Synthesis, Structure, Solid‐State NMR Spectroscopy, and Electronic Structures of the Phosphidotrielates Li3AlP2 and Li3GaP2

Abstract The lithium phosphidoaluminate Li9AlP4 represents a promising new compound with a high lithium ion mobility. This triggered the search for new members in the family of lithium phosphidotrielates, and the novel compounds Li3AlP2 and Li3GaP2, obtained directly from the elements via ball milling and subsequent annealing, are reported here. It was unexpectedly found through band structure calculations that Li3AlP2 and Li3GaP2 are direct band gap semiconductors with band gaps of 3.1 and 2.8 eV, respectively. Rietveld analyses reveal that both compounds crystallize isotypically in the orthorhombic space group Cmce (no. 64) with lattice parameters of a=11.5138(2), b=11.7634(2) and c=5.8202(1) Å for Li3AlP2, and a=11.5839(2), b=11.7809(2) and c=5.8129(2) Å for Li3GaP2. The crystal structures feature TrP4 (Tr=Al, Ga) corner‐ and edge‐sharing tetrahedra, forming two‐dimensional ∞2TrP23- layers. The lithium atoms are located between and inside these layers. The crystal structures were confirmed by MAS‐NMR spectroscopy.


Introduction
Lithium ion solid electrolytes have been intensively studied for years due to the promisinge nhanced safety and electrochemical performances of all-solid-state-batteries. [1][2][3] Thus,m any new materials with potentially high lithium ion conductivity have been discussed in the literature. [3][4][5][6][7] Recently,w ith Li 14 SiP 6 , Li 8 SiP 4 and a/b-Li 8 GeP 4 ,w ei ntroduced group 14 phosphidebased lithium ion conductors, which achieve ionic conductivities up to 1m Scm À1 . [8][9][10] Theirs tructures are built by group 14 phosphorous tetrahedra [TtP 4 ] 8À (Tt = Si, Ge). In the case of Li 8 SiP 4 and a/b-Li 8 GeP 4 ,i solated [TtP 4 ] 8À tetrahedra occur.A t lower Li contentsw ef ound that the tetrahedra are connected in different ways and form dimers as in Li 10 Si 2 P 6 ,t wo-dimensional slabs as in Li 3 Si 3 P 7 ,o rt hree-dimensional networks as in Both compounds are characterizedb yR ietveld analysis and MAS-NMR spectroscopy.I na ddition, electronic band structure calculations are discussed.
Powder X-ray diffraction:F or powder X-ray diffraction (PXRD) measurements, the samples were grounded in an agate mortar and sealed inside 0.3 mm glass capillaries. PXRD measurements were performed at room temperature on aS TOES tadi Pd iffractometer equipped with aG e(111)m onochromator for Cu Ka 1 radiation (l = 1.54056 )a nd aD ectris MYTHEN DCS 1K solid-state detector.T he raw powder data were processed with the software package WinXPOW. [19] Structure determination and Rietveld refinement:T he structures of Li 3 TrP 2 were determined by Rietveld refinements of the powder X-ray diffraction data using JANA2006. [20] The initial structure model was gained using the subprogram Superflip. [21] The space group Cmce (no. 64) and the cell parameters were determined with the subprogram system evaluation of WinXPOW. [19] All cell parameters, all atom positions and the isotropic displacement parameters of Tr and Pwere refined freely.The isotropic displacement parameters of all Li atoms were refined coupled in Li 3 AlP 2 and uncoupled in Li 3 GaP 2 .
CCDC 1979150 (Al) and 1979151 (Ga) contain the supplementary crystallographic data for this paper.T hese data are provided free of charge by The Cambridge Crystallographic Data Centre through the CCDC/FIZ Karlsruhe deposition service.
