Synthesis and Crystal Structure of Three Ga-rich Lithium Gallides, LiGa₆, Li₁₁Ga₂₄, and LiGa₂

Abstract

Three new binary phases have been synthesized in the Ga-rich part of the Li− Ga system: LiGa 6 , Li 11 Ga 24 , and LiGa 2 . Their crystal structures and the respective phase formation conditions have been investigated with X-ray single crystal structure refinements, Rietveld refinements of X-ray powder diffraction data, and thermal analyses. They complete the Ga-rich part of the Li−Ga phase diagram together with the reported phases Li 6−x Ga 14 with 2 ≤ x ≤ 3 and LiGa 3.42 . The compositions of two of the new gallides, LiGa 6 and LiGa 2 , had been predicted in previous thermoanalytical studies, but their crystal structures remained unknown. All three new binary main group compounds adopt new structure types. LiGa 6 crystallizes with the trigonal space group R3̅ c (No. 167, a = 6.1851(8) Å, c = 23.467(4) Å), Li 11 Ga 24 crystallizes with the hexagonal space group P6 3 mc (No. 186, a = 13.7700(19) Å, c = 23.250(5) Å), and LiGa 2 crystallizes with the orthorhombic space group Cmce (No. 64, a = 8.51953(4) Å, b = 14.44163(7) Å, c = 15.29226(7) Å). All phases form air-and moisture-sensitive crystals of bright metallic luster. They can be synthesized starting from the pure elements and taking into account their incongruent melting behavior by adequate tempering sequences derived from differential scanning calorimetry (DSC) studies of the system. Lithium gallides do not form electron-precise Zintl phases. The electronic structures of these polar intermetallic phases combine ionic, covalent, and metallic bonding contributions and have been analyzed by density functional theory (DFT) calculations in the cases of LiGa 6 and LiGa 2 . Measurements of the specific electronic resistivities have also been performed and prove the metallic behavior.

Na2Ga7: A Zintl–Wade Phase Related to “α-Tetragonal Boron”

“…However, this counting scheme does not strictly apply to all alkali-metal gallides. For gallides of lithium, both electron-deficient and electron-excess compositions have been reported as well (p + stands for a positive and e – for a negative charge). ,, Such cases are worth investigating in depth. …”

“…However, this counting scheme does not strictly apply to all alkali-metal gallides. For gallides of lithium, both electron-deficient and electron-excess compositions have been reported as well (p + stands for a positive and e – for a negative charge). ,, Such cases are worth investigating in depth. 2 Li 2 Ga 7 = Li 4 Ga 14 = false[ Li + false] 4 [ false( 12 normalb false) ( Ga 12 ) 2 − ] false[ ( 4 b ) Ga − false] 2 2 Li 1.5 Ga 7 = Li 3 Ga 14 = false[ Li + false] 3 [ false( 12 normalb false) ( Ga 12 ) 2 − ] false[ ( 4 b ) Ga − false] 2 · normalp + ...…”

“…For gallides of lithium, both electron-deficient and electron-excess compositions have been reported as well (p + stands for a positive and e – for a negative charge). 13 , 14 , 45 Such cases are worth investigating in depth. …”

“…Gallium transforms to an fcc packing like aluminum only at extreme conditions, 9 , 10 and it has a much lower tendency than boron to form icosahedra, which have only been observed in δ-Ga. 11 However, the topology of the α-Ga structure has been described by motifs related to open icosahedra. 12 The crystal structures of alkali-metal gallides typically comprise Ga 12 icosahedral units, which can be connected by four-bonded Ga atoms as in the cases of LiGa 2 , 13 Li 3 Ga 14 , 14 RbGa 7 , 15 and CsGa 7 . 16 The atomic connectivity in the mentioned compounds is thus reminiscent of the structure formerly considered as “α-tetragonal boron” 17 − 20 or the Mg 2 B 24 C type.…”

Ordering by cation replacement in the system Na2−xLixGa7

“…For the sample with x = 1.5, the majority phase was Na 1 Li 1 Ga 7 , accompanied by foreign phases (Table 1). During annealing, the sample melted partially, indicating that the identified phases, Na 1 Li 1 Ga 7 , Li 3 Ga 14 , LiGa 2 and LiGa 6 , 13 were not in equilibrium upon cooling. Therefore, we conclude that the upper limit for substitution of Na atoms is reached at x = 1.…”