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

Abstract

Thel ithium phosphidoaluminate Li 9 AlP 4 represents apromising new compound with ahigh lithium ion mobility. This triggered the searchf or new members in the familyo f lithium phosphidotrielates, and the novel compounds Li 3 AlP 2 and Li 3 GaP 2 ,o btained directly from the elements via ball milling ands ubsequenta nnealing, are reported here. It was unexpectedly found through band structure calculations that Li 3 AlP 2 and Li 3 GaP 2 are directb and gap semiconductors with band gaps of 3.1 and 2.8 eV,r espectively.R ietveld anal-yses revealt hat both compounds crystallize isotypically in the orthorhombic space group Cmce (no. 64) with lattice parameters of a = 11.5138(2), b = 11.7634(2)a nd c = 5.8202(1) for Li 3 AlP 2 ,a nd a = 11.5839(2), b = 11.7809(2) and c = 5.8129(2) for Li 3 GaP 2 .T he crystal structures feature TrP 4 (Tr = Al, Ga) corner-and edge-sharing tetrahedra, forming two-dimensional 1 2 TrP 2 3À ½ layers. The lithium atoms are located between and inside these layers. The crystal structures were confirmed by MAS-NMR spectroscopy.Li 2 SiP 2 . [8,11] Interestingly,t he phases Li 8 SiP 4 ,L i 5 SiP 3 (= Li 10 Si 2 P 6 ), Li 2 SiP 2 ,a nd LiSi 2 P 3 are connected by af ormal reduction of the formula by units of Li 3 P. [11] Al ower Li 3 Pc ontent leads to a higherc onnectivity of the tetrahedra.Compared to the related sulfide-basedl ithium ion conductors, [3,6,7,12] the anionic substructure of phosphido-based conductorsc arry one additional charge( formal "P 3À "v ersus a formal "S 2À "), and thus the Li content that is required for chargeb alance is higher.R ecently,w ee xpanded this concept of highly charged tetrahedra to lithium phosphidoaluminates by replacing the centralg roup 14 metal by aluminium. [13] Li 9 AlP 4 contains highly charged[TrP 4 ] 9À tetrahedra andreaches high ionic conductivities of % 3.0 mS cm at room temperature. Besides this first report of astructurally characterized lithium phosphidoaluminate, another compound of the composition Li 3 AlP 2 was mentioned already in 1952 and described with an orthorhombic distorted CaF 2 -type structure, in which the phosphorus atoms form ad istorted cubic close packing, althoughw ithoutr eliable crystallographic data. [14] Twoy ears later,t he corresponding gallium compound Li 3 GaP 2 was also postulated. [15] Despite the poorly characterized structure model, quantum-chemical calculations of Li 3 AlP 2 and Li 3 GaP 2 were performed, anticipating the model of vertex-sharing AlP 4 tetrahedra. [16][17][18] As forl ithium phosphidotetrelates, lithium phosphidoaluminates can also be connected on al ine in a Gibbs composition triangle (Finetti diagram). Li 3 AlP 2 is located on the line in the phase system Li-Al-P connecting Li 3 Pa nd AlP ( Figure S7, Supporting Information) by reducing Li 9 AlP 4 by two units of Li 3 P( Li 3 AlP 2 = Li 9 AlP 4 À2 Li 3 P). Assuming ac harge balanced valence compound, the degree of connectivity of the AlP 4 tetrahedra in Li 3 AlP 2 mustb eh igher, and isolated tetrahedra as observed in Li 9 AlP 4 cannot occur.Here...

Supertetrahedral polyanionic network in the first lithium phosphidoindate Li3InP2 – structural similarity to Li2SiP2 and Li2GeP2 and dissimilarity to Li3AlP2 and Li3GaP2

“…This range is typical for chemical shifts of two-fold connected P 1− atoms such as in Li 3 AlP 2 (−300 and −308.7 ppm) or Li 3 GaP 2 (−234.8 and −280.5 ppm). 16 However, the signals of two-fold connected P 1− atoms in the related phosphidotetrelates are much more downfield shifted (Li 2 SiP 2 : −129.1 and −241.5 ppm and Li 2 GeP 2 : −59.9, −164.8 and −178.4 ppm) due to the deshielding of the more electronegative tetrel elements compared to indium. 8,13 …”

“…The 6 Li NMR spectrum shows only one signal with a chemical shift of 7 Li MAS-NMR spectroscopy), and Li 2 GeP 2 (3.6 and 2.4 ppm). 8,13,15,16 Compared to the abovementioned compounds with two signals in the 6 Li NMR spectrum, the difference in local coordination, which is expressed by the P-Li-P angles, is the lowest for -59.9 , -164.8 and -178.4 ppm) due to the deshielding of the more electronegative tetrel elements compared to indium. 8,13 Chemical Science Accepted Manuscript In order to gain additional insight into the experimentally observed structure types, we performed DFT-based structural optimizations for the Al, Ga and In compounds using the PBEsol functional 24 as implemented in CASTEP 25 (computational details are given in the ESI).…”

“…In recent investigations we expanded this class of compound further to phosphidoaluminates, which contain tetrahedral AlP 4 building units, and we discovered the fast lithium ion conductor Li 9 AlP 4 , which shows ionic conductivities of 3 × 10 −3 S•cm −1 . 15 In addition, we also obtained Li 3 AlP 2 , which is built up by 2 ∞ [AlP 2 3 -] layers of corner-and edge-sharing AlP 4 tetrahedra, 16 and we then also introduced the isotypic gallium compound Li 3 GaP 2 as the first phosphidogallate. 16 Both trielate (Tr = Al, Ga) compounds do not show moderate lithium ion conductivity but unexpectedly turned out to be direct band gap semiconductors with optical band gaps of 3.1 and 2.8 eV, respectively.…”

“… 15 In addition, we also obtained Li 3 AlP 2 , which is built up by layers of corner- and edge-sharing AlP 4 tetrahedra, 16 and we then also introduced the isotypic gallium compound Li 3 GaP 2 as the first phosphidogallate. 16 Both trielate (Tr = Al, Ga) compounds do not show moderate lithium ion conductivity but unexpectedly turned out to be direct band gap semiconductors with optical band gaps of 3.1 and 2.8 eV, respectively.…”

“…This correlates with a larger increase of the size of the InP 4 tetrahedron (8.8857 Å 3 ) compared to AlP 4 (7.0897 Å 3 ) and GaP 4 (7.1334 Å 3 ). Comparison of the shortest interatomic Tr-Tr (Tr = Al, Ga, In) distances, the cell volume and the volume per formula unit in Li 3 AlP 2 , Li 3 GaP 2 and Li 3 InP 2 from Rietveld refinements at room temperature 16.…”