High-pressure diamond synthesis in the presence of rare-earth metals

•The effect of rare-earth metals on diamond crystallization in the Mg-C system is studied.•Sc, La, Pr, and Dy do not affect diamond synthesis compared to the undoped system.•Yb, Y, and Nd significantly reduce the degree of conversion of graphite to diamond.•Diamonds synthesized with Y, Nd, and Yb additives exhibit new photoluminescence features. The effect of rare-earth metals on diamond crystallization processes in the Mg-REM-C system (REM = Sc, Y, La, Pr, Nd, Dy, and Yb) was studied at 7.8 GPa and 1800 °C. It was found that the rare-earth metals added in an amount of 10 wt% had different effects on diamond crystallization. Sc, La, Pr, and Dy, only slightly affected the intensity of diamond synthesis as compared to the undoped Mg-C system. Yb, Y, and Nd significantly reduced the degree of conversion of graphite to diamond. The effect of higher contents of rare-earth metals on diamond crystallization was studied for the Mg-Pr-C system with a Pr content varied from 0 to 80 wt%. It was found that with increasing Pr content the degree of the graphite-to-diamond conversion decreased and at 80 wt% Pr diamond synthesis was terminated and mass crystallization of metastable graphite took place. The morphology of diamonds synthesized in the Mg-REM-C system was determined by the {1 0 0} faces. The surface microrelief of the faces was connected with crystal defectiveness and characterized by specific features, depending on the system composition and the degree of graphite-to-diamond conversion. From the spectroscopic characterization it was found that similarly to their action on diamond synthesis, different REM additives produced different effects on photoluminescence characteristics of the synthesized diamonds. Those rare-earth metals which only slightly affected diamond crystallization, showed no significant effects on the PL spectra relative to the undoped Mg-C system. Diamonds produced with Y, Nd and Yb additives showed a number of new photoluminescence features, which could be related to defects inhibiting diamond crystal growth.