Metal-insulator transition due to charge ordering inR1/2Ba1/2

Lattice constants, resistivity, and Raman scattering spectra have been investigated for ${R}_{1/2}{\mathrm{Ba}}_{1/2}{\mathrm{CoO}}_{3},$ where $R$ is a trivalent rare-earth ion, with systematically changing the averaged ionic radius ${r}_{R}$ of the rare-earth ion. For $R=\mathrm{Gd},$ Eu, Sm, prominent metal-insulator (MI) transition is observed at ${T}_{\mathrm{MI}}\ensuremath{\sim}360\mathrm{K},$ but the transition is suppressed for $R=\mathrm{La}.$ The MI transition becomes also blurred for ${\mathrm{Gd}}_{0.48}{\mathrm{Ba}}_{0.52}{\mathrm{CoO}}_{3},$ suggesting that the transition is due to charge ordering. We have observed enhancement of intensity of Raman scattering from the Co-O stretching mode in the insulating state $(T<~{T}_{\mathrm{MI}}),$ and have interpreted it in terms of folding of the oxygen phonon branch due to doubling of the cell size.