Energy-dispersive X-ray spectroscopy (EDX):P ieces of Li 3 TrP 2 were measured on aH itachi TM-1000 Ta bletop (15 kV) scanning electron microscope equipped with an energy dispersive X-ray an- alyser (SWIFT-ED-TM). The samples were mounted onto an aluminium stub using graphite tape. To exclude aluminium impurities of the stub in the EDX spectra, measurements of the graphite tape on the aluminium stub without the samples were carried out, and no aluminium signal was registered. All samples were measured three times, and the values were averaged. Differential scanning calorimetry (DSC):F or thermal analysis samples of Li 3 TrP 2 were sealed in niobium ampules and measured on a DSC instrument (Netzsch, DSC 404 Pegasus) under ac onstant gas flow of 75 mL min À1 .T he samples were heated to 750 8Ca nd then cooled to 150 8Ct wice at ar ate of 10 8Cmin À1 .F or the determination of the onset temperatures of the DSC signals, the PROTEUS Thermal Analysis software was used. [22] Impedance spectroscopy: The electrochemical impedance spectroscopy for Li 3 TrP 2 was performed in an in-house designed cell. The detailed setup and procedure are described in Restle et al. [13] Impedance spectra were recorded on aB io-Logic potentiostat (SP-300) in af requency range from 7MHz to 50 mHz at ap otentiostatic excitation of AE 50 mV.D ata were treated using the software EC-Lab (V 11.27). The measurements were performed in an Ar-filled glove box at 26 8C. NMR spectroscopy:M agic-angle spinning (MAS) NMR spectra have been recorded on aB ruker Avance3 00 NMR device operating at 7.04 Ti na4mmZ rO 2 rotor.T he resonance frequencies of the nuclei are 44.17, 78.21, 91.53, and 121.46 MHz for 6 Li, 27 Al, 71 Ga, and 31 P, respectively.T he rotational frequency was set to 15 kHz for all nuclei. The MAS spectra have been obtained at room temperature with relaxation delays of 10 s( 6 Li), 2s( 27 Al), 2s( 71 Ga), and 30 s ( 31 P), and 800 scans ( 6 Li), 280 scans ( 27 Al), 200 scans ( 71 Ga), and 720 scans ( 31 P). All 6 Li spectra were referenced to LiCl (1 m,a q) and LiCl (s) with chemical shifts of 0.0 ppm and À1.15 ppm, respectively.T he 27 Al spectrum is referred to aluminium nitrate nonahydrate (s) with ac hemical shift of À0.54 ppm with reference to Al(H 2 O) 6 3 + in aqueous solution. The 71 Ga spectrum is referred to gallium nitrate monohydrate (1 m,a q) with ac hemical shift of 0ppm. The 31 P spectra were referred to ammonium dihydrogen phosphate (s) with ac hemical shift of 1.11ppm with reference to concentrated H 3 PO 4 .All spectra were recorded using single-pulse excitation.
Electronic structure calculations: The computational analysis for the structures Li 3 AlP 2 and Li 3 GaP 2 was performed using the Crys-tal17 program package and hybrid density functional methods. [23,24] Ah ybrid exchange correlation functional after Perdew,B urke and Ernzerhof (PBE0) [25,26] and triple-zeta valence + polarization level basis sets derived from the Karlsruhe basis sets for the elements Li, Al, Ga, and Pw ere applied (further details are in the Supporting Information). [27][28][29] The starting geometry was taken from the experimental findings, and all structures were fully optimized within the constraints imposed by the space group symmetry.Band structures and density of states (DOS) were calculated for both structures. The nature of as tationary point on the potential energy surface was confirmed to be am inimum by af requency calculation for each compound at G-point. No imaginary frequencies were observed. For data processing and visualization Jmol was used. [30] Results and Discussion Synthesis and characterization of Li 3 TrP 2 Phase-pure Li 3 AlP 2 and almost phase-pure Li 3 GaP 2 were synthesized from the elements via at wo steps procedure. Firstly,s toichiometric amountso fL i, Tr and Pw ere ball milled resulting in reactive mixtures which showedt he mosti ntense reflections with large half width of the corresponding compound in the X-ray powder diffractogram (see Figures S1 and S2 in Supporting Information). Subsequently,p ellets of the reactive mixtures were annealed in niobium ampules at 700 8Cfor one day,yielding phase-pure Li 3 AlP 2 and Li 3 GaP 2 ,w hich contained small amountso fG aP as as ide phase (see Figure 1). Powdered Li 3 AlP 2 is yellow-ochre,p owderedL i 3 GaP 2 is brick-red (see Figure S3). Energy dispersive X-ray spectroscopy( EDX) investigations of the products showt he absence of Wa nd Nb and are in very good accordance with the Al/P und Ga/P ratios used in syntheses (see Ta ble S1). Li 3 AlP 2 and Li 3 GaP 2 can also be synthesized by heating stoichiometrica mounts of the respective elements at 550 8Cf or one week. However,a nu nknown phase accompanied by TaPr emains as impurity in Li 3 AlP 2 ,w hereas the sample of Li 3 GaP 2 contains GaP plus another unknown phase (see Figures S4 and S5). Due to the good quality of the powderd iffractograms the structures of Li 3 AlP 2 and Li 3 GaP 2 could be solved and refined from the powder X-ray diffraction data. The results from the Rietveld refinement are shown in Figure 1, and parameters are listed in Table 1.
