tt = Si, Ge, Sn) had to be quenched faster in water to ensure phase pure samples. The X-ray powder diffraction patterns of the stoichiometric mixtures 14 : 1 : 6 show intense reflections which can be assigned to the product already after ball milling (Fig. S1–S3†). Li₃As and unreacted tetrel are present in small amounts as side phases (broadening of reflections is due to micro crystallinity after ball milling). In case of Si, increasing numbers of ball mill cycles lead to lower intensities of Li₃As. Therefore, the completion of the reaction between the elements in the milling process depends on the duration (higher energy input over time) of the ball mill experiment. For Li₁₄GeAs₆ only unreacted Ge is present after ball milling. Performing additional milling cycles showed no improvement, leading to the conclusion that the ball milled mixture needs subsequent thermal treatment for total conversion into the ternary phase. The reaction mixture with Sn shows no side phase after ball milling.

Li₈SiAs₄ and Li₈GeAs₄ are isotypic and crystalize in the cubic space group Pa3̄ (no. 218) with the lattice parameters 12.096(1) Å and 12.170(1) Å, respectively (Fig. 1 and S4†). Crystallographic data of all compounds are displayed in the ESI (Table S1†) together with displacement parameters (Table S2†) and atomic coordinates (Table S3†). All crystal structures were refined from powder diffraction data by Rietveld analysis using the corresponding isotypic phosphide structures as starting models (Fig. 3).^3,7 The structures can be derived from the antifluorite structure type (Fig. 4) leading to a 2 × 2 × 2 supercell (Z = 8) which is built up by a cubic close packed arrangement of As atoms. The formation of covalent bonds between arsenic and the tetrel atom form isolated [TtAs₄]⁸⁻-tetrahedra, which distort the ideal arrangement of the cubic close packed As atoms. To balance the negative charge these [TtAs₄]⁸⁻ tetrahedra are surrounded by 7 + 1 Li⁺ cations occupying all other tetrahedral voids (Li/Ge tetrahedra occupancy 7 : 1) and 25% of the octahedral voids. The [SiAs₄]⁸⁻-tetrahedra in Li₈SiAs₄ are slightly distorted with interatomic distances of d(Si–As) = 2.371 Å (1×), 2.398 Å (1×) and 2.399 Å (3×). These distances are comparable to related compounds like Na₅SiAs₃ (2.351–2.428 Å),¹⁶ KSi₃As₃ (2.323–2.414 Å)¹⁷ and SiAs (2.364–2.414 Å).¹⁸ The [GeAs₄]⁸⁻-tetrahedra of the isotypic compound Li₈GeAs₄ have interatomic distances d(Ge–As) of 2.468 Å (3×) and 2.471 Å (1×) similar to related compounds as Na₅GeAs₃ (2.433–2.520 Å),¹⁹ KGe₃As₃ (2.414–2.509 Å)²⁰ and GeAs (2.449–2.488 Å).²¹ All coordination polyhedra are shown in the ESI (Fig. S5 and S6†).

Fig. 1 Cubic crystal structure of Li₈SiAs₄ with [SiAs₄]⁸⁻-tetrahedra coloured in green.

