Ordinary-pressure phase transition from graphite to diamond induced by Ta atoms

High pressure high temperature (HPHT) was conventionally required for the phase transition from graphite to diamond. Here, we deposited monodispersed Ta atoms on pressed pure graphite substrate in hot filament chemical vapor deposition system and then performed annealing on the sample under ordinary pressure, letting graphite transit to diamond. Specifically, along the longitudinal direction the unannealed sample is divided into three regions of TaC particles, amorphous carbon and graphite, and 2-3 nm sized diamond grains are only observed among the TaC particles. After annealing the sample under 1000 degrees C for 15 min, diamond grains not only form among TaC particles, but also appear in amorphous carbon region. It is observed that more diamond grains with larger size of 3-10 nm and diamond variant structures i-Carbon (i-C) with the size larger than 30 nm are formed in amorphous carbon region. And more monodispersed Ta atoms are distributed in the amorphous carbon region of the annealed sample. This suggests that monodispersed Ta atoms firstly change graphite into amorphous carbon and then induce its transformation into i-C and diamond. Our results give a different way from HPHT method to prepare diamond based on graphite under ordinary pressure.