Extent of stacking disorder in diamond

TL;DR

This study quantitatively analyzes the stacking disorder in diamond samples, revealing that what is often called hexagonal diamond is actually a complex mixture of cubic and hexagonal stacking, with less than 60% hexagonality.

Contribution

It provides the first detailed quantitative analysis of stacking sequences in diamond, challenging the notion of pure hexagonal diamond and describing it as stacking disordered diamond.

Findings

Hexagonal stacking fractions are below 60% in samples labeled as hexagonal diamond.

Cubic and hexagonal sequences are interlaced in complex ways.

Lonsdaleite is best described as stacking disordered diamond, not a pure phase.

Abstract

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 we find in materials which were previously referred to as hexagonal diamond are below 60%. The remainder of the stacking sequences are cubic. We show that the cubic and hexagonal sequences are interlaced in a complex way and that naturally occurring Lonsdaleite is not a simple phase 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.