The lithium richer arsenidotetrelates Li₁₄TtAs₆ (Tt = Si, Ge, Sn) crystalize with even higher symmetry in space group Fm3̄m (no. 225) with lattice parameters of a = 6.131(1) Å (Li₁₄SiAs₆), 6.143(1) Å (Li₁₄GeAs₆) and 6.193(1) Å (Li₁₄SnAs₆). XRPD patterns with Rietveld analysis are displayed in Fig. 3 for the Si compound and the corresponding Ge and Sn diffraction patterns are shown in the ESI (Fig. S8 and S9†). For Rietveld refinements the isotypic phosphidotetrelates were used as initial structure models with As localized on the P positions.¹⁸ The crystal structure can be described as a face-centered cubic lattice of arsenic atoms where all tetrahedral voids are occupied by a mixed site of Li1 and the corresponding tetrel with a ratio of tetrel/lithium 1 : 11 (Fig. 2). A second Li position (Li2) occupies all octahedral voids with a SOF of 0.5. A detailed discussion of the Rietveld refinement is given in the ESI.† Due to the highly symmetric crystal structure several interatomic distances are identical like the distance of Tt/Li1–Tt/Li1 and Li2–As1. Selected interatomic distances are listed in Table 1. Arsenidotetrelates and phosphidotetrelates show similar behaviour with respect to the increase of lattice parameter and interatomic distances. The lattice parameters of the arsenidotetrelates increase from Si to Ge by 0.20% and from Ge to Sn by 0.81%. A similar increase is observed for the phosphidotetrelates: from Si to Ge by 0.29% and from Ge to Sn by 1.02%. Fässler et al. assign the smaller increase of lattice parameters from Li₁₄SiP₆ to Li₁₄GeP₆ compared to Li₁₄GeP₆ to Li₁₄SnP₆ to the fact, that Ge could compensate the larger atomic radius of Ge compared to Si by its higher electronegativity.⁸ The overall slightly larger lattice parameters and interatomic distances of the arsenidotetrelates compared to their corresponding phosphidotetrelates can be explained with the larger atomic radius of arsenic compared to phosphorus. It must be noted that to the best of our knowledge no reliable values for the ionic radii of P³⁻ and As³⁻ can be found in textbooks or scientific literature.

Fig. 2 Cubic crystal structure of Li₁₄TtAs₆ (Tt = Si, Ge, Sn). Tetrahedral voids are occupied by lithium and tetrel cation in a ratio of 11 : 1 and the octahedral voids are half occupied by lithium.

Fig. 3 X-ray powder diffraction data with Rietveld analyses of Li₈SiAs₄ (top), Li₈GeAs₄ (middle) and Li₁₄SiAs₆ (bottom). Observed intensities Y_obs are displayed with black crosses, the red line indicates the calculated intensities Y_calc, Bragg positions are marked with black lines and the difference plot (Y_obs − Y_calc) is drawn in blue.

Table 1 Interatomic distances in Li₁₄TtAs₆ (Tt = Si, Ge, Sn)

Formula	Tt/Li1–Li2/Å	As1–As1/Å	Li2–As1/Å
Li14SiAs6	2.654(1)	4.335(1)	3.065(1)
Li14GeAs6	2.660(1)	4.343(1)	3.071(1)
Li14SnAs6	2.681(1)	4.379(1)	3.096(1)

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The crystal structures of Li₈TtAs₄ (Tt = Si, Ge) with space group Pa3̄ (no. 218) and the crystal structures of Li₁₄TtAs₆ (Tt = Si, Ge, Sn) with space group Fm3̄m (no. 225) are closely related and can be both derived from the antifluorite structure type (stuffed type) or alternatively from the Li₃Bi structure type (defect type). The main structural difference between the lithium poorer and lithium richer arsenidotetrelates is that the lower amount of tetrel atoms in Li₁₄TtAs₆ (1 of 21 atoms) is evenly distributed over all tetrahedral voids creating mixed occupancy with lithium whereas the higher amount of tetrel atoms in Li₈TtAs₄ (1 of 13 atoms) is distributed in an ordered way occupying 1/8 of all tetrahedral voids by 100%. The structural relationship between Li₈TtAs₄ and Li₁₄TtAs₆ and the relation to the antifluorite structure type (or alternatively to the Li₃Bi structure type) are visualized in Fig. 4. The ordering of tetrel cations in Li₈TtAs₄ reduces the space group symmetry from Fm3̄m in Li₁₄TtAs₆ to Pa3̄ (Fm3̄m → Pm3̄m → Fm3̄c → Fm3̄ → Pa3̄). A detailed description of these symmetry relations is given by Fässler et al.⁷

