Aliovalent substitution in phosphide‐based materials – Crystal structures of Na₁₀AlTaP₆ and Na₃GaP₂ featuring edge‐sharing EP₄ tetrahedra (E=Al/Ta and Ga)

Abstract

Recently, ternary lithium phosphides have been studied intensively owing to their high lithium ion conductivities. Much less is known about the corresponding sodium-containing compounds, and during investigations aiming for sodium phosphidotrielates, two new compounds have been obtained. The sodium phosphidoaluminumtantalate Na 10 AlTaP 6 , at first obtained as a by-product from the reaction with the container material, crystallizes in the monoclinic space group P2 1 /n (no. 14) with lattice parameters of a = 8.0790(3) Å, b = 7.3489(2) Å, c = 13.2054(4) Å, and β = 90.773(2)°. The crystal structure contains dimers of edge-sharing [(Al 0.5 Ta 0.5 )P 4 ] tetrahedra with a mixed Al/Ta site. DFT calculations support the presence of this type of arrangement instead of homonuclear Al 2 P 6 or Ta 2 P 6 dimers. The 31 P and 23 Na MAS NMR as well as the Raman spectra confirm the structure model. The assignment of the chemical shifts is confirmed applying the DFT-PBE method on the basis of the ordered structural model with mixed AlTaP 6 dimers. The sodium phosphidogallate Na 3 GaP 2 crystallizes in the orthorhombic space group Ibam (no. 72) with lattice parameters of a = 13.081(3) Å, b = 6.728(1) Å, and c = 6.211(1) Å and is isotypic to Na 3 AlP 2 . Na 3 GaP 2 exhibits linear chains of edge-sharing 1 1 [GaP 4/2 ] tetrahedra. For both compounds band structure calculations predict indirect band gaps of 2.9 eV.

Electronic Structure Analysis of the A10Tt2P6 System (A=Li−Cs; Tt=Si, Ge, Sn) and Synthesis of the Direct Band Gap Semiconductor K10Sn2P6

“…All atoms are located on 4e Wyckoff positions and are fully occupied (Table 2). As mentioned for previous compounds of this structure type, the P atoms form a distorted hexagonal packing with partially occupied tetrahedral voids [3] . Since the unit cell harbours 3 P atoms per stoichiometric unit, with Z=4, a total of 12 octahedral and 24 tetrahedral voids result.…”

“…As mentioned for previous compounds of this structure type, the P atoms form a distorted hexagonal packing with partially occupied tetrahedral voids. [3] Since the unit cell harbours 3 P atoms per stoichiometric unit, with Z = 4, a total of 12 octahedral and 24 tetrahedral voids result. All octahedral voids are filled with atoms K3 to K5, while 50 % of the tetrahedral voids are filled by Sn, K1 and K2.…”

“…While for many ternary alkali‐metal phosphidotetrelates the ionic conductivity was intensively investigated, their electronic structures were merely briefly mentioned [1–5] . Recently interesting electronic properties such as direct band gaps, which make these compounds interesting candidates for electronic applications, have been reported [6] .…”

“…While for many ternary alkali-metal phosphidotetrelates the ionic conductivity was intensively investigated, their electronic structures were merely briefly mentioned. [1][2][3][4][5] Recently interesting electronic properties such as direct band gaps, which make these compounds interesting candidates for electronic applications, have been reported. [6] The class of alkali-metal phosphidotetrelates, comprises many representatives and thus forms a good basis for the search of structure property relationships which may lead to an insight on why direct band gaps occur in some phases.…”