Investigation of double perovskites Sr_(2)SmNbO_(6) and X_(2)CoNbO_(6)(X=Sr,Ba)with SCAN functional and plus U correction

The double perovskites Sr_(2)SmNbO_(6),Sr_(2)CoNbO_(6)and Ba2CoNbO_(6)were investigated with first principles computations based on the density functional theory and plus U treatment.Firstly,different calculation methods were examined in order to quantitatively approach the exact band gap.It was found that neither the strongly constrained and appropriately normed(SCAN)semilocal density functional nor the hybrid HSE06 functional can well address the semiconducting nature of the investigated double perovskites,while PBEþU or SCANþU with appropriately determining the U value can have good performance,which paves the way for future studies of double perovskites.With self-consistently calculated electron correlation strength,the magnetic states and the band gaps of the Sr_(2)CoNbO_(6)and Ba_(2)CoNbO_(6)were more precisely determined.The electronic,optical and thermoelectric properties were then investigated and discussed for possible applications.