ANALYSIS OF INFRARED REFLECTIVITY OF PR2NIO4 SINGLE-CRYSTAL

The reflectivity spectra of a single crystal of stoichiometric and non-stoichiometric Pr2NiO4+delta (0 < delta < 0.3, measured by thermogravimetry) are reported for the polarizations parallel and perpendicular to the c axis. The crystal structure is found to depend upon the oxygen stoichiometry, which itself controls the transition from insulator to semiconducting. The semiconducting behaviour is checked by the temperature dependence of resistivity. In the non-stoichiometric compound, the phonons are partially screened by the plasma background, which is limited to the conducting basal (a, b) plane, as in La2CuO4. An annealing process at 650 K under 5% of hydrogen in argon provides the stoichiometric compound which is an insulator and shows an orthorhombic distortion with respect to the tetragonal structure.