Direct Band Gap Semiconductors with Two- and Three-Dimensional Triel-Phosphide Frameworks (Triel=Al, Ga, In)

Recently, several ternary phosphidotrielates and -tetrelates have been investigated with respect to their very good ionic conductivity, while less focus was pointed towards their electronic structures. Here, we report on a novel series of compounds, in which several members possess direct band gaps. We investigated the known compounds Li₃AlP₂, Li₃GaP₂, Li₃InP₂, and Na₃InP₂ and describe the synthesis and the crystal structure of novel Na₃In₂P₃. For all mentioned phosphidotrielates reflectance UV-Vis measurements reveal direct band gaps in the visible light region with decreasing band gaps in the series: Li₃AlP₂ (2.45 eV), Li₃GaP₂ (2.18 eV), Li₃InP₂ (1.99 eV), Na₃InP₂ (1.37 eV), and Na₃In₂P₃ (1.27 eV). All direct band gaps are confirmed by quantum chemical calculations. The unexpected property occurs despite different structure types. As a common feature all compounds contain EP₄ tetrahedra, which share exclusively vertices for E=In and vertices as well as edges for E=Al and Ga. The structure of the novel Na₃In₂P₃ is built up by a polyanionic framework of six-membered rings of corner-sharing InP₄ tetrahedra. As a result, the newly designed semiconductors with direct band gaps are suitable for optoelectronic applications, and they can provide significant guidance for the design of new functional semiconductors.