Shift of CO2-I absorption bands in diamond: a pressure or compositional effect? A FTIR mapping study

TL;DR

This study uses high-resolution IR mapping to analyze CO2 absorption in diamonds, revealing that impurity presence, not just pressure, influences spectral shifts, impacting the reliability of spectroscopic barometry.

Contribution

It demonstrates that impurity effects, rather than pressure alone, explain CO2 IR band shifts in diamonds, challenging previous assumptions and refining spectroscopic barometry methods.

Findings

IR band shifts are influenced by impurities, not just pressure.

Spectroscopic barometry can be biased by impurities in CO2 inclusions.

High purity CO2 shows clear Davydov splitting, enabling accurate pressure estimates.

Abstract

Infra-red maps and profiles with high spatial resolution were obtained for two single crystal diamonds with pronounced CO2 IR absorption peaks. Detailed examination allows unambiguous assignment of the spectral features to solid CO2-I phase. It is shown that the distribution of IR band positions, intensities and widths in the sample follows regular patterns and is not chaotic as was suggested in previous works where spectra of a few individual spots were analysed. Consequently, pressure effects alone fail to explain all observed features and shifts of the CO2 bands. Experimental data can be explained by presence of impurities (such as water, N2, etc.) in the trapped CO2. This implies that spectroscopic barometry of CO2 microinclusions in diamond may be subject to poorly controlled bias. However, barometry is still possible if Davydov splitting of the CO2-I {\nu}2 band is unequivocally observed, as this indicates high purity of the CO2 ice.