Fig. 4 Representation of the structural relation of the compounds Li₈TtAs₄ and Li₁₄TtAs₆ to the antifluorite structure type and the Li₃Bi structure type. (a) Li₃Bi structure type, (b) antifluorite structure type, middle: structure of Li₁₄TtAs₆ which can be derived from the Li₃Bi structure type by removing half of the occupation from the octahedral voids forming a half-occupied lithium position and adding a mixed position of lithium and tetrel cation in the tetrahedral voids (deficient Li₃Bi structure). Second, the structure of Li₁₄TtAs₆ can be understood as a filled antifluorite structure, where the octahedral voids are half filled with lithium and tetrahedral voids are occupied with the mixed position. In the structure of Li₈TtAs₄ the mixed positions in the tetrahedral voids are replaced by an ordered arrangement of the tetrel cation occupying one of the eight octahedral voids in the yellow cell. The formation of ordered [TtAs₄]⁸⁻-tetrahedra leads to a distortion in the ccp of the arsenide anions. The other seven tetrahedral voids are fully occupied by lithium and the lithium occupancy in the octahedral voids is reduced. Right: unit Cell of Li₈TtAs₄. The yellow cell (former equivalent of the unit cell of Li₁₄TtAs₆) is highlighted to underline the structural relation of both structures. The unit cell of Li₈TtAs₄ can be understood as a 2 × 2 × 2 supercell of eight yellow cells and an additional origin shift of (½, 0, 0).

Thermal analysis

Quenching is crucial for phase pure samples of Li₁₄TtAs₆ (Tt = Si, Ge, Sn) indicating the stability of this cubic phase only at high temperatures. This is proven by high temperature powder diffraction data of previously quenched samples up to 305 °C. A decomposition process is most pronounced for Li₁₄GeAs₆ (Fig. 5) by the reformation of Li₃As and Li₈GeAs₄ starting at 125 °C (Fig. S13†). The diffraction pattern of the Si compound shows weak intensity reflections at a temperature of 305 °C belonging to Li₈SiAs₄ (Fig. S10 and S12†) and the Sn compound decomposes at a temperature of 285 °C to Li₃As and additional reflections which could possibly be assigned to yet unknown Li₈SnAs₄ (Fig. S11†). Reasonable diffraction data for Li₈SnAs₄ suitable for structural characterization are not yet accessible.

Fig. 5 High temperature powder diffraction data (25 °C → 305 °C → 25 °C) of Li₁₄GeAs₆ in steps of 20 °C. Additional reflexes are emerging at a temperature of 165 °C which can be assigned to Li₃As and Li₈GeAs₄. Please note that artifacts of the heating device are present in a 2θ range of 32°–38° (grey background).

For temperatures above 305 °C differential thermal analysis (DTA) (Fig. S14–S16†) was performed showing significant and clear endothermal effects for Li₁₄SiAs₆ at an onset point of 489 °C (Li₁₄GeAs₆ at 464 °C, and Li₁₄SnAs₆ at 533 °C) in the heating cycles and exothermal effects in the cooling cycles at onset points of 473 °C and 442 °C for Li₁₄SiAs₆ (Li₁₄GeAs₆ at 480 °C and Li₁₄SnAs₆ at 534 °C and 465 °C). We assume a behaviour similar to the recently reported homologous phosphides, for which the following interpretation was suggested.¹⁸ The endothermal effects can be explained by a reformation process of Li₃As and Li₈TtAs₄ to Li₁₄TtAs₆ after gradual decomposition of Li₁₄TtAs₆ during the previous annealing period. The initial decomposition which is detected in the high temperature powder data must therefore be too slow to be detectable by DTA. While cooling the metastable phase Li₁₄TtAs₆ from high temperatures the samples again undergo decomposition to Li₃As and Li₈TtAs₄ indicated by the exothermal effects. Powder diffraction data of the Si compound after DTA (Fig. S11†) shows a mixture of Li₁₄SiAs₆, Li₈SiAs₄ and Li₃As proofing the decomposition of the lithium rich arsenidotetrelate after the cooling cycle (Li₁₄SiAs₆ → Li₈SiAs₄ + 2Li₃As). The described decomposition and reformation processes are also observed for the lithium phosphidotetrelates Li₁₄TtP₆ (Tt = Si, Ge, Sn) which decompose to Li₈TtP₄ (Tt = Si, Ge, Sn) and Li₃P upon decreasing temperatures and therefore confirm the strong similarity between the arsenidotetrelates and phosphidotetrelates not only in structural aspects but also in the thermal properties.^1,8

