Polymorphism of Ba2SiP4

The three-dimensional SiP₄ network in the known phosphidosilicate Ba₂SiP₄-tI28 is analogous to β-Cristobalite if oxygen is formally replaced by P–P dimers. Here we report a second polymorph Ba₂SiP₄-oP56 Pnma, a = 12.3710(4) Å, b = 14.6296(7) Å, c = 7.9783(3) Å; Z = 8] with chains of SiP₄ tetrahedra connected by P–P bonds, reminiscent to the elusive fibrous SiO₂. Ba₂SiP₄ is enantiotropic. The high temperature polymorph Ba₂SiP₄-oP56 transforms to the low-temperature phase Ba₂SiP₄-tI28 at 650 °C and reconstructs to the high-temperature modification at 1100 °C. DFT calculations predict an indirect optical bandgap of about 1.7 eV.