Superstructure formation at the metal-insulator transition inRBaCo2O5.5(R=Nd,Tb)as seen from reciprocal space mapping

Abstract

Single crystal synchrotron x-ray diffraction study has been performed on the layered perovskites $R{\text{BaCo}}_{2}{\text{O}}_{\ensuremath{\sim}5.5}$ $(R=\text{Tb},\text{Nd})$ over a wide temperature range $100lTl375\text{ }\text{K}$. 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 $Pmmm({a}_{c}\ifmmode\times\else\texttimes\fi{}2{a}_{c}\ifmmode\times\else\texttimes\fi{}2{a}_{c})\ensuremath{\leftrightarrow}Pmma(2{a}_{c}\ifmmode\times\else\texttimes\fi{}2{a}_{c}\ifmmode\times\else\texttimes\fi{}2{a}_{c})$ transformation. The detailed structural analysis shows that all four different ${\text{Co}}^{3+}$ ions possible within the space-group $Pmma$ have different oxygen polyhedron distortions, thus, indicating ordering of different spin states.