Triple CoII, III, IV charge ordering and spin states in modular cobaltites: a systematization through experimental and virtual compounds

Abstract

The series of modular compounds [Ba_nCo_2+nO_3n+2][BaCo₆O₉] (n = 1 to 3) including experimental and hypothetical terms, was investigated using DFT calculations and several experimental results. A systematic evolution of the electronic and magnetic states was evidenced along the series leading to ordered Co^II/Co^IIIversus mixed Co^III/IV charge segregation in two distinct structural motifs. In essence, using different packing modes within the labile [Ba_nCo_2+nO_3n+2] block, we have systematized the spin state dependence on the CoO₆ connectivity, i.e. corner-sharing (HS states) against face-sharing (LS states). We also show that the electronic and magnetic features of the [BaCo₆O₉] blocks do not vary trough the series, (i.e. HS-Co^II and LS-Co^III charge ordering) whereas the [Ba_nCo_2+nO_3n+2] blocks hold drastic changes from n = 1 to 3. In particular, the later carries a mixed III/IV cobalt charge for n ≥ 2. It leads to a triple valence cobalt state. For n = 2, we experimentally observe at 4 K a superstructure (2a, 2c) superstructure accompanied by a perfect Co^II/Co^III/Co^IV charge ordering. The charge ordering occurs at Tt = 160 K and is accompanied by a transition in the electronic transport leading to a 2D-VRH behaviour below Tt.