In this paper, a series of high-quality hydrogen-doped diamonds is successfully synthesized in Ni70Mn25Co5-C system by using Fe(C5H5)2 as hydrogen source at pressures ranging from 5.5 GPa to 6.0 GPa and temperatures of 1280–1400 ◦C. We find that both pressure and temperature conditions strengthen with adding the Fe(C5H5)2. Scanning electron microscope micrographs show that the obtained diamonds at low levels of Fe(C5H5)2 additive have smooth surfaces. However, many defects are found and some pores appear on the diamond surface with increasing the Fe(C5H5)2 additive in the system. From the obtained Fourier transform infrared (IR) spectrum, we notice that there is no significant change of nitrogen concentration in the synthesized diamond with the Fe(C5H5)2 additive lower than 0.3 wt%, while the nitrogen concentration gradually decreases with the further increase of Fe(C5H5)2 additive. In the system with 0.5 wt%Fe(C5H5)2 additive, the nitrogen concentration in synthesized diamond is only half that of system without Fe(C5H5)2 additive. Meanwhile, the hydrogen associated IR peaks of 2850 cm−1 and 2920 cm−1 are gradually enhanced with the increase of Fe(C5H5)2 additive in the system, indicating that most of the hydrogen atoms in the synthesized diamond are incorporated into the crystal structure as sp3-CH2-symmetric (2850 cm−1) and sp3 CH2-antisymmetric (2920 cm−1) vibrations. From the obtained Raman spectrum, we find the incorporation of hydrogen impurity leads to a significant shift of the Raman peak towards higher frequencies from 1333.90 cm−1 to 1334.42 cm−1 with increasing the concentration of Fe(C5H5)2 additive from 0.1 wt% to 0.5 wt%, thereby giving rise to some compressive stress in the diamond crystal lattice. This is the first time that the gem-grade hydrogen-doped diamond single crystal, with size up to 3.5 mm has been successfully synthesized by using new hydrogen source Fe(C5H5)2 additive. We believe that our work can provide a new method to study the influence of hydrogen impurity on diamond synthesis and it will help us to further understand the genesis of natural diamond in the future.
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