First-principles Study On The Physical Properties Of Double Perovskites Sr₂CuIrO₆ And A₂CoNbO_6-?(A=Sr,Ba)

A₂B?B?O₆ double perovskites have attracted much attention of researchers due to their excellent physical and chemical properties.Moreover,they are widely used in electronic devices,sensors,fuel cells and solar cells,etc.At present,researchers mainly focus on the experimental studies of double perovskite Sr₂CuIrO₆ and A₂CoNbO_6-?(A=Sr,Ba),and they are lack of theoretical investigations.They may have other properties need to be discovered.Therefore,theoretical studies on double perovskites Sr₂CuIrO₆ and A₂CoNbO_6-?(A=Sr,Ba)are important to reveal their properties and predict their potential applications.Moreover,the effects of A,B site and oxygen vacancies on the properties of double perovskite are analyzed.The calculation results show that double perovskite Sr₂CuIrO₆ belongs to the tetragonal system,and the magnetic interaction in the C-type antiferromagnetic ground state can be explained by the semicovalent-exchange mechanism.Cu_2b and Ir_2a atoms are antiferromagnetically coupled in the XOY-plane through the magnetic exchange chains of Cu₂¹ _b-O₈² _h-Ir₂² _a and Cu₂² _b-O₈¹ _h-Ir₂¹ _a,while Cu_2b and Ir_2a atoms are ferromagnetically coupled along the Z-axis through the magnetic exchange chains of Ir₂¹ _a-O₄¹ _e-Cu₂¹ _b and Ir₂² _a-O₄² _e-Cu₂² _b.The electronic structure calculated by the hybrid functional HSE06 indicates that double perovskite Sr₂CuIrO₆ is an indirect semiconductor with a band gap of 1.170 e V,and that it has high absorption coefficient in the ultraviolet region.The electron transitions from O-2p orbitals in the valence bands to Ir-5d orbitals in the conduction bands play an important role in the optical properties of double perovskite Sr₂CuIrO₆.The crystal structures,mechanical properties,lattice dynamics,electronic structures and optical properties of double perovskites A₂CoNbO₆(A=Sr,Ba)have been studied by the first principles of density functional theory.The theoretically obtained crystal parameters of double perovskites A₂CoNbO₆(A=Sr,Ba)are consistent with their experimental ones.Both Sr₂CoNbO₆ and Ba₂CoNbO₆belong to the I4/m 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.Double perovskites A₂CoNbO₆(A=Sr,Ba)are indirect band gap semiconductors,and their band gaps are respectively 2.916 and 3.050 e V.The close band gaps are mainly dominated by the similar<Co O₆>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 double perovskites A₂CoNbO₆(A=Sr,Ba).Due to the same point group,Sr₂CoNbO₆ and Ba₂CoNbO₆have the same five active lattice vibration modes of A_g,B_g,E_g,A_u and E_u and one static lattice vibration mode of B_u,and the typical displacement patterns are also analyzed in details.The theoretical results show that A site in double perovskite A₂CoNbO₆(A=Sr,Ba)has a little influence on their physical properties.For double perovskites Sr₂CoNbO_6-?,the oxygen vacancy has a great influence on their properties.The relaxed 1×1×1 unit cell,2×1×1,3×1×1 and 2×2×1 supercells by the GGA+U method indicate that oxygen vacancies will distort the crystal lattice of double perovskites Sr₂CoNbO_6-?.The higher oxygen vacancy concentration,the more lattice distortion and the higher energy.The Young's modulus,bulk modulus and shear modulus of double perovskites Sr₂CoNbO_6-?decrease with oxygen vacancies,while their anisotropy increase.Moreover,double perovskites Sr₂CoNbO_6-?may change from brittle to ductile behavior.The electronic structures show that double perovskites Sr₂CoNbO_6-?with oxygen vacancies considered in this paper maintain the semiconductor character.When the oxygen vacancy concentration is higher,the band gap decreases smaller.The reason for this phenomenon has been explained qualitatively.In addition,the magnetic properties induced by oxygen vacancies in double perovskites Sr₂CoNbO_6-?are complicated.Theoretical analysis shows that double perovskites Sr₂CoNbO_6-?with oxygen vacancies exhibit the macroscopic spin glass-like magnetic behavior.