Neutron diffraction studies of Stoichiometric and oxygen intercalated La_2NiO_4 single crystals

摘要

Single crystal neutron diffraction was used to characterize the structural evolution of electrochemically oxidized La_2NiO_(4+delta) crystals as a function of temperature. In order to better describe the disorder of the apical and interstitial oxygen atoms we used the maximum entropy method. There is structural evidence not to attribute a peroxide character between the apical and interstitial oxygen atoms situated in exactly ((1/4)(1/4)(1/4)) of the F4/mmm unit cell. We have also reinvestigated the structural phase transition of stoichiometric La_2NiO_4 crystals at 74 K. The symmetry changes from Bmab to Pccn and remains orthorhombic down to 5 K.