Novel meta-phase arising from large atomic size mismatch

Discovery and regulation of new phases of matter drive science and technology forward. Here, we report unique meta-phase Cu2(S,Te), Ag2(S,Te), and Mg2(Si,Sn) over a wide composition range. In the formation of meta-phase, the large diffusion coefficient mismatch counterbalances the large atomic size and electronegativity mismatches, thereby escaping phase separation and creating stable single-phased exquisite atomic structures beyond the classic Hume-Rothery (H-R) rule. Featured by the highly tunable contrasting degree of order/disorder of the sublattice, the meta-phase showcases electron localization/delocalization in Cu2(S,Te), a rare co-existence of mechanical plasticity and amorphous characteristics in Ag2(S,Te), along with inherently low lattice thermal conductivity and high thermoelectric performance in all three compounds. We envisage that the concept of meta-phase will usher in more innovations in materials research and technical applications.