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

In this work, a detailed study of the structural, electronic and absorption properties of crystalline 2,6-dimethyl-4-(diphenylmethylene)-2,5-cyclohexadienone with α form (α-DDCD) in the pressure range of 0–250GPa 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 α-DDCD under compression. In addition, it can be see that the b-direction is much stiffer than the a- and c-axes in the structure of α-DDCD, suggesting the compressible crystal of α-DDCD has anisotropy. Then, by analyzing the bandgap and density of states (DOS) of α-DDCD, it is found that the crystal undergoes a phase transformation from semiconductor to metal at 90GPa and it becomes more sensitive under compression. Besides, in the pressure range 110–170GPa, repeated transformations between metal and semiconductor occur four times, suggesting the structural instability of α-DDCD in this pressure range. Finally, the relatively high optical activity with the pressure increases of α-DDCD is seen from the absorption spectra, and two obvious structural transformations are also observed at 130GPa and 140GPa, respectively.