Li5SnP3 – a Member of the Series Li10+4x Sn2−x P6 for x=0 Comprising the Fast Lithium‐Ion Conductors Li8SnP4 (x=0.5) and Li14SnP6 (x=1)

Abstract The targeted search for suitable solid‐state ionic conductors requires a certain understanding of the conduction mechanism and the correlation of the structures and the resulting properties of the material. Thus, the investigation of various ionic conductors with respect to their structural composition is crucial for the design of next‐generation materials as demanded. We report here on Li5SnP3 which completes with x=0 the series Li10+4x Sn2−x P6 of the fast lithium‐ion conductors α‐ and β‐Li8SnP4 (x=0.5) and Li14SnP6 (x=1). Synthesis, crystal structure determination by single‐crystal and powder X‐ray diffraction methods, as well as 6Li, 31P and 119Sn MAS NMR and temperature‐dependent 7Li NMR spectroscopy together with electrochemical impedance studies are reported. The correlation between the ionic conductivity and the occupation of octahedral and tetrahedral sites in a close‐packed array of P atoms in the series of compounds is discussed. We conclude from this series that in order to receive fast ion conductors a partial occupation of the octahedral vacancies seems to be crucial.


Introduction
Solid-state electrolytes (SE) are predicted to dominate mainly in electric vehicles and future lithium battery chemistry. [1] Therefore, extensive efforts are made aiming for the discovery of SE materials that are suitable to meet demanded properties for application in all-solid-state batteries. [2][3][4] Another approach focuses on a better understanding of the origin of materials' properties, such as ionic conductivity. The elaboration of structure-property relationships by comparison of a variety of crystalline candidate materials that comprise diverse structural differences with respect to their electronic properties allows for designing and tailoring of materials' properties as demanded. [5][6][7][8] Searching for high-performance ionic conductors, a large number of innovative SEs featuring ever increasing ionic conductivities has been reported over the last decades. [2][3][4]9,10] But a thorough investigation of structureproperty relationships also demands the evaluation of less powerful materials in order to unveil the reasons for favorable or unfavorable properties.
The recently introduced family of lithium phosphidotetrelates and the closely related lithium phosphidotrielates are wellsuited for further analyses of ionic conduction mechanisms as this class of materials offers a broad structural variety as well as a corresponding large variety of properties. For example, several compounds with a fast ionic conduction of up to 3 × 10 À 3 S cm À 1 have been reported [3,4,7] next to materials that feature a band gap of about 3 eV, indicative of semiconducting behavior. [11] In addition the compound LiGe 3 P 3 shows moderate electric conductivity and an unprecedented stability when exposed to water and air. [12] Apart from the latter and a few other exceptions, most of the so far discovered lithium phosphidotetrelates and -trielates are based on tetrahedral [TtP 4 ] 4 ] 9À anions that are charge-compensated by the corresponding amount of Li + or they build frameworks of condensed tetrahedra and supertetrahedra, respectively. [8,[11][12][13][14][15] The compound Li 10 Si 2 P 6 comprises pairs of edge-sharing [SiP 4 ] units resulting in the polyanion [Si 2 P 6 ] 10À . [16] Due to these building blocks, the formula is commonly given as Li 10 Si 2 P 6 rather than Li 5 SiP 3 to express the molecular-anionic character.
