Insulator-to-metal transition in Co-doped La3Ni2O7− <i>δ</i> with high oxygen pressure annealing

The effects of Co doping in a series of La3Ni2−xCoxO7−δ samples before and after high oxygen pressure annealing have been investigated. The structural refinements suggest that the Co ions could be incorporated into the La3Ni2O7−δ matrix with a high solution level of x = 0.3. Despite of substantial increase in electrical conductivity with increasing Co doping, the as-prepared La3Ni2−xCoxO7−δ samples exhibit insulator-like conductivity. Noticeably, the high oxygen pressure annealing treatment results in an increase in the oxygen content and the average valence state of Ni ions of the La3Ni2−xCoxO7−δ samples. As a result, the annealed La3Ni2−xCoxO7−δ samples exhibit an insulator-to-metal transition, and the metallic-like conductivity could be achieved above ∼20 K in Co-doped samples.