Oxygen vacancy ordering in La2-xSrxNiO4-delta(0<=x<=0.5): The crystal structure and defects investigated by neutron diffraction

The oxygen deficient and formally Ni1+-Ni2+ mixed-valence La2-xSrxNiO4-delta system has been studied by means of neutron diffraction. It is found that, for the described synthesis procedure, there exists a critical Sr concentration x(c) approximate to 0.135 separating the stability regions of two different crystallographic phases. For small Sr doping (x < x(c)), La2-xSrxNiO4-delta samples have basically the same structure as stoichiometric La2NiO4, undergoing similar phase transitions. For x > x(c), the partial ordering of the oxygen vacancies produces a change of the crystallographic symmetry (from Bmab to Immm) and prevents the two structural phase transitions observed in the x < x(c) region. In contrast to the behavior observed in hole-doped nickelates, the local fluctuations of the oxygen content produce lattice microstrains, the presence of which is clearly detected by the strong broadening of a family of Bragg reflections. A simple statistical model has been used to quantitatively characterize the microstrain distribution, supposed to be gaussian, from the observed (hkl)-dependence of the reflection broadening.