Local Environments in Iron-Bearing Clay Minerals by DFT Approaches: The Case of Structural Fe in Kaolinite

Abstract

Technological properties of kaolins depend on the internal structure of the particles that constitute them. For this reason, unraveling the structural features from the micro to the nanoscale is a permanent matter of interest, even in the case of raw samples. From the experimental point of view, because naturally-occurring kaolins contain iron, Mössbauer spectroscopy is a very convenient technique to reach the nanoscopic scale avoiding experimental difficulties related to the sample's lack of structural order at the crystallographic scale. In this work, first-principles calculations based on the Density Functional Theory (DFT) were used to model such iron environments in kaolinite, and to assess the performance of the Gauge-Included Projector Augmented Waves (GIPAW) method to describe the changes to the host structure and the electronic modifications produced by the iron atoms. To this purpose, structural relaxation, Grimme's D2 dispersion, and Hubbard corrections (DFT+U approach) were considered. A detailed analysis was done for the obtained predictions for the Fe local structure, oxidation state, and Mössbauer quadrupole splitting, including comparisons with available experimental data. The results contribute to better understand the naturally-occurring kaolins, and support the DFT+U approach for the description of the layer structure and the electronic properties of iron-containing clay minerals.