Gold and silver: Noble solvents for the synthesis of superconducting boron-doped diamonds

The synthesis of diamonds in traditional growth systems based on Fe, Co and Ni does not allow overcoming the semiconductor level of boron doping of diamond, presumably because of a high affinity of boron to borideforming metals. For the first time, noble metals that do not form borides-silver and gold-are used as growth media for the synthesis of heavily boron-doped diamonds at 8-9 GPa. Gold and silver are not catalysts for the transformation of graphite to diamond, whereas elemental boron is catalytically active at eutectic melting temperatures above 2500 K. It is found that the synthesis of diamond in Ag-B and Au-B growth media starts at temperatures close to the melting temperatures of Ag and Au, 1600 and 1800 K, respectively. Calculations show that molten Au and Ag dissolve boron atoms without a significant change in the electronic structure of the solution, suggesting that the metal melts serve as a carrier for the boron catalyst. The resulting polycrystalline diamond contains dispersed inclusions of Ag and Au, as well as traces of boron carbide; the formation of borides is not detected. The transport measurements of the diamond samples reveal a transition to a superconducting state in the temperature range from 4.5 to 2.5 K. Raman spectroscopy confirms the heavy boron doping of diamond. Thus, we show that melts of "non-catalytic" metals that do not form borides can be effectively used to synthesize boron-doped conductive/superconducting diamonds at relatively low temperatures.