Extent of stacking disorder in diamond

• The percentages of cubic/hexagonal stacking in diamond samples are quantified.
• No sample displayed a ‘hexagonality’ greater than 60%.
• Physical mixtures of cubic and hexagonal diamond have not been observed.
• Diamond displays a tendency of staying in a given stacking sequence.
• Lonsdaleite is best described as stacking disordered diamond.

Hexagonal diamond has been predicted computationally to display extraordinary physical properties including a hardness that exceeds cubic diamond. However, a recent electron microscopy study has shown that so-called hexagonal diamond samples are in fact not discrete materials but faulted and twinned cubic diamond. We now provide a quantitative analysis of cubic and hexagonal stacking in diamond samples by analysing X-ray diffraction data with the DIFFaX software package. The highest fractions of hexagonal stacking in materials previously referred to as hexagonal diamond are below 60%. The remainder of the stacking sequences is cubic. We show that the cubic and hexagonal sequences are interlaced in a complex way and that naturally occurring Lonsdaleite is not a simple physical mixture of cubic and hexagonal diamond. Instead, it is structurally best described as stacking disordered diamond. The future experimental challenge will be to prepare diamond samples beyond 60% hexagonality and towards the so far elusive ‘perfect’ hexagonal diamond.

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