Indeed, a compound with the composition Li 5 SiP 3 has been reported in the 1950s, and characterized by the determination of the cubic space group Fm � 3m with a lattice parameter of a = 5.852 Å. [17] The structure is closely related to the antifluorite type or a defect variant of the Li 3 Bi structure, [18] which can be described by a ccp of P atoms in which all tetrahedral voids are statistically occupied by Li + and Si 4 + in a mixing ratio of 5 : 1. So far, all attempts to reproduce these findings have failed. [13,16] However, about 20 years later the same structure was reported for the heavier homologue Li 5 SnP 3 [19] with identical cubic space group Fm � 3m (no. 225) and a lattice parameter of a = 5.97 Å. Further investigations of the material's properties are still pending, whereas the lithium-rich phosphidostannates αand β-Li 8 SnP 4 as well as Li 14 SnP 6 have been recently reported to show superionic lithium-ion conductivities of about 1 × 10 À 3 S cm À 1 . Interestingly in αand β-Li 8 SnP 4 , the Li and Sn atoms are fully ordered in the tetrahedral voids of the ccp of P atoms, thus leading to polyanionic SnP 4 8À units. By contrast in Li 14 SnP 6 , the Li and Sn atoms are statistically distributed over the tetrahedral sites. Analysis of Li + diffusion pathways based on powder neutron diffraction data unveiled structural variations, which were directly connected to the different values of the ionic conductivities of the three compounds. [7,8] In the following we report on the systematic investigation of the system Li 10 + 4x Sn 2À x P 6 (x = 0.0 to 1.0) including the compounds Li 5 SnP 3 , (α-and β-) Li 8 SnP 4 and Li 14 SnP 6 that arise for x = 0.0, 0.5, and 1.0, respectively. Following a well-established synthesis route for lithium phosphidotetrelates including mechano-chemical milling allows for the first time the isolation and detailed characterization of the compound Li 5 SnP 3 by single-crystal and powder X-ray diffraction data completed by Rietveld refinement as well as 6 Li, 31 P and 119 Sn solid-state magic angle spinning (MAS) NMR measurements. Differential scanning calorimetry (DSC) and isothermal annealing experiments of the reactive mixtures obtained via mechanical alloying were carried out to investigate the thermal properties of the materials. Furthermore, the Li + mobility and its activation energy, as well as the ionic and electronic conductivity were determined via temperature-dependent 7 Li NMR spectroscopy and electrochemical impedance spectroscopy (EIS). Finally, all data and the associated properties are compared to that of the recently reported lithium phosphidostannates αand β-Li 8 SnP 4 and Li 14 SnP 6 , which allows for the formulation of new structureproperty relationships regarding the ionic conductivity in solidstate Li + conductors.

Experimental Section
All syntheses were carried out under Ar atmosphere in glove boxes (MBraun, 200B) with moisture and oxygen levels below 0.1 ppm, or in containers, which were sealed under Ar atmosphere and vacuum (< 2 · 10 À 2 mbar), respectively. Lithium phosphidostannates are sensitive to oxygen and moisture; in particular, contact with water results in a vigorous reaction including the formation of flammable and toxic gases (e. g., phosphine). Therefore, disposal must be addressed in small amounts at a time and under proper ventilation.
In the first step, a "reactive mixture" (m = 5.0 g) was prepared by mechano-chemical milling using a Retsch PM100 Planetary Ball Mill (350 rpm, 18 h, 10 min interval, 3 min break) with a tungsten carbide milling jar (V = 50 mL) and three balls with a diameter of 15 mm.
In the second step, the "reactive mixture" was pressed into pellets, sealed in batches of 0.3 to 1.0 g in carbon-coated silica glass ampules and heated in a muffle furnace (Nabertherm, L5/11/P330) to 673, 773 or 973 K (heating rate: 4 K min À 1 ) for 24 h, followed by quenching of the hot ampules in water.
The data analysis of Li 5 SnP 3 was performed using the full profile Rietveld method implemented in the FullProf program package. [21] To model the peak profile, the pseudo-Voigt function was chosen. The background contribution was determined using a linear interpolation between selected data points in non-overlapping regions. The scale factor, zero angular shift, profile shape parameters, resolution (Caglioti) parameters, asymmetry and lattice parameters as well as fractional coordinates of atoms and their displacement parameters were varied during the fitting. Free refinement of the occupancy of the 8c site by Sn and Li exhibited only marginal deviations from the electron-precise formula Li 5 SnP 3 (Z = 1.33) or Li 6.67 Sn 1.33 P 4 (Z = 1). The corresponding data are given as Supporting Information. In addition, a second refinement was carried out with site occupancies set to the exact stoichiometry. Since the results of both refinements were in very good agreement, the electron precise stoichiometry Li 5 SnP 3 is assumed. All structures were visualized using DIAMOND. [22] Synthesis of powdery and single-crystalline Li 5 SnP 3 : Li 5 SnP 3 is obtained as black powder on a gram scale and in high purity by annealing of the "reactive mixture" of the nominal composition "Li 5 SnP 3 " (Li 10 + 4x Sn 2À x P 6 with x = 0.0) in carbon-coated silica glass ampules at 773 K for 24 h, followed by quenching of the hot ampule in water. The weight fraction of remaining β-Sn was determined via Rietveld refinement to 0.8(1) %.
