Interstitial and apical oxygen order-disorder in La2CoO4+δ observed by single-crystal neutron and X-ray diffraction
La2CoO4+δ single crystals with δ=0.09 and 0.20 have been studied by neutron diffraction at room temperature. The nuclear density distribution has been reconstructed using the maximum entropy method. For δ=0.09, the tilt direction is found to be along the  direction with respect to the F centred unit cell, whereas the CoO6 octahedra are distorted along  for δ=0.20. The related distances between the interstitial oxygen atoms and the apical oxygen atoms are about 2.75 and 2.6Å, respectively, which exclude any peroxide character, even for higher oxygen doping. For La2CoO4.20, weak incommensurate satellite reflections, with a 2D incommensurate vector, have been found for the first time by X-ray diffraction indicating a more complex 3D ordering of the interstitial oxygen atoms.