Potential structures for the H1a defect

Two proposals existed for the structure of the defect responsible for H1a but nei- ther appear at first inspection to satisfy both the isotopic evidence for H1a and its annealing behaviour. Isotopic substitution studies of H1a have been reported previously [2, 7]. Comparative work between 14_{N and} 15_{N doped diamonds has} shown that the defect responsible for the H1a band involves the vibration of a

(a) (b) (c)

Figure 6-2: Potential interstitial structures involving nitrogen in diamond. Figure (a) is a schematic of the simple nitrogen interstitial defect structure in diamond; (b) represents the puckered bond-centred nitrogen interstitial proposed as the source of the H1a absorption band by Kiflawi et al. [2]; (c) is the alternative structure of the di-nitrogen split interstitial as originally proposed by Goss et al. [17].

single nitrogen atom. Two sharp peaks at 1450.3(1) cm−1 _{and 1426.0(1) cm}−1 _were

recorded for a sample with approximately equal concentrations of 14_{N and} 15_N (Figure 6.3(a)) [7]. The immediate conclusion from this would be that the vibra- tion involves a single nitrogen atom, since no mode from a defect of mixed isotopic composition was identified.

In addition, work with12_{C and}13_{C enriched diamond has indicated the involve-} ment of two equivalent carbon atoms in the H1a local vibrational mode (LVM) (Figure 6.3(b)) [2]. Spectra from a range of samples doped with various percent- ages of13_{C, identified the 1450 cm}−1_{peak to shift to 1424 cm}−1_{in the 97% enriched}

13_{C sample. For samples of mixed isotopic composition, three absorption peaks} were identified to be associated to H1a, with the additional band occurring at 1438.3 cm−1_{. Throughout annealing studies made by Kiflawiet al., the relative in-}

tensities of the three peaks remained constant, confirming them to originate from the same centre.

It must though be noted that a minor peak at 1438 cm−1 _{has been previously}

identified with alternative annealing behaviour and hence is attributable to a dif- ferent defect [3]. Care must therefore be taken not to confuse this with evidence of a mixed isotope sample. This feature is seen to arise post electron and neutron irradiation and annealing in type Ia and Ib samples and is often hidden by the

(a) (b)

Figure 6-3: Spectra from the isotopic substitution investigations of the H1a de- fect. (a) illustrates the 50:50 14_N:15_{N enriched sample spectra for the H1a defect} (adapted from a paper by Woods et al. [7]) and (b) illustrates results considering a range of isotopic ratios of 12_C:13_{C. This is adapted from a paper by Kiflawi et} al. [2].

strong 1450 cm−1 _{absorption. During annealing at increasing temperature, the in-}

tensity is seen to decrease, at a different rate to that of the 1450 cm−1 _{line but is}

still present post 4 hours annealing at 1400‰[3].

The results of Woods led to the initial proposal for H1a to originate from a nitrogen interstitial defect of the ⟨001⟩-split site form [3]. However, upon the un- derstanding of the isotopic work for12_{C and}13_{C enriched diamond, indicating that} two equivalent carbon atoms are involved in the vibration, theoretical modelling based upon semi-empirical Hartree-Fock theory, led to the hypothesis that the nitrogen atom was occupying a position midway between the two nearest neigh- bour carbon atoms, displaced off the axis between them at a bond angle of 115○

with the carbon-nitrogen bond having a length of 1.47 ˚A (Figure 6.2(b)) [2]. The proposed formation mechanism in type Ia material requires that an approaching self-interstitial interacts with the N-pair allowing it to dissociate at relatively low temperatures into NI and NS. The different behaviour in type Ib material was tentatively explained by the role of charge state effects not present in Ia material (Equations (6-5) and (6-6)).

Calculations by density-functional theory of the potential interstitial nitrogen structures and a range of associated complexes, cast doubt on this model [17]. Indeed, the puckered bond-centred structure was found to be unstable relative

to the formation of a ⟨001⟩-oriented split-interstitial, where nitrogen is three fold co-ordinated, consistent with simple chemical principles. In addition, the stabil- ity of H1a at high temperatures was viewed as inconsistent with the calculated migration barrier of the simple nitrogen interstitial. The calculations showed NI to be mobile at the production temperature of 300‰ in type Ia material, which would contradict the stability of the absorption feature at 1400‰. Furthermore, recent EPR studies [13] have identified, as discussed, the NIto adopt an⟨001⟩-split nitrogen interstitial configuration, thus rebutting the claim by Kiflawi et al. [2], that the nitrogen interstitial adopts a bond-centred configuration.

It was proposed that the H1a defect was a complex of two nitrogen atoms sharing a single ⟨001⟩-split configuration (Figure 6.2(c)) [17]. This explains both the thermal stability and vibrational properties seen experimentally but super- ficially contradicts the lack of additional bands in 14_N-15_{N mixed-isotope doped} material, which was seen as evidence of just one N atom present in the H1a cen- tre [2]. However, the calculated vibrational modes exhibit negligible interaction between the nitrogen atoms for the degenerate pair of modes in the vicinity of H1a, so in any given mode, only one of the nitrogen atoms is involved, leading to the nitrogen isotopic effects observed. Investigations by electron paramagnetic resonance have to date not revealed any analogue to the H1a feature. In light of this, the proposal of H1a to be a di-nitrogen interstitial seems reasonable due to its non-paramagnetic nature in the neutral charge state.

The equilibrium structure of N2I is tetragonal, transforming under the D2d point-group. The proposed structure is illustrated in Figure 6.2(c) with the prin- cipal axis vertically aligned. The predicted high symmetry of the defect and the degeneracy of the local-vibrational mode, lends this centre to a convincing as- signment using the splitting patterns and rates under uniaxial-stress along high- symmetry crystallographic directions.