Single crystals were obtained by a high-temperature reaction of lithium (Rockwood Lithium, 99 %), tin (Merck, 99.9 %) and red phosphorus (Sigma-Aldrich, 97 %) in a ratio corresponding to "Li 5 SnP 6 ". The elements were annealed for 18 h at 873 K (heating Table 1. Overview of the prepared "reactive mixtures" according to the formula Li 10 + 4x Sn 2À x P 6 (x = 0.0 to 1.0).

Chemistry-A European Journal
Research Article doi.org/10.1002/chem.202104219 rate: 4 K min À 1 ) in a sealed tantalum ampule and subsequently quenched in water.

Single-crystal X-ray Diffraction Data Collection:
A single crystal of Li 5 SnP 3 was isolated and sealed in a glass capillary (0.1 mm). For diffraction data collection, the capillary was positioned in a 150 K cold N 2 gas stream. Data collection was performed with a STOE StadiVari (Mo Kα1 radiation) diffractometer equipped with a DECTRIS PILATUS 300 K detector. Structures were solved by Direct Methods (SHELXS-2014) and refined by full-matrix least-squares calculations against F 2 (SHELXL-2014). [23] Further details of the crystal structure investigations may be obtained from the joint CCDC/FIZ Karlsruhe online deposition service: Deposition Number(s) CSD-2074706 (Li 6.6667 Sn 1.33 P 4 , single crystal), CSD-2074707 (Li 6.6667 Sn 1.33 P 4 , powder), CSD-2074709 (Li 6.74 Sn 1.29 P 4 , single crystal), and CSD-2074710 (Li 6.70 Sn 1.30 P 4 , powder) contain(s) the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.

Differential Scanning Calorimetry (DSC):
The thermal behavior of the compounds was studied with a Netzsch DSC 404 Pegasus device. Niobium crucibles were filled with the samples and sealed by arc-welding. Empty sealed crucibles served as a reference. Measurements were performed under an Ar flow of 75 mL min À 1 and a heating/cooling rate of 10 K min À 1 . Data collection and handling were carried out with the Proteus Thermal Analysis program, [24] and visualization was realized using OriginPro 2020. [25] Solid-State NMR Spectroscopy: Magic angle spinning (MAS) NMR spectra were recorded on a Bruker Avance 300 NMR device operating at 7.04 T by the use of a 4 mm ZrO 2 rotor. The resonance frequencies of the measured nuclei are 44.2, 121.5 and 111.9 MHz for 6 Li, 31 P and 119 Sn, respectively. The rotational frequency was set to 15 kHz. The MAS spectra were acquired at room temperature with recycle delays of 10 to 30 s and 1000 to 2736 scans. All 6 Li spectra were referenced to LiCl (1 m, aq) and LiCl (s) offering chemical shifts of 0.0 ppm and À 1.15 ppm, respectively. The 31 P spectra were referred to (NH 4 )H 2 PO 4 (s) (ammonium dihydrogen phosphate) with a chemical shift of 1.11 ppm with respect to concentrated H 3 PO 4 (aq) (phosphoric acid). SnO 2 (s) (Cassiterite) was used as a secondary standard for the 119 Sn spectra, showing a chemical shift of À 604.3 ppm [26,27] referred to (CH 3 ) 4 Sn(l) (tetramethylstannane). All spectra were recorded using single-pulse excitation.
Static 7 Li NMR experiments were performed using a Bruker Avance III spectrometer operating at a magnetic field of 7 T employing a 4 mm WVT MAS probe. The resonance frequency of the 7 Li nucleus is 116.6 MHz. The sample was sealed in a 4 mm glass tube to avoid contact with air and moisture. The temperature calibration for the measurements was performed using the temperature-dependent 207 Pb NMR shift of lead nitrate (Pb(NO 3 ) 2 ) as chemical shift thermometer, which was also measured in a sealed glass tube. A saturation comb had been used prior to the 7 Li data acquisition. The spectra were recorded in the temperature range from room temperature to 200 K with recycle delays of 60 s and 4 scans. All spectra were referenced to LiCl (9.7 M, aq).

