Spin Blockade, Orbital Occupation, and Charge Ordering in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>La</mml:mi><mml:mn>1.5</mml:mn></mml:msub><mml:msub><mml:mi>Sr</mml:mi><mml:mn>0.5</mml:mn></mml:msub><mml:msub><mml:mi>CoO</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:math>

Using Co-L2,3 and O-K x-ray absorption spectroscopy, we reveal that the charge ordering in La1.5Sr0.5CoO4 involves high spin (S=3/2) Co2+ and low spin (S=0) Co3+ ions. This provides evidence for the spin-blockade phenomenon as a source for the extremely insulating nature of the La2-xSrxCoO4 series. The associated e{g}{2} and e{g}{0} orbital occupation accounts for the large contrast in the Co-O bond lengths and, in turn, the high charge ordering temperature. Yet, the low magnetic ordering temperature is naturally explained by the presence of the nonmagnetic (S=0) Co3+ ions. From the identification of the bands we infer that La1.5Sr0.5CoO4 is a narrow band material.