When carbon impurities trigger the synthesis of alpha boron at high pressure and high temperature

TL;DR

This study explores how carbon impurities facilitate the synthesis of alpha boron at high pressure and temperature through experiments and DFT calculations, revealing defect structures, optical properties, and a method to control carbon content.

Contribution

It demonstrates the role of carbon in alpha boron formation at high pressure and temperature, identifying specific defect structures and providing a calibration method for carbon concentration.

Findings

Carbon impurities enable alpha boron formation at higher temperatures than previously reported.

The (B11C) defect accounts for volume contraction and optical color changes in alpha boron.

Synthesis temperature can be used to control carbon concentration in alpha boron.

Abstract

The role of carbon in the formation of alpha boron at high pressure and high temperature (HPHT) has been investigated by combining HPHT experiments and density functional theory (DFT) calculations. Starting from beta rhombohedral or amorphous boron and amorphous carbon at 5 GPa, the alpha boron phase has been repeatedly observed between 1473 K and 2273 K, at temperatures that are much higher than those reported in the phase diagram of boron. The DFT investigation of the effect of carbon insertion into the alpha rhombohedral boron atomic structure on the formation enthalpy, volume and optical properties shows that only the (B11C) substituted icosahedron defect accounts for the slight volume contraction observed in {\alpha} boron. It is also compatible with the observed red colour of {\alpha} boron crystals, and consistently has a very low formation energy. Our calculations also provide a calibration tool to evaluate the carbon concentration in the samples. Finally, the synthesis temperature is found to be an easy means to tune the carbon concentration on demand.