Magnetic ordering inHoBaCo2O5.5

Abstract

The crystal and magnetic structures of $\mathrm{Ho}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5.5}$ have been studied by neutron powder diffraction in the temperature range from $20\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}330\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The crystal structure of $\mathrm{Ho}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5.5}$ was found to be best described in space group $Pmma$ on a $2{a}_{p}\ifmmode\times\else\texttimes\fi{}2{a}_{p}\ifmmode\times\else\texttimes\fi{}2{a}_{p}$ unit cell where ${a}_{p}$ refers to the lattice parameter of the cubic perovskite unit cell. The $a$ and $b$ axes were found to decrease and increase abruptly within the temperature range from $290\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}315\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ as the temperature increases, and the unit-cell volume exhibits an excess thermal expansion in the temperature range from $230\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}290\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The anomalous changes in lattice parameters and unit-cell volume were attributed to the metal-insulator transition and spin-state changes of the ${\mathrm{Co}}^{3+}$ ions. The magnetic structure of $\mathrm{Ho}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5.5}$ was shown to be antiferromagnetic with a $2{a}_{p}\ifmmode\times\else\texttimes\fi{}2{a}_{p}\ifmmode\times\else\texttimes\fi{}4{a}_{p}$ magnetic unit cell containing four crystallographically independent Co ions, two octahedrally coordinated and two pyramidally coordinated. The refined low temperature values of the magnetic moments of the Co ions indicated that one set of octahedrally coordinated ${\mathrm{Co}}^{3+}$ ions were in the high-spin state while the other set of octahedrally coordinated ${\mathrm{Co}}^{3+}$ ions were in mixed intermediate- and low-spin states. The pyramidally coordinated ${\mathrm{Co}}^{3+}$ ions were found to be in the intermediate-spin states.