Kaolin group minerals under pressure: The study of their structural and electronic properties by DFT methods

Kaolinites have a wide impact in many technological applications, and also are promising for their potential new uses in the future. Because these clay minerals naturally exist as fine particles, there are difficulties in making measurements of their bulk structural and electronic properties. Also, the occurrence of stacking faults and the presence of structural defects at the nanoscale, make these clay minerals quite complex systems to propose accurate models at the crystallographic scale. This work presents a study of six different kaolin group polytypes using calculations based on the Density Functional Theory (DFT), and includes the naturally occurring clay minerals kaolinite, dickite, and nacrite, and also other three pressure-induced polytypes that have been experimentally observed. The idealized structures were optimized using DFT methods, and their responses to hydrostatic pressure were theoretically analyzed from energy vs. volume and enthalpy vs. pressure data. From these, the bulk modulus for each polytype was predicted, and their relative stability and the potential transformations between structures were discussed. Finally, the electronic density of states for each polytype was calculated and compared to each other. The results indicate that although structurally these polytypes present significant differences between each other that lead to different elastic responses, from the perspective of the electronic properties they may be indistinguishable. The analysis performed in this work can be helpful as guidance and for interpretation in future experiments.