DFT crystal and electronic structure of the direct bandgap Cu_(1-x)Na_xPF₆: (x = 0.125n, n = 1–7)

DFT study on the crystal and electronic structure of the hexafluorophosphates Cu_(1-x)Na_xPF₆ (x = 0.125n, n = 1–7) is presented. Both parent compounds CuPF₆ and NaPF₆ constituting the co-crystals have the same (FCC) spatial symmetry, very similar lattice constants (Δa = 0.017 Å), and according to periodic PBE density functional computations possess direct electronic bandgaps of 0.82 eV and 8.01 eV, respectively. Geometry optimization of the co-crystals Cu_(1-x)Na_xPF₆ (x = 0.125n, n = 1–7) retain FCC symmetry. All the resulting electronic bandgaps are direct. Their values are intermediate between those of the parent compounds falling into the range from 0.97 eV (Cu_0.875Na_0.125PF₆) to 2.22 eV (Cu_0.125Na_0.875PF₆). All the compounds feature very small hole velocities. In contrast, electron velocities are high varying from 2.05*10⁵ m/s (Cu_0.875Na_0.125PF₆) to 1.12*10⁵ m/s (Cu_0.125Na_0.875PF₆) and showing descending trend as the content of sodium in the co-crystals increases. Taking into account the values of direct electronic bandgaps and presumed poor solubility in water the co-crystals seem good candidates for solar light harvesting applications.