Li 3 AlP 2 and Li 3 GaP 2 crystallize in the orthorhombic space group Cmce (no. 64) with five independent crystallographic positions (P1, P2, Tr1, Li1, and Li2) (Table S2). Compared to the earlier reported cell (a = 11.47, b = 11.61 and c = 11.73 ), which corresponds to a2 2 2o rthorhombic distorted supercell of the anti-CaF 2 structure type, [14] we observe ac orresponding 2 2 1o rthorhombic supercell.A3 1 1u nit cell of the orthorhombicc rystal structure is displayed in Figure 2a.T he crystal structure is built up by an orthorhombic distorted cubic close packing of Pa toms. The Tr atoms occupy one quarter of the tetrahedral voids, forming AlP 4 tetrahedra.T he occupation occursi nafully ordered manner and is found only in every second layer.W ithint he layer the Tr atoms occupy 50 %o ft he tetrahedral voids. Pairs of the resulting AlP 4 tetrahedra are connectedb ys haring edges through P1, and the resulting dimers share corners throughP 2, resulting in at wo-dimensional ½ layer (Figure 2b). All remaining tetrahedral voids based on a ccp packing of Pa toms are occupied with lithium, whereby Li1 is located within the 1 2 TrP 2 3À ½ layers, and Li2 occupies the tetrahedral voids between the layers. The stacking sequence of the 1 2 TrP 2 3À ½ layers is ABAB, as shown in Figure S6, in which the edge-sharing Al 2 P 6 dimers of the adjacent layers are located above the neighbouring tetrahedral sites, which are occupied by Li (shift along a by a/2). Li 3 TrP 2 has a similar structure as LiNa 2 AlP 2 ,i nw hich exclusively Na atoms are located between, and Li atoms within the 1 2 AlP 2 3À ½ layers, resultingi nalarger separation of the layers (longer b-axis with 13.592(3) in LiNa 2 AlP 2 compared to 11.7634(2) in Li 3 AlP 2 ). [31] The bond lengthsi nt he TrP 4 tetrahedra are in the narrow range between 2.398(3) and 2.410(3) for the Al1ÀP1 and Al1ÀP2 distances, respectively,a nd between 2.404(2) and 2.419(2) for the Ga1ÀP1 and Ga1ÀP2 distances, respectively. As expected, the AlÀPb onds are slightly shorter than the GaÀ Pb onds. The AlÀPb ond lengths are very similar to those in Li-Na 2 AlP 2 (2.410(3)-2.426(3) )a nd are in the range of other known compounds with strongA l ÀPi nteractions like in AlP (2.360 ), Na 3 AlP 2 (2.376(4) )o ri nS r 3 Al 2 P 4 (2.377(3)-2.417(2) )a nd weaker AlÀPi nteractions like in Li 9 AlP 4 (2.423(2)-2.434(1) ). [13,[31][32][33][34] In the case of Tr = Ga, the GaÀP bonds are longert han in GaP (2.3601(1) )a nd slightly shorter than in Ba 3 GaP 3 (2.43(1) ), where [Ga 2 P 6 ] 6À dimers occur. [32] Due to the edge-und corner-sharing situation of the TrP 4 tetra-hedra,t hese TrP 4 units are distorted. This distortion is exemplified by the deviation of the P-Al-P angles between 101.70(1)8 and 111.57(1)8 and of the P-Ga-P angles between 100.6(1)8 and 112.18(1)8 from the ideal tetrahedral angle of 109.478.T he Li-P bond lengths in Li 3 AlP 2 range from 2.51(1) to 2.61(1) and from 2.50(1) to 2.65(1) in Li 3 GaP 2 .O verall,t hesed istances are in good agreement comparedt ootherb inary or ternary phases containing Li and P. [8][9]11] Considering three positively chargedl ithium atoms and the threefold negatively charged 2D layer,L i 3 TrP 2 can be written as an electronically balanced formula (Li + ) 31 2 TrP 2 3À ½ . The lattice parameters of Li 3 AlP 2 and Li 3 GaP 2 vary only slightly in the b-a nd c-axes, whereas the a-axes are significantly different. Hence, the substitutiono ft he aluminium atoms within the tetrahedra by the larger gallium atoms (ionic radii 0.53 for Al 3 + and 0.61 Ga 3 + by Shannona nd Prewitt) [35] leads to an anisotropic enlargement of the 1 2 TrP 2 3À ½ 2D layer due to the fact that in a-direction the tetrahedra share edges and corners, whereas in c-direction they are exclusivelyc onnected by corners (as shown in Figure 2b and Figure S6).
