Crystal and Electronic Structure and Optical Properties of AE2SiP4 (AE = Sr, Eu, Ba) and Ba4Si3P8

Three new compounds in the AE-Si-P (AE = Sr, Eu, Ba) systems are reported. Sr2SiP4 and Eu2SiP4, the first members of their respective ternary systems, are isostructural to previously reported Ba2SiP4 and crystallize in the noncentrosymmetric I42d (no. 122) space group. Ba4Si3P8 crystallizes in the new structure type, in P2(1)/c (no. 14) space group, mP-120 Pearson symbol, Wyckoff sequence e(30). In the crystal structures of Sr2SiP4 and Eu2SiP4 all SiP4 tetrahedral building blocks are connected via formation of P-P bonds forming a three-dimensional framework. In the crystal structure of Ba4Si3P8, Si-P tetrahedral chains formed by corner-sharing, edge-sharing, and P-P bonds are surrounded by Ba cations. This results in a quasi-one-dimensional structure. Electronic structure calculations and UV/Vis measurements suggest that the AE(2)SiP(4) (AE = Sr, Eu, Ba) are direct bandgap semiconductors with bandgaps of ca. 1.4 eV and have potential for thermoelectric applications.