DFT study of the structural transformations and absorption properties of crystalline 2,6-dimethyl-4-(diphenylmethylene)-2,5-cyclohexadienone under hydrostatic compression

Abstract

In this work, a detailed study of the structural, electronic and absorption properties of crystalline 2,6-dimethyl-4-(diphenylmethylene)-2,5-cyclohexadienone with [Formula: see text] form ([Formula: see text]-DDCD) in the pressure range of 0–250[Formula: see text]GPa is performed by density-functional theory (DFT) calculations. The particular analysis of the variation tendencies of the lattice constants, bond lengths and bond angles under different pressures shows that there occur complex transformations in [Formula: see text]-DDCD under compression. In addition, it can be see that the [Formula: see text]-direction is much stiffer than the [Formula: see text]- and [Formula: see text]-axes in the structure of [Formula: see text]-DDCD, suggesting the compressible crystal of [Formula: see text]-DDCD has anisotropy. Then, by analyzing the bandgap and density of states (DOS) of [Formula: see text]-DDCD, it is found that the crystal undergoes a phase transformation from semiconductor to metal at 90[Formula: see text]GPa and it becomes more sensitive under compression. Besides, in the pressure range 110–170[Formula: see text]GPa, repeated transformations between metal and semiconductor occur four times, suggesting the structural instability of [Formula: see text]-DDCD in this pressure range. Finally, the relatively high optical activity with the pressure increases of [Formula: see text]-DDCD is seen from the absorption spectra, and two obvious structural transformations are also observed at 130[Formula: see text]GPa and 140[Formula: see text]GPa, respectively.