Impedance spectroscopy

Lithium ion mobility was measured using ac-impedance spectroscopy with ion blocking electrodes (indium foil) in a temperature range of 50 °C to 100 °C. Measurements at higher temperatures were not investigated, because of the limited thermal stability of the lithium rich arsenidotetrelates. Conductivities of the title compounds were obtained from Nyquist plots (Fig. S17 and S18†) showing semi-circles with low frequency tails, typical for lithium ion conductors. The temperature dependent ionic conductivities are displayed in the Arrhenius plot shown in Fig. 6. The lithium poor arsenidotetrelates Li₈SiAs₄ and Li₈GeAs₄ show specific ion conductivities of σ_spec(50 °C) = 1.9 × 10⁻⁵ S cm⁻¹ (E_a = 0.39 eV) and σ_spec(50 °C) = 3.0 × 10⁻⁵ S cm⁻¹ (E_a = 0.40 eV). In the system Li–Si–As only Li₂SiAs₂ is characterized in terms of ion conductivity with a specific ion conductivity of σ_spec(25 °C) = 8 × 10⁻⁸ S cm⁻¹.¹¹ Thus, Li₈SiAs₄ exceeds Li₂SiAs₂ in lithium conductivity by three orders of magnitude. The homologous lithium phosphidotetrelate Li₈SiP₄ shows similar ion conducting behaviour with a specific ion conductivity of σ_spec(75 °C) = 1.2(2) × 10⁻⁵ S cm⁻¹ and the corresponding Ge containing compound with a specific ion conductivity of σ_spec(25 °C) = 1.8 × 10⁻⁵ S cm⁻¹ shows also similar values like Li₈GeAs₄.^3,7 The lithium rich arsenidotetrelates show specific ion conductivities of σ_spec(50 °C) = 4.6 × 10⁻⁴ S cm⁻¹ (E_a = 0.28 eV), σ_spec(50 °C) = 6.6 × 10⁻⁴ S cm⁻¹ (E_a = 0.34 eV) and σ_spec(50 °C) = 3.9 × 10⁻⁴ S cm⁻¹ (E_a = 0.40 eV) for Li₁₄SiAs₆, Li₁₄GeAs₆, and Li₁₄SnAs₆, respectively. These specific ion conductivities in the range of 10⁻⁴ S cm⁻¹ at 50 °C are quite high and only about one order of magnitude lower than for the corresponding phosphides (σ ∼ 1 × 10⁻³ S cm⁻¹ at room temperature^1,7). Conductivity measurements originating from different experimental setups can differ for several orders of magnitude. Therefore, a direct comparison of the conductivity values of the phosphides and the arsenides under discussion is difficult.²² Nevertheless, in all conductivity measurements we observed lower specific lithium ion conductivities for the arsenides. Overall, impedance spectroscopy shows that Li₈SiAs₄ and Li₈GeAs₄ exhibit lower lithium ion mobility than the Li₁₄TtAs₆ (Tt = Si, Ge, Sn) compounds which could be explained by the higher mole fraction of lithium, since Li₈SiAs₄ and Li₈GeAs₄ contain 61.5% lithium whereas Li₁₄SiAs₆ and Li₁₄GeAs₆ contain 66.6% lithium.