The size of the alkali-metal atom has as trong impact on the connectivity of the AlP 4 tetrahedra. In LiNa 2 AlP 2 ,t he identical 2D 1 2 AlP 2 3À ½ layerso fA lP 4 tetrahedra exist like in Li 3 AlP 2 ,a lthoughw ith larger distances between the layers due to the larger Na ions that are located between the layers.T he same appliest oL iK 2 AlP 2 .I nterestingly,i nN a 3 AlP 2 ao ne-dimensional chain with exclusively edge-sharing tetrahedra is observed.T he reason might be that the large Na atoms do not allow af illing of the tetrahedra. More space is provided, when the Na atoms are located between the chains. [33] In the case of Cs 3 AlP 2 ad ifferent structure is adopted. [36] The Al atoms are coordinated in at rigonal planar manner by phosphorus atoms, leadingt od imers of edge-sharing triangles. In the case of related galliumc ompounds, only ternary alkali metal phosphidogallates with trigonal planar GaP 3 triangles are reported (Na 6 GaP 3 ,K 2 GaP 2 ,R b 3 GaP 2 ,C s 6 Ga 2 P 4 ), [37][38][39][40] and Li 3 GaP 2 represents the first ternary alkali metal-based phosphidogallate with gallium in at etrahedral coordinatione nvironment. Further,i n quaternary mixed-alkali metal phosphidogallates,s uch GaP 4 tetrahedra already exist, fore xample, in K 2 NaGaP 2 and Cs 2 NaGaP 2 . [41,42] As observed for phosphidoaluminate derivatives with larger alkali metals like Na 3 AlP 2 ,i nK 2 NaGaP 2 and Cs 2 NaGaP 2 ,t he GaP 4 tetrahedra are arranged in edge-sharing 1D chains. Related alkaline earth metal phosphidotrielatesc ontain the same polyanion 1 2 TrP 2 3À ½ .F ormally three Li ions are replaced by one and ah alf alkaline earth metal, such as in Ca 3 Al 2 P 4 ,C a 3 Ga 2 P 4 ,S r 3 Ga 2 P 4 and Ba 3 Al 2 P 4 . [34,43] As tructural change dependingo nt he size of the alkaline earth metal atom can also be observed in these species. In the case of the smaller Ca and Sr atoms the structures contain distorted 2D layers of edge-and corner-sharing TrP 4 tetrahedra. However,i n Ba 3 Al 2 P 4 ,t he larger Ba atoms lead to as egregation into twisted chainsw ith only edge-sharing AlP 4 tetrahedra. Differential scanning calorimetry DSC measurements of both compounds were performed (Figures S9 and S10) and show that Li 3 AlP 2 is stable up to 750 8C, whereas Li 3 GaP 2 is stable only up to about 710 8C. Above this temperature Li 3 GaP 2 might melt or decompose into other unknown phases,a sa lso supported by the PXRD data after the measurement (Figures S11and S12).

Impedances pectroscopy
The Nyquist-plots for Li 3 AlP 2 andL i 3 GaP 2 are shown in Figure S17 and Figure S18. The Nyquist-plots display only the be-haviouro facapacitor.H ence, no lithium diffusion was observed by electrochemical impedance spectroscopy.

