The mid-infrared spectrum of a new double carbonate, BaFe[CO3]2

BaFe[CO3](2), a new double carbonate from the norsethite-type family of carbonates, was recently discovered and synthesized at high-pressure-temperature (high-PT) conditions. Two polymorphs of the new double carbonate were identified belonging to the R (3) over barm and R (3) over barc space groups, respectively(1). In the present communication we present for the first time the mid-infrared (IR) spectra (measured with transmission infrared spectroscopy and attenuated total reflectance infrared spectroscopy) and assign the observed bands to the respective fundamental and combined IR-active modes. This study, therefore, adds new spectroscopic results to the previous crystal chemical characterization of the new double carbonate. Results are compared to those of other Ba-bearing double carbonates with the norsethite-type structures. With substitution of Ba2+ in the double carbonate lattice by increasingly smaller cations a decrease in wavenumbers of absorption bands is observed. Due to similar crystal structures, and masses and radii of the cations, the IR spectra of both BaFe[CO3](2) carbonates are similar to the ones observed for BaMn[CO3](2) and BaMg[CO3](2), with some characteristic deviations. The carbon isotope shift coefficient of 0.969 for upsilon(2) of the carbonate ion group agrees with previous theoretical predictions. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

In this study, the mid-infrared spectra of a newly discovered double carbonate BaFe[CO3](2) were reported for the first time. The spectra were compared with other Ba-bearing double carbonates, showing a decrease in absorption band wavenumbers with substitution of smaller cations. The carbon isotope shift coefficient agreed with theoretical predictions.