Electronic structures and physical properties of double perovskite A₂CoNbO₆ (A  =  Sr, Ba) crystals

The crystal structures, mechanical properties, lattice dynamics, electronic structures and optical properties of Sr₂CoNbO₆ and Ba₂CoNbO₆ have been studied by the first principles of density functional theory. The theoretically obtained crystal parameters of Sr₂CoNbO₆ and Ba₂CoNbO₆ are consistent with their experimental ones. Both Sr₂CoNbO₆ and Ba₂CoNbO₆ belong to the [Formula: see text] space group at the low-temperature limit and have very weak elastic anisotropy. The former is slightly brittle while the latter is more brittle. Their electronic structures are similar to each other, and Co-3d and O-2p  orbitals constitute the top valence bands while Co-3d orbitals form the bottom conduction bands. Sr₂CoNbO₆ and Ba₂CoNbO₆ are indirect band gap semiconductors, and their band gaps are respectively 2.916 and 3.050 eV. The close band gaps are mainly dominated by the similar [Formula: see text] octahedrons in their crystal structures. The electron transitions from O-2p  orbitals in the valence bands to Co-3d orbitals in the conduction bands play important roles in the optical properties of Sr₂CoNbO₆ and Ba₂CoNbO₆. Due to the same point group, Sr₂CoNbO₆ and Ba₂CoNbO₆ have the same five active lattice vibration modes of [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] and one static lattice vibration mode of [Formula: see text], and the typical displacement patterns are also analyzed in detail.