Low to high spin-state transition induced by charge ordering in antiferromagnetic YBaCo2O5

The oxygen-deficient double perovskite YBaCo2O5, containing corner-linked C oO5 square pyramids as principal building units, undergoes a paramagnetic t o antiferromagnetic spin ordering at 330 K. This is accompanied by a tetrag onal to orthorhombic distortion. Below 220 K orbital ordering and long-rang e Co2+/Co3+ charge ordering occur as well as a change in the Co2+ spin stat e from low to high spin. This transition is shown to be very sensitive to t he oxygen content of the sample. To our knowledge this is the first observa tion of a spin-state transition induced by long-range orbital and charge or dering.