Theoretical study of the octahedral substitution effect in delaminated pyrophyllite: physicochemical properties and applications

Abstract

We have studied, using DFT calculations, some geometrical and electronic properties of delaminated pyrophyllite (D-P) and the corresponding layers that result from three isomorphic substitutions on octahedral sheets (Mg²⁺, Fe²⁺ and Fe³⁺). Bond lengths, layer thickness (d_L), band gap (E_g), work function (WF), magnetic moment (μ), density of states and charge distributions are reported. These properties are important to control the behaviour of electronic devices. The results show that the layer thickness increases according to the ionic radius of the considered substituent. In the case of the three substitutions a reduction of the forbidden band is observed. Mg²⁺ induces a decrease in the E_g value of about 16.5% with respect to D-P, whereas for Fe this reduction is more significant due to the presence of trap states in the forbidden zone. For Fe²⁺ (Fe³⁺) the reduction in the E_g is around 62% (51%). Regarding the WF, our results showed that there is a decrease in its value after substitution. D-P has the highest WF value (8.15 eV), whereas the delaminated clay with Fe²⁺ has the lowest value (2.22 eV). Finally, D-P and D-P substituted with Mg²⁺ have a diamagnetic behaviour (μ = 0), whereas the presence of Fe²⁺ and Fe³⁺ induces a paramagnetic behaviour. The computed magnetic moment is 4 μ_B and 1 μ_B for D-P substituted with Fe²⁺ and Fe³⁺, respectively.