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 thermograv imetry) are reported for the polarizations parallel and perpendicular to the c axis. The crystal structure is found to depend upon the oxyge n stoichiometry, which itself controls the transition from insulator t o semiconducting. The semiconducting behaviour is checked by the tempe rature 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 anneal ing process at 650 K under 5% of hydrogen in argon provides the stoich iometric compound which is an insulator and shows an orthorhombic dist ortion with respect to the tetