DFT crystal and electronic structure of the direct bandgap Cu(1-x)NaxPF6: (x = 0.125n, n = 1–7)

Abstract

DFT study on the crystal and electronic structure of the hexafluorophosphates Cu(1-x)NaxPF6 (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)NaxPF6 (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 (Cu0.875Na0.125PF6) to 2.22 eV (Cu0.125Na0.875PF6). All the compounds feature very small hole velocities. In contrast, electron velocities are high varying from 2.05*105 m/s (Cu0.875Na0.125PF6) to 1.12*105 m/s (Cu0.125Na0.875PF6) 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.