Na₃Ge₂P₃: A Zintl Phase Featuring [P₃Ge-GeP₃] Dimers as Building Blocks

Recently, ternary lithium phosphidotetrelates have attracted interest particularly due to their high ionic conductivities, while corresponding sodium and heavier alkali metal compounds have been less investigated. Hence, we report the synthesis and characterization of the novel ternary sodium phosphidogermanate Na₃Ge₂P₃, which is readily accessible via ball milling of the elements and subsequent annealing. According to single crystal X-ray structure determination, Na₃Ge₂P₃ crystallizes in the monoclinic space group P2₁/c (no. 14.) with unit cell parameters of a = 7.2894(6) Å, b = 14.7725(8) Å, c = 7.0528(6) Å, β = 106.331(6)° and forms an unprecedented two-dimensional polyanionic network in the b/c plane of interconnected [P₃Ge-GeP₃] building units. The system can also be interpreted as differently sized ring structures that interconnect and form a two-dimensional network. A comparison with related ternary compounds from the corresponding phase system as well as with the binary compound GeP shows that the polyanionic network of Na₃Ge₂P₃ resembles an intermediate step between highly condensed cages and discrete polyanions, which highlights the structural variety of phosphidogermanates. The structure is confirmed by ²³Na- and ³¹P-MAS NMR measurements and Raman spectroscopy. Computational investigation of the electronic structure reveals that Na₃Ge₂P₃ is an indirect band gap semiconductor with a band gap of 2.9 eV.