Solid solution perovskite substrate materials with indifferent points

Single-crystal substrate materials with crystal structures and lattice parameters matching a desired epitaxial film are an enabling technology for many critical materials. Such substrates are best grown by bulk techniques that benefit from the substrate material being congruently melting. The shortage of congruently melting perovskites in critical lattice parameter ranges has been addressed herein by a search for new congruently melting compositions. A solid solution of two perovskites can be congruently melting at a minimum temperature under specified thermodynamic conditions where the coefficient matrix has a zero determinant. This is called an indifferent point. A wide variety of perovskite solid solutions were investigated to identify compounds that have an indifferent melting minimum and a cubic crystal structure with favorable lattice constants in the range of 0.390–0.412 nm. Solid solution pairs that form such indifferent points include, but are not limited to, seven compositions identified in this study. The lattice parameters of the new materials show a broad range of desirable lattice constants. A perovskite tolerance factor close to unity gave a favorable contribution to the free energy and makes a cubic crystal structure favorable. These solid solutions constitute a broad and important class of compounds not previously anticipated. The only significant drawback is the high vapor pressure of sodium, which is present in most of these materials and requires special growth conditions and equipment for safety.