New family of direct band gap semiconductors. The new compound Na₃In₂P₃ is a semiconductor with a direct band gap of 1.27 eV and exhibits a three-dimensional polyanionic framework of corner-sharing InP₄ tetrahedra. Additionally, the related phosphidotrielates Li₃AlP₂, Li₃GaP₂, Li₃InP₂ and Na₃InP₂, which are all built up by triel phosphorus tetrahedra as well, have been proven to be semiconductors as well, with direct band gaps ranging from of 1.37 eV to 2.45 eV.

Abstract

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.