Quantum behaviour of hydrogen in vacancy-defects in diamond

Amongst the key observations regarding the presence of H in as-grown CVD diamond are those of the paramagnetic resonance centres exhibiting trogonal symmetry. Initial interpretation of one data set also involving nitrogen was hydrogen trapped at the well knonw vacancy-nitrogen centre, with the hydrogen atom lying on the trigonal axis. However, this would require the rather unusual case of four-fold coordiated N in diamond, even more unexpected given the presence of under-coordinated carbon in the defect.

[Fig 1: NVH- centre in diamond indicating schematically the tunnelling average yielding C3v symmetry]

We showed using a range of calculations that, as one migth expect from the usual chemistry of these elements, that the H atom prefers to bond to a carbon atom in a N-V-H complex. The simplest way to correlate this centre with the experiment was to assume that the H atom could hop between carbon sites on the time-scale of the measurement, as shown schematically in the figure

Another center, this time not involving nitrogen was also linked to a vacancy-hydrogen complex. Again, the centre is trigonal and the experimental data was interpreted in terms of a static V-H complex. However, if the H atom is rapidly reorienting in the N-V-H complex, why would V-H be so different. If H could tunnel between sites in V-H, one would expect a cubic, isotropic spectrum. We showed that in collaboration with theoreticians in UCL that a tunnelling mode of hydrogen at one end of a di-vacancy fits well with the published data.

[Fig 2: VH- and V2H- centres in diamond]

Fig 2: VH- and V2H- centred in diamond