Metal-insulator transition due to charge ordering in R1/2Ba1/2CoO3

Lattice constants, resistivity, and Raman scattering spectra have been investigated for R1/2Ba1/2CoO3, where R is a trivalent rare-earth ion, with systematically changing the averaged ionic radius r(R), of the rare-earth ion. For R = Gd, Eu, Sm, prominent metal-insulator (MI) transition is observed at T-MI similar to 360 K, but the transition is suppressed for R = La. The MI transition becomes also blurred for Gd0.48Ba0.52CoO3. 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 less than or equal to T-MI), and have interpreted it in terms of folding of the oxygen phonon branch due to doubling of the cell size. [S0163-1829(98)51444-0].