Impedance Spectroscopy and DC Conductivity Measurements:
Potentiostatic impedance spectroscopy was carried out using a Biologic SP-300 potentiostat in a frequency range of 7 MHz to 100 mHz with an excitation amplitude of � 10 mV. All measurements were conducted in an argon-filled glove box. Powder samples of Li 5 SnP 3 (300 mg) were measured in a custom-built symmetric cell (Ø = 8 mm) with hardened steel electrodes under blocking conditions. Pressure was applied by six M14 screws, fastened with a defined torque of 30 Nm each, translating to proximately 480 MPa, so that the sample was compressed to 88 % of its crystallographic density. A more detailed description can be found in the literature. [3] The temperature was controlled via a Julabo Dyneo DD 1000 Thermostat feeding an aluminum heating block, which enclosed the measurement cell. The electric conductivity was measured in the same cell setup with three polarization steps of 50, 100 and 150 mV, each held for 6 h to ensure equilibrium conditions.

Syntheses
For the systematic investigation of lithium-rich ternary lithium phosphidostannates, mixtures with nominal compositions according to Table 1 were alloyed mechanically in a ball mill. The compositions were chosen according to the formula Li 10 + 4x Sn 2À x P 6 (x = 0.0 to 1.0) including also the compounds Li 5 SnP 3 , [19] αand β-Li 8 SnP 4 , [8] as well as Li 14 SnP 6 [7] for x = 0.0, 0.5 and 1.0, respectively.
In order to detect further phases within this family of materials and to reveal existing phase widths of the compounds, the "reactive mixtures" were annealed at 673, 773 and 973 K, respectively. All "reactive mixtures" and products were analyzed using powder X-ray diffraction (PXRD) data for the identification of the occurring phases within the samples as well as for the determination of the cell parameters of the lithium phosphidostannates. The data showed the formation of the known compounds Li 5 SnP 3 , [19] αand β-Li 8 SnP 4 [8] and Li 14 SnP 6 [7] as well as the presence of Li 3 P and remaining β-Sn. Consequently, no phase widths were observed for these compounds. Details of the results and all PXRD patterns are given as Supporting Information.
For the structural reinvestigation of Li 5 SnP 3 the corresponding "reactive mixture" obtained by ball milling of the elements in stoichiometric amounts was annealed at 773 K for 24 h, followed by quenching of the hot ampule in water. By this method, the material is accessible on a gram scale and in high purity as indicated by Rietveld analysis (Figure 1). Details of the refinement are shown in Table 2.
Differential scanning calorimetry, followed by PXRD measurements of the samples indicate the decomposition of Li 5 SnP 3 at high temperatures, resulting in a mixture of β-Sn and another cubic phase, indicated by additional reflections assignable to a superstructure as observed for the ordered structures of αand/ or β-Li 8 SnP 4 . Since the additional reflections are broadened, a partial ordering of the cations is assumed. The corresponding thermograms and PXRD patterns as well as a detailed discussion of the results is given as Supporting Information.
In accordance with previous reports, [19] the single-crystal data of Li 5 SnP 3 indicate the cubic space group Fm � 3m (no. 225) and a lattice parameter of a = 5.9541(7) Å at 150 K ( Figure 2 and Table 3).
The structure of Li 5 SnP 3 can be described as a ccp of P atoms (4a site) with the Sn and Li atoms statistically distributed in all tetrahedral voids (8c site) with a Sn : Li ratio of 1 : 5. The structure is thus closely related to the antifluorite structure with P and Li/Sn on Ca and F atom positions, respectively. The structure was also confirmed by powder X-ray diffraction and Rietveld refinement at 293 K. Atomic coordinates and anisotropic displacement parameters as well as the results from the powder X-ray diffraction at 293 K and the single-crystal X-ray diffraction at 150 K are given in the Supporting Information.