Fig. 6 Arrhenius plots of the ion conductivities of Li₈SiAs₄, Li₈GeAs₄, Li₁₄SiAs₆, Li₁₄GeAs₆ and Li₁₄SnAs₆. It becomes obvious that the conductivity is strongly increasing with the Li content of the materials.

Conclusions

We report on new members of the family of lithium arsenidotetrelates. Li₈SiAs₄, Li₈GeAs₄, Li₁₄SiAs₆, Li₁₄GeAs₆, and Li₁₄SnAs₆ can be synthesized via ball milling and phase pure samples of the compounds are obtained after annealing at high temperatures. The crystal structure (space group Pa3̄ (no. 218)) of the lithium poorer arsenidotetrelates Li₈SiAs₄ and Li₈GeAs₄ is built up by a distorted ccp of arsenide ions. The tetrel cation occupies 1/8 of all tetrahedral voids by 100% and lithium cations occupy the remaining tetrahedral voids and partly some octahedral voids. Both crystal structures feature slightly distorted [TtAs₄]⁸⁻-tetrahedra enclosed by lithium atoms. The compounds Li₁₄TtAs₆ (Tt = Si, Ge, Sn) crystallize in a higher symmetric space group Fm3̄m (no. 225). In contrast to the lithium poorer compounds all tetrahedral voids are occupied with a mixture of lithium and the tetrel cation in a ratio of 1 : 11. The higher tetrel content leads to an occupation of 50% of the octahedral voids by lithium. The structural relation between Li₈TtAs₄ and Li₁₄TtAs₆ can be understood looking at the corresponding unit cells in detail.

The main difference between the crystal structures is that in Li₈TtAs₄ the tetrel cations occupy 1/8 of the tetrahedral voids by 100% in a well-ordered arrangement whereas in Li₁₄TtAs₆ the tetrel cations are statistically distributed over all tetrahedral voids and therefore forming a mixed occupied site in addition with lithium in a ratio of 1 : 11 (tetrel/lithium). The unit cell of Li₈TtAs₄ can be derived from a 2 × 2 × 2 supercell of Li₁₄TtAs₆ unit cells (additional origin shift of the Li₈TtAs₄ unit cell by (1/2, 0, 0) is necessary) considering the different tetrel cation arrangement in the tetrahedral voids, the varied lithium content and distribution, and the distortion of the ccp of As anions. Both crystal structures can be derived from the anti-fluorite structure type or alternatively from the Li₃Bi structure type.

Considering the sum formulae of the arsenidotetrelates under discussion the ternary phases can also be described as an formal aliovalent substitution of lithium in Li₃As by an tetrel cation (Li₁₂As₄ → Li₈Tt⁴⁺As₄, Tt = Si, Ge; Li₁₈As₆ → Li₁₄Tt⁴⁺As₆, Tt = Si, Ge, Sn) exchanging four lithium cations with one tetrel cation. The introduced tetravalent tetrel cation forces the former hexagonal structure of Li₃As into the cubic ternary phases Li₈TtAs₄ and Li₁₄TtAs₆, depending on the synthesis route and exact composition.

From high temperature diffraction data and DTA, it can be concluded that the compounds Li₁₄TtAs₆ (Tt = Si, Ge, Sn) are only stable at high temperatures and therefore are only accessible via quenching samples from high temperatures to ambient conditions. At moderate temperatures a decomposition to lithium arsenide and Li₈TtAs₄ (Li₁₄TtAs₆ → Li₈TtAs₄ + 2Li₃As) is observed. This decomposition shows that the mixed occupancy of a site by tetrel and lithium in the higher symmetrical structure is thermodynamically unstable at room temperature. It is replaced by an ordered distribution of the tetrel cations at lower temperatures in the thermodynamically stable structure of Li₈TtAs₄. Impedance spectroscopy reveals lithium ion mobility for all compounds under discussion with specific ion conductivities for Li₈SiAs₄ and Li₈GeAs₄ in the range of 10⁻⁵ S cm⁻¹ showing one order of magnitude lower in conductivity compared to the lithium richer compounds.

