We create new materials that cannot be obtained as thermodynamically stable phases under conventional conditions by combining high-pressure synthesis, elemental substitution, cation and anion ordering, and mixed-anion chemistry. In particular, we focus on the relationship between the free energy stored in metastable structures and electronic, magnetic, orbital, and lattice degrees of freedom, with the aim of discovering functions such as quantum magnetism, anomalous transport, exchange bias, magnetocaloric effects, ionic conduction, and giant lattice responses. By integrating materials synthesis, crystal-structure analysis, and physical-property measurements with synchrotron X-ray diffraction, neutron scattering, electron microscopy, and first-principles calculations through collaborative research, we clarify the mechanisms underlying these functions. Our goal is to establish new materials-design principles that exploit metastable structures as functional resources.