Low to High Spin-State Transition Induced by Charge Ordering in AntiferromagneticYBaCo2O5

The oxygen-deficient double perovskite ${\mathrm{YBaCo}}_2{\mathrm{O}}_5$, containing corner-linked ${\mathrm{CoO}}_5$ square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range ${\mathrm{Co}}^{2+}/{\mathrm{Co}}^{3+}$ charge ordering occur as well as a change in the ${\mathrm{Co}}^{2+}$ spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering.