Plane-wave density functional theory study on the structural and energetic properties of cation-disordered Mg-Al...

Periodic solid models have been employed to investigate the structural and energetic properties of cation-disordered Mg-Al layered double hydroxides (LDHs), including composition, ordering, and stacking pattern of layers. The geometry, lattice energy and density of states (DOS) of the periodic models were computed using the plane-wave pseudopotential implementation of density functional theory (DFT) with the virtual crystal approximation (VCA). The calculation results for the MgAl-Cl-LDH model show that a pure LDH phase is difficult to sustain when the Mg/Al ratio (R) is R > or = 4. The stability of the cation-disordered MgAl-Cl-LDH unit cell increases upon increasing R, owing to the decrease in the number of Al(III)-O-Al(III) linkages in the LDH sheets. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of MgAl-Cl-LDH receive contributions from the 3s and 3p orbitals of Mg and Al and the 2p orbitals of O and Cl, respectively, implying significant host-guest interactions. The increase of R leads to the decrease in electron density of 3s and 3p of metal cation and that of 2p in Cl. Consequently, the HOMO-LUMO energy gap as well as the systematical stability increase upon increasing R. From this point of view, it has been demonstrated that the cations distribute in an ordered arrangement with the absence of Al(III)-O-Al(III) linkages if R is in the low range (R = 2-3). It was also found that the 3R polytype is the most stable stacking pattern with the same Mg/Al ratio, owing to the fact that the conductive band energy levels decline with the increase of electron density of 3s and 3p of the metal cation in the 3R-stacking system. These findings agree well with the experimental results and provide a profound understanding of critical factors influencing the structure of LDHs, including metal composition and ordering, the stacking sequence, as well as the host-guest interactions.