Garnet Polyhedron in the Isobaric–Isothermal Y2O3–Bi2O3–Fe2O3–Ga2O3 Tetrahedron

X-ray diffraction data have been used to construct the composition polyhedron of the (Y,Bi)3(Fe,Ga)5O12 garnet phase (18 vertices and 11 faces) in the isobaric–isothermal Y2O3–Bi2O3–Fe2O3–Ga2O3 composition tetrahedron. We have identified nine crystalline phases involved in multiphase equilibria with the garnet phase, paying particular attention to equilibria involving the garnet and perovskite phases.