New nickel-related optical absorption in high-pressure synthetic diamond

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Abstract
Diamonds grown by the temperature-gradient method using a nickel catalyst and sufficient nitrogen getter to produce barely detectable nitrogen absorption in the defect-induced one-phonon region have a deep brown colour which we show results from continuum absorption whose threshold lies around 1.7eV. In these diamonds we observe, for the first time, the presence of zero-phonon structure lying between 1.2 and 1.25 eV in the near-infrared spectral region of the absorption spectrum. This structure exhibits interesting photochromic effects and we observe a correlation between partial decay of the 1.40eV system (whose transition has previously been ascribed to nickel) and growth of the new zero-phonon structure. Like the 1.40eV system, the new zero-phonon structure shows strong polarization effects and is confined exclusively to the {111} growth sectors of diamond. From these results we propose that the new structure results from a different charge state of the 1.40 eV nickel system.