Recently, extensive studies on the ceria-based two-phase composites as functional electrolytes have created excellent 300–600°C fuel cell technology. There is an emergence need to deepen the knowledge and to develop theoretical methodologies in this field. The feasibility to design and develop two-phase materials as superionic conductors for 300–600°C solid oxide fuel cells (LTSOFCs) is reported. The superionic conductivity at 300–600°C in two-phase materials where the interfaces between the constituent phases are constructed as “superionic highways” resulting in interfacial high ionic conduction. The material architecture and design presented in this report thus reaches beyond the conventional molecular way to synthesize new compounds.