In this study, {111}-oriented diamond crystals with different nitrogen concentrations were successfully synthesized in a series of experiments at 5.8 GPa pressure and 1380–1400 °C temperature. The nitrogen-vacancy (NV) centers were investigated in diamonds with different nitrogen concentrations. The Raman measurements indicated that the nitrogen atoms caused the Raman peaks to shift to a lower frequency. The photoluminescence (PL) results showed that the intensity of the negatively-charged nitrogen-vacancy (NV⁻) centers increased with the increase in the nitrogen content when the diamonds had low nitrogen content (372 ppm), the intensity of the NV⁻ centers gradually decreased until it became undetectable. Additionally, we found that the neutral nitrogen-vacancy (NV⁰) centers were more common in diamonds with low nitrogen content. We believe that the results of this study provide a better understanding of the relationship between the nitrogen content and the NV centers in high pressure and high temperature (HPHT) synthetic diamonds.

We report a significant enhancement in the figure of merit ZT of bulk BiSbTe₃ fabricated by high pressure sintering technique under variable pressures. High pressure intervention during synthesis processes can effectively adjust the thermoelectric transport properties and crystal structures of the reaction products. The novel vibration modes produced by high pressure in Raman spectra can induce a positive role in the reduction of thermal conductivity. Meanwhile, the electrical resistivity can also obtain an efficient decrease due to the effect of high pressure on the texture and the preferred orientation. Because of these positive effects, the maximum ZT has reached 1.4 at 432 K from synthesized BiSbTe₃ at 2.5 GPa in a short period. Therefore, we present that the appropriate high-pressure has a positive effect on the thermoelectric transport properties to produce the significant enhancement in thermoelectric properties of BiSbTe₃.

To investigate the effect of the hydrogen element on diamond crystallization, hydrogen doped diamond crystals are synthesized at 5.0–6.0 GPa and 1250–1600 °C by adding ferrocene (C₁₀H₁₀Fe) to a system of carbon and Fe-based solvent–catalyst. The essential dependence of diamond morphology and nucleation on the composition of the crystallization medium is established in the P–T diagram. It is found that the growth region of the {111} face becomes wider, while the region of the {100} face becomes narrower and almost disappears with increasing concentration of C₁₀H₁₀Fe. The changing of the diamond growth habits can be explained by the effect of hydrogen from the decomposition of C₁₀H₁₀Fe on diamond crystallization. Fourier transform infrared absorption spectra reveal that the hydrogen-related peaks increase with an enhancement in the ratio of C₁₀H₁₀Fe additive. The observed Raman shift is due to the doping of hydrogen atoms into the diamond lattice. In addition, we find that the change of characteristics of the growth medium of the diamond crystal induced by hydrogen is an important factor for the changes of the synthesis conditions, the growth rate and the nitrogen concentration of the synthesized diamond.

In this paper, diamond single crystals co-doped with H and B were successfully synthesized under a fixed pressure of 6.0 GPa and temperature ranging from 1560 to 1600 K. In the synthesized diamond, hydrogen was found to be mainly incorporated as sp³ CH₂, which had been detected by its characteristic stretching absorptions at 2920 cm⁻¹ and 2850 cm⁻¹ from the Fourier transform infrared spectra. Raman measurements indicated that the diamonds co-doped with B and H had a more compatible lattice structure than B-doped diamond. Hall effect measurements indicated that the co-doped diamonds showed p-type semiconductor behavior. Besides, the Hall mobility was nearly equivalent between B-doped and co-doped diamond crystals, while the concentrations of the carrier and conductivity of the co-doped diamonds were higher than that of the B-doped diamond crystals.