The structure of the lithium-rich compound Li 14 SnP 6 is almost isotypic to that of Li 5 SnP 3 , but with a slightly larger lattice parameter (a = 6.01751(3) Å) and a different occupation of the mixed Li/Sn positions in the tetrahedral voids and -due to the higher Li amount -partially occupied octahedral sites (4b). The same Li 3 Bi-type structure was also observed for the lighter homologues Li 14 SiP 6 and Li 14 GeP 6 . [3,7] All interatomic Li/SnÀ P (2.5782(2) Å), Li/SnÀ Li/Sn (2.9771(3) Å) and PÀ P distances (4.2102(3) Å) are within the range of those found for related ternary or binary compounds like Li 14 TtP 6 (Tt = Si, Ge, Sn), [3,7] (α-/β-)Li 8 TtP 4 (Tt = Si, Ge, Sn) [8,13,14] and Li 3 P. [28] The 31 P MAS NMR spectrum of Li 5 SnP 3 shows one very broad resonance (~17 kHz) at a chemical shift of À 220.3 ppm (Figure S8). A comparable broadening was also observed in case of the structurally related and highly disordered compounds Li 14 TtP 6 (Tt = Si, Ge, Sn). [3,7,29] Furthermore, it is assumed that the in some extend very complex coupling of Sn and P atoms also leads to a merging of signals with a related chemical shift as recently reported for example, for αand β-Li 8 SnP 4 . [8] In   comparison to the resonances of the latter and of other closely related lithium phosphidostannates such as Li 14 SnP 6 , [7] the maximum of the signal appertaining to Li 5 SnP 3 shows a downfield shift of about 20 to 40 ppm. This indicates a lower shielding of the P atoms and hints for a lower formal charge (< À 2) and to a higher coordination number of P by Sn atoms (Figure 3). At the local level, all P atoms in Li 5 SnP 3 are covalently bound to at least one Sn atom, whereas in Li 14 TtP 6 also P 3À anions are present according to [(Li + ) 14 (TtP 4 ) 8À (P 3À ) 2 ]. In analogy to the structures of αand β-Li 8 SnP 4 and Li 14 SnP 6 , respectively, the Sn atoms are occupying tetrahedral voids, resulting in SnP 4 units. [7,8] Regarding the electron-precise stoichiometry (Li 5 SnP 3 or Li 6.67 Sn 1.33 P 4 ), each P atom is statistically coordinated by 1.33 Sn atoms. Thus, one would expect the coordination of 1/3 of all P atoms by two Sn atoms, and all the others coordinate to one Sn atom. In other words, two Sn atoms occupy adjacent tetrahedral voids and form edge-sharing tetrahedra with the formula [Sn 2 P 6 ] 10À . Such units are also observed in the homologous lithium phosphidosilicate Li 10 Si 2 P 6 . [16] However, the [Sn 2 P 6 ] 10À units are not ordered, and thus, the chemically different P atoms do not appear with a distinct difference in the chemical environment. A further resolution of the broad signal in order to distinguish P atoms located next to only one Sn atom (1b-P 2À ) and P atoms surrounded by two Sn atoms (2b-P 1À ) was not feasible. Possible reasons are given after the discussion of the 119 Sn NMR spectrum.
Regarding the 119 Sn NMR spectrum of Li 5 SnP 3 , the high level of cation disorder also results in only one very broad (~13 kHz) tin resonance at a chemical shift of 124.6 ppm ( Figure S9). In analogy to the 31 P NMR measurements, this effect has also been observed in the 119 Sn spectrum of Li 14 SnP 6 . [7] In comparison to the latter, the maximum of the signal for Li 5 SnP 3 shows a downfield shift of 26.5 ppm, and the resonances of the lithium phosphidostannates Li 5 SnP 3 , αand β-Li 8 SnP 4 and Li 14 SnP 6 shown in Figure 4 are upfield shifted depending on the Sn to P ratio indicating the slightly different bonding situations within the compounds discussed above. [7,8] Regarding the electron-precise stoichiometry of the so far known lithium phosphidostannates the structure of Li 14 SnP 6 shows both, [SnP 4 ] 8À and P 3À units, whereas the two modifications of Li 8 SnP 4 only contain [SnP 4 ] 8À tetrahedra, and the basic structure of Li 5 SnP 3 consists of [Sn 2 P 6 ] 10À units. [7,8] This, in combination with the crystal structures and the just discussed NMR data, at first appears counterintuitively since neither the NMR data nor the crystallographic data clearly resolve the presence of edge-sharing [SnP 4 ] tetrahedra in Li 5 SnP 3 , which are correspondingly reported for Li 10 Si 2 P 6 . In contrast to the strongly covalent character of the SiÀ P bonds leading to molecule-like [Si 2 P 6 ] units [16] the bonding situation in Li 5 SnP 3 is assumed to be mainly dominated by the ionic character of the SnÀ P bonds resulting in much weaker bonds and, thus, in more uniform chemical environments.

Lithium-ion mobility
The Li + mobility, the activation energy and the ionic as well as the electronic conductivity are evaluated and compared to recent results of the related compounds αand β-Li 8 SnP 4 and Li 14 SnP 6 .
