“…The discovery of interfacial phase‐like behaviors provided new insights into the understandings of a spectrum of long‐standing scientific mysteries, for example, origins and atomic mechanisms of activated sintering of ceramics and refractory metals, [ 5–11 ] liquid metal embrittlement of Ni–Bi and Al–Ga [ 52,53,63–65 ] as well as the classical GB embrittlement of Bi versus S‐doped Ni (Figure 2), [ 52–54 ] and abnormal grain growth in Al 2 O 3 and Ni–S. [ 54,56,57 ] IGFs and other GB complexions are also known to affect the toughness, strength, fatigue, and wear resistance of Si 3 N 4 , SiC, and Al 2 O 3 and other ceramics, [ 9,23,24,27,44,66,67 ] the hot strength and creep and oxidation resistance of various structural ceramics, [ 68–76 ] superplasticity of zirconia, [ 77 ] grain growth and mechanical properties of WC‐based cermets, [ 55,78–80 ] the stability and mechanical properties of nanocrystalline alloys, [ 81–91 ] corrosion of synroc, [ 92 ] the electrical resistance of ruthenate thick‐film resistors, [ 93 ] the coercivity of Nd–Fe–B magnets, [ 94 ] the nonlinear I–V character of ZnO‐based varistors, [ 1,42,43 ] the critical current of YBCO superconductors, [ 95 ] the ionic conductivity of solid electrolytes, [ 96–98 ] and performance of various battery electrode materials, [ 99–102 ] amongst other structural and functional properties. [ 12,13,27,50,103 ]…”