Interplay of structural, magnetic and transport properties in the layered Co-based perovskite LnBaCo(2)O(5) (Ln = Tb, Dy, Ho)

The oxygen deficient cobaltites LnBaCo(2)O(5) (Ln = Tb, Dy, Ho) exhibit two successive crystallographic transitions at TN similar to 340 K and at T-CO similar to 210 K. Whereas the first transition (P4/mmm to Pmmm) is related to the long-range antiferromagnetic ordering of the Co ions (spin ordering ), the second transition (Pmmm to Pmmb) corresponds to the long-range order ing of the Co2+ and Co3+ species (charge ordering) occurring in 1:1 ratio i n the structure. The charge ordered (CO) state was directly evidenced by th e observation of additional superstructure peaks using neutron and electron diffraction techniques. The CO state was also confirmed indirectly from re finement of high resolution neutron diffraction data as well as from resist ivity and DSC measurements. From the refined saturated magnetic moment valu es only, similar to3.7 mu (B) and similar to2.7 mu (B), the electronic conf iguration of the Co ions in LnBaCo(2)O(5) remains conjectural. Two pictures , with Co3+ ions either in intermediate spin state (t(2g)(5)e(g)(1)) or in high spin state (t(2g)(4)e(g)(2)), describe equally well our experimental d ata. In both cases, the observed magnetic structure can be explained using the qualitative Goodenough-Kanamori rules for superexchange. Finally, in co ntrast to the parent Ln = Y compound [Vogt et al., Phys. Rev. Lett. 84, 296 9 (2000)], we do not report any spin transition in LnBaCo(2)O(5) (Ln = Tb, Dy, Ho).