Ordered Arrangement of Planar Faults with Picoscale Perfection in Titanium Oxide Natural Superlattices

Coherent control of thermal phonons by atomic-scale periodic structures is of great interest due to the potential for advanced thermal management. However, the coherence is easily broken by interface roughness and interfaces with subatomic-scale perfection, which have never been achieved even by the advanced synthesis of artificial superlattice thin films, are strongly needed. Here, we demonstrated that the ordered arrangement of planar faults in titanium oxide natural superlattices has picoscale structural perfection. High-angle annular dark-field scanning transmission electron microscopy observation revealed that the average periodicity disorder was 36 pm for interface spacings of 2.64-2.85 nm, and the interface roughness was estimated to be 15 pm. Calculation of the specularity parameter indicates that the interface behaves coherently for phonons with a frequency of less than 23 THz, which includes almost all phonons in rutile TiO2. Coherent phonon transport should become apparent in the titanium oxide NSL system owing to these smooth interfaces, whose roughness is 1 order of magnitude lower than that in artificial superlattice thin films.

Automated summary

The ordered arrangement of planar faults in titanium oxide natural superlattices exhibits picoscale structural perfection, with an average periodicity disorder of 36 pm and interface roughness estimated to be 15 pm. Calculation of the specularity parameter indicates that the smooth interfaces in the titanium oxide NSL system can coherent control phonons with frequencies less than 23 THz.