For a rough estimation of the activation barrier for Li + mobility in crystalline Li 5 SnP 3 the dynamic behavior of Li + was investigated by temperature-dependent evolution of the static 7 Li NMR line width. Since the central transition of the I = 3/2 7 Li nucleus is broadened by homonuclear ( 7 LiÀ 7 Li) and heteronuclear ( 7 LiÀ 31 P) dipolar coupling, both of which scale with the second Legendrian (3 cos 2 β-1), any dynamic process leads to a Figure 3. Overview of the 31 P MAS NMR spectra of Li 5 SnP 3 (olive), α-Li 8 SnP 4 (blue), β-Li 8 SnP 4 (red), [8] and Li 14 SnP 6 (teal). [7] Spinning sidebands and Li 3 P (impurity) are indicated by * and + , respectively. (blue), β-Li 8 SnP 4 (red), [8] and Li 14 SnP 6 (teal). [7] (partial) averaging of this orientational dependence and, thus, to a narrowing of the NMR line. The corresponding results are depicted in Figure 5. At 213 K a single Gaussian line was obtained at 3.9 ppm with a line width of about 7.6 kHz. At temperatures above 263 K the signal becomes more heterogeneous and increasingly Lorentz-shaped, combined with a stronger narrowing of the line. The resonance remains heterogeneous up to 300 K, with a line width of 5.0 kHz. Application of the empirical Waugh-Fedin relation, E NMR A ¼0.156 · T onset [30] allows for a rough estimation of the activation energy. Since the high-temperature plateau is not reached at 300 K an activation energy of E NMR A ¼47 kJ mol À 1 or higher can be assumed. In comparison with the corresponding values determined for the lithium-richer phosphidostannates αand β-Li 8 SnP 4 ðE NMR A ¼34 and 28 kJ mol À 1 , respectively) and Li 14 SnP 6 ðE NMR A ¼28 kJ mol À 1 ) the estimated activation energy for Li 5 SnP 3 is by far the highest. Moreover, since the onset temperature T onset is estimated to be at 300 K or higher, no or only an extremely low conductivity is expected in electrochemical impedance measurements.
The ionic conductivity of Li 5 SnP 3 was determined by electrochemical impedance spectroscopy (EIS) in a blocking electrode configuration. The results obtained at temperatures between 298 and 353 K � 0.5 K are shown in Figure 6a. The Nyquist plots exhibit well resolved but slightly broadened semicircles and the onset of a branch at low frequencies. For the evaluation of the ionic conductivity only the high-frequency semicircle was fitted, using two serial R/C elements, revealing two processes involved in the ionic conduction mechanism, that is, a dominant process with a capacity of 3(5) · 10 À 10 F and a minor process with a capacitance of 5(1) · 10 À 7 F. The first process can be assigned, according to Irvine et al., [31] to grain boundary-controlled ionic conductivity, while the latter resembles the contribution of a surface layer. The overall ionic conductivity at 298 K was determined to 3.2(2) · 10 À 7 S cm À 1 . Calculated from the slope of the Arrhenius plot in Figure 6b, the activation energy of the ionic mobility was determined to E PEIS A ¼47.6(6) kJ mol À 1 (~0.49 eV). The electric conductivity of the sample was studied by polarization of the sample in three different potential steps of 50, 100 and 150 mV, each held until stationary conditions were approached, monitoring the current in the same cell setup as for impedance spectroscopy (Figure 6c). Application of Ohm's law results in an electronic conductivity of 2.1(9) · 10 À 8 S cm À 1 , which is approximately one order of magnitude lower than the ionic conductivity.  with spectra recorded at temperatures between 298 and 353 K according to the color code of the inset. Circles and triangles indicate data collection during heating and cooling, respectively. The equivalent circuit used for fitting is also shown; b) Arrhenius plot of the product of conductivity and temperature (σ Li T) obtained in one heating-cooling cycle, with error bars for each temperature based on the standard deviation from independent measurements with three cells; the shown linear fit was used to obtain the activation energy E PEIS A ; c) polarization curves of Li 5 SnP 3 for the determination of the electronic conductivity. The black line referring to the left y axis shows the recorded current, while the red line (right y axis) shows the applied potential steps.

