Ab initio exploration of layer slipping transformations in kaolinite up to 60 GPa

Abstract

Kaolinite transformation into other kaolin layer polytypes under pressure is of interest in many materials or industrial applications as well as in seismology. Assuming that this transformation would involve a layer slipping mechanism similar to that of the reversible dickite to HP dickite transformation observed at 2 GPa, the authors undertake here an exploratory study of corresponding transformations. Neglecting contributions from third neighbour layers and beyond to differences in total energy, it is concluded that 19 stacking models of kaolin layers are prime candidates for the lowest enthalpy under moderate pressure. Ab initio compression of those models up to 60 GPa shows that, although several of the above models come close to kaolinite in enthalpy, kaolinite probably survives compression up to ∼ 12 GPa. Beyond this pressure, a new family of kaolin polytypes with lower enthalpy than kaolinite, resulting from a different layer slipping mechanism, is spontaneously produced by ab initio compression. The silicon coordination transforms gradually from tetrahedra to triangular dipyramids with no drastic change to layer architecture. Corresponding distinguishable transformations between adjacent layers resulting from this new coordination are translations -a/3 and (a+b)/3, which are not possible translations at zero pressure. Numerous low enthalpy new polytypes based on those translations are possible. Compression to 20 GPa of two kaolin polytypes among the 19 models created above have spontaneously resulted, one into the repeated (a+b)/3 polytype with symmetry Cm, and the other one into the repeated -a/3 polytype with symmetry P1. As both models derive from kaolinite by a layer slipping mechanism and have very similar enthalpies, both considerably lower than that of kaolinite, those phases and the polytypes from the same family are prime candidates for post-kaolinite phases beyond ∼ 12 GPa.