Experimental

Warning: Since elemental arsenic and its compounds are highly toxic all manipulations should be executed with extreme caution. In addition, safety considerations on a high level are advised, because all arsenic containing samples can reach a high vapor pressure at elevated temperatures and lithium is able to damage quartz ampoules by corrosion phenomena.

Since all synthesised compounds are highly sensitive against air and moisture all manipulations were carried out in a Glovebox (M Braun) with oxygen and moisture levels below 0.5 ppm.

Synthesis

All compounds were synthesized in a two-step synthesis. First a pre-reacted powder mixture was obtained by ball milling (FRITSCH Pulverisette 7 premium line, 25 mL zirconia grinding bowls, 10 zirconia grinding balls (∅ = 10 mm)) the elements lithium (Merck, 99%), silicon (Alfa Aesar, 99.9%), germanium (Chempur, 99.9%), tin (Chempur, 99.999%) and arsenic (Chempur, sublimed) in stoichiometric amounts (batch size: 2 g) for 12 milling cycles (24 cycles for Li₁₄SiAs₆). One milling cycle was 5 minutes of milling (rotation speed: 600 rpm) followed by 4 minutes of letting the sample equilibrate. Lithium was used in excess (up to 15%) to compensate for evaporation during the second step. The pre-reacted mixture obtained from ball milling was placed in a graphite container (∅ = 6 mm) that was closed with a lid. The loaded container was then placed in an evacuated silica tube. Phase pure samples of the compounds were obtained by heating the mixtures to the reaction temperature (Li₈SiAs₄ and Li₈GeAs₄: 650 °C, 12 h; Li₁₄SiAs₆, Li₁₄GeAs₆, Li₁₄SnAs₆: 700 °C, 24 h) followed by quenching the silica tube in water for Li₁₄TtAs₆ (Tt = Si, Ge, Sn). The silica tubes for Li₈TtAs₄ (Tt = Si, Ge) were removed from the furnace and cooled on air.

Powder diffraction

The samples were ground in a mortar and filled in a glass capillary (∅ = 0.3 mm), which was sealed with glue in the glove box. The subsequent flame sealed capillary was mounted on a STOE STADI P diffractometer (Stoe & Cie) equipped with a Mythen 1K detector for data collection and a graphite furnace for high temperature measurements. CuKα₁-radiation (λ = 1.540598 Å) was used in all measurements. Raw data were processed with the WinXPOW software package (Stoe & Cie), Rietveld refinements were performed with Jana2006.^23,24

DTA

Samples were ground in argon atmosphere and prepared in flame sealed quartz tubes (∅ = 2 mm). Measurements were carried out using a SETARAM TG–DTA 92.16.18.

Impedance spectroscopy

Impedance measurements were carried out on a Zahner Zennium impedance analyzer coupled with an Eurotherm controlled furnace located in a Glovebox (M Braun) with oxygen and moisture levels below 0.5 ppm. The cold-pressed pelletized samples (∅ = 8 mm) were sandwiched between indium foil and contacted with platinum electrodes. Measurements were carried out in a temperature range of 50–100 °C in steps of 10 °C and a frequency range of 1 MHz to 100 mHz. The measurement of ionic conductivities at room temperature was not possible since our measuring cell is located in an argon filled glovebox and cannot be actively cooled. The software Zahner Analysis was used for raw data processing.²⁵

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors express their gratitude to the Free State of Bavaria and the University of Regensburg for excellent working conditions. Dedicated to Prof. H. D. Lutz on the occasion of his 90th birthday.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2332047–2332051. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt00664j

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