Spectroscopic studies of a thermochromic centre in synthetic diamonds grown by the temperature-gradient method

Abstract Synthetic diamonds grown by the temperature-gradient method at temperatures just above the melting point of the solvent-catalyst often have an unattractive dull-green colour. This green colour, which is prominent at room-temperature, disappears if the diamond is cooled to below 120 K and is restored on warming the crystal back to room temperature. We show that the behaviour of the absorption band responsible for the colour is consistent with the transition occurring from the upper level of a centre with a split ground state, for which transitions from the lower level are forbidden. Temperature-dependence measurements indicate that the energy splitting in the ground state is about 50 meV. The broad featureless absorption band, with a maximum near 1·9eV, is assumed to be vibronic in origin. The absence of structure in the band indicates that the electronic transition is strongly coupled to the lattice and, as expected, the zero-phonon line, estimated to be at around 1eV, is too weak to detect. The same absorption band is also shown to be present in some individual crystals of synthetic diamond abrasive grit.