MAS-NMRs pectroscopy
For Li 3 AlP 2 and Li 3 GaP 2 , 6 Li, 27 Al, 71 Ga, and 31 PM AS-NMR measurements were performed (see Figure 3). In agreement with the crystallographic multiplicity,t wo independent 6 Li signals occur in the expected ratio of 1:2( 4.00 and 2.96 ppm in Li 3 AlP 2 and 4.14 and 3.39 ppm in Li 3 GaP 2 ). The lithium atoms inside the 1 2 TrP 2 3À ½ layers are shifted more downfield than the others. In comparison to the signals of the aluminium phase, the resonances of both lithium signals in the gallium phase are shiftedt ol ower fields. Hence, the layer itself andt he more electronegative metal galliuml ead to ah igherd eshielding of the signals. For both compounds the chemical shift of the Li atoms are in the same range as those for related phosphidosilicates like Li 8 SiP 4 and Li 3 Si 3 P 7 . [8,11] The 27 Al, respectively 71 Ga NMR spectra show only one signal in accordance with the crystal structure. The Al shift of 137 ppm utterly fits to the one of tetrahedral aluminium phosphines in solution andm atches almostp erfectly to the tetrahedrally coordinated Al in AlP (142 ppm). [44,45] The chemical shift of 304 ppm of Ga also is in good agreement with the tetrahedral environmento fG ai n GaP (307 ppm). [45] The shape of the 71 Ga signali ss lightly asymmetric due to small GaP impurities at 307 ppm. Li 3 AlP 2 shows two singlets in the 31 PM AS-NMR spectrum. Both signals can be integrated with av alue of one. Their chemical shifts are in the range of isolatedP 3À in Li 3 Pa nd tetrahedrally coordinated Pi nL i 8 SiP 4 . [8,46] For Li 3 GaP 2 two main signals occur with almost the same integrated intensity.T he 31 Ps ignals are shifted slightly more to lower fieldst han in Li 3 AlP 2 .T he smalls ignal at À143 ppm can be assignedt oG aP. [47] Summing up, the NMR measurementsa re in very good agreement with the crystal structure evaluation on the basis of the Rietveld analyses.

Electronicstructures
For Li 3 AlP 2 and Li 3 GaP 2 ac omputational analysisw as carried out at aD FT-PBE0/TZVP level of theory.T he optimized structure for Li 3 AlP 2 and Li 3 GaP 2 exhibit am aximum deviationo f 1.95 %f or all parameters and average atomicd istances, which reassures the experimental findings (Table2). Band structure calculations show that both compoundsa re semiconductors with direct band gaps (Figure 4). Due to the usage of hybrid functional the calculated band gaps are typicallyi ng ood agreement with the experiment. The calculated band gap of Li 3 GaP 2 of 2.8 eV is significantly smaller than the one of Li 3 AlP 2 (3.1 eV). This divergence is in accordance with the two different colours of the phases:L i 3 AlP 2 with al arger band gap is of yellow-ochre colour, whereas Li 3 GaP 2 with as maller band gap is brick red. The densities of states revealt hat the contribution of phosphorus is the highest at the valence band maximum, whereas in the conduction band minimum aluminium and gallium have the highest contributions. The calculated band structure is typicalofadirect band gap semiconductor.

Conclusions
Li 3 AlP 2 is an ew representative of lithium phosphidoaluminates. It is the first lithium phosphidoaluminate with interconnected AlP 4 tetrahedra. In the orthorhombic distorted lattice, the AlP 4 tetrahedra are connected via edges and corners to give 2D layers.T he lithium atomsa re located between and within these layers. Li 3 GaP 2 represents the first lithium phosphidogallate. Both phases are easily accessible through ball millingo ft he elements and subsequent annealing and show thermals tabilityu pt o7 00 8C. In the respective MAS-NMR spectra all different positions can be assigned individually. Though both compounds are poor ion conductors, band structure calculations revealt hat Li 3 AlP 2 and Li 3 GaP 2 are directb and gap semiconductors with band gaps of 3.1a nd 2.8 eV,r espectively.T hese resultsd emonstrate that lithium phosphidotrielates can-depending on the content of Li 3 P-also exhibit structures with connected TrP 4 tetrahedra.