Superstructure formation at the metal-insulator transition in R BaCo2 O5.5 (R=Nd,Tb) as seen from reciprocal space mapping

Single crystal synchrotron x-ray diffraction study has been performed on the layered perovskites R BaCo2 O∼5.5 (R=Tb,Nd) over a wide temperature range 100<T<375K. By means of reciprocal space mapping and symmetry analysis, it is shown that the first-order structural transition, accompanied by a metal-insulator transition, turns out to be a Pmmm (ac ×2 ac ×2 ac) ↔Pmma (2 ac ×2 ac ×2 ac) transformation. The detailed structural analysis shows that all four different Co3+ ions possible within the space-group Pmma have different oxygen polyhedron distortions, thus, indicating ordering of different spin states.