The formation conditions, structure, and properties of the recently discovered phosphide, Si12P5 (initially assigned Si7P5), have been studied using x-ray diffraction, high-resolution transmission electron microscopy, photospectrometry, and a four-point probe. The phosphide formed in amorphous Si-P alloy thin films with 30, 35 and 44 at. % P after annealing at 1000 degrees C for 30 min. During annealing at 1100 degrees C, the phosphide dissociated through P evaporation. The Si12P5 phase is proposed to have rhombohedral symmetry with a P<(31)over bar>m space group and a C5W12 structure. The hexagonal lattice parameters measured were n=6.16+/-0.05 Angstrom and c=13.17+/-0.01 Angstrom. The optical band gap and the electrical resistivity of the phosphide were determined to be 1.55 eV and 5 m Ohm cm, respectively. The Gibbs free energy diagram for the Si-P system was modified to include this new Si12P5 phase.