Discussion and Conclusion
The straightforward synthesis of single crystals and phase-pure microcrystalline powders finally allows for a comparison of the structure and properties of Li 5 SnP 3 with that of the recently reported compounds αand β-Li 8 SnP 4 and Li 14 SnP 6 , which contain an increasing percentage of Li + . [7,8] On both the Li-poor and Li-rich sides, mixed Li/Sn positions in a small cubic unit cell occur. The cell parameters shown in Table 4 increase with a higher content of Li + because the exchange of one Sn 4 + requires the insertion of four Li + to keep the electronic preciseness of the structures. The relatively small amount of Li + in Li 5 SnP 3 is found to occupy all tetrahedral voids, whereas the octahedral voids remain completely empty, and thus, the octahedral voids must be regarded as energetically less favored. With increasing Li + content, however, also the octahedral voids are progressively filled reaching an occupancy of 25 % in αand β-Li 8 SnP 4 and of 50 % in Li 14 SnP 6 .
In addition, Figure 7 reveals a correlation between the occupancy of the octahedral voids and the ionic conductivity, and the latter increases from Li 5 SnP 3 to Li 14 SnP 6 by more than three orders of magnitude. The relatively low ionic conductivity of Li 5 SnP 3 is attributed to the absence of occupied octahedral voids and corroborates the assumption that these vacancies are energetically less favored. As a consequence, Li + diffusion does not occur via octahedral sites but through edge-sharing tetrahedral voids that require a higher activation energy if compared to the diffusion along face-sharing tetrahedral and octahedral voids, as recently shown by the investigation of Li + diffusion pathways in αand β-Li 8 SnP 4 and Li 14 SnP 6 .
The systematic investigation of the ternary Li/Sn/P system within the formula Li 10 + 4x Sn 2À x P 6 (x = 0.0 to 1.0) did not lead to compounds with other Li/Sn ratios than that of the previously reported phases Li 5 SnP 3 , [19] αand β-Li 8 SnP 4 [8] and Li 14 SnP 6 . [7] Interestingly, no ordered structure is observed for Li 5 SnP 3 and Li 14 SnP 6 , whereas two polymorphs with distinctly ordered cation positions are found for the Li 8 SnP 4 . In addition, there is no evidence of a phase width of the compounds. The disorder in Li 5 SnP 3 and Li 14 SnP 6 is in accordance with the recorded 31 P and 119 Sn MAS NMR spectra, which exhibit extremely broad resonances. Such broad resonances hint for a vague chemical environment of the P and Sn atoms. Nevertheless, the chemical shift of the signals is within the range of the resonances reported for the ordered structures of αand β-Li 8 SnP 4 , indicating the presence of [SnP 4 ] tetrahedra in Li 5 SnP 3 , which in accordance with the charge are expected to form edge-sharing [Sn 2 P 6 ] 10À dimers as found as ordered variant in Li 10 Si 2 P 6 . [16] A two-step synthesis route, including mechanical alloying and subsequent annealing of the samples, yields all four compounds in high purity and on a gram scale allowing for a profound determination of the properties.
The low ionic conductivity of σ Li = 3.2(2) · 10 À 7 S cm À 1 of Li 5 SnP 3 in combination with vacant octahedral sites on the one hand, and the high ionic conductivity of αand β-Li 8 SnP 4 as well as of Li 14 SnP 6 with partially filled octahedral sites on the other unequivocally proof the importance of the participation of the octahedral voids in ion motion. In order to lower the activation energy one can either lower the energy barrier for Li + motion between neighboring sites or raise the energy level of the respective sites. The partial occupation of the energetically unfavorable octahedral voids in the Li-rich phosphidotetrelates corresponds to the latter case and leads to an overall flattening of the energy landscape. In this context the investigation of less promising Li + -conducting materials with insufficient ionic conductivities for application, plays a key role in the understanding of the criteria to design and tailor nextgeneration ionic conductors.

Supporting Information Summary
Details of crystal structure determination of Li 5 SnP 3 , details on the investigation of the system Li 8À 4x Sn 1 + x P 4 (x = À 0.333 to + 0.333), differential scanning calorimetry (DSC), 6 Li, 119 Sn, and 31 P MAS NMR spectroscopy.  7. Correlation between the occupation of the tetrahedral and octahedral voids and the resulting ionic conductivity of the phases Li 5 SnP 3 , αand β-Li 8 SnP 4 and Li 14 SnP 6 . The percentages of Sn and Li in the tetrahedral voids are shown in teal and olive, respectively, and the partial occupation of the octahedral voids is indicated in blue. The corresponding ionic conductivity at room temperature is shown in red according to the scale on the right.