Subsurface reconstruction and saturation of surface bonds

Surface reconstructions and stabilization mechanisms have been great challenges for insulators. Based on accurate determination of the long-sought atomic structure of the spinel (1 1 1) surface, here we show that the surface is stabilized by an unconventional mechanism. In general, solid surfaces have unsaturated chemical bonds and are prone to atomic reconstruction to saturate them. The spinel (1 1 1) surface, however, has the surface bonds fully saturated, while the unsaturated bonds remain only in the subsurface. It undergoes a reconstruction that keeps the topmost atomic layer unchanged, but has the subsurface atoms completely rearranged. Such a reconstruction results in a perfect compensation of the surface polarity and a large reduction in the surface energy. This work provides surprising insights into the surface stability and physical and chemical behaviors of complex oxides and insulators.