Pr3+ crystal-field excitation study of apical oxygen and reduction processes in Pr2-xCexCuO4 +/-delta

We present an infrared transmission Pr3+ crystal-field study of as-grown, reduced, and oxygenated Pr2-xCexCuO4+/-delta single crystals and thin films. Excitations from the ground-state multiplet H-3(4) to the H-3(5), H-3(6), F-3(2), and F-3(3) excited multiplets are observed in all samples. In addition to the Pr3+ regular sites, which remain unperturbed following the cerium doping or the oxygen content modifications, Pr3+ sites are detected. A precise set of crystal-field parameters, which reproduces the energy and the symmetry of the levels, is determined. The reduction process, which drives the electron-doped cuprates superconducting, is discussed in detail and scenarios for the reduction mechanism and induced vacancies are proposed. In contrast to the common belief, the apical oxygen, which is clearly detected in all samples, is not removed in Pr1.85Ce0.15CuO4 following reduction. This observation questions the role attributed to the apical oxygen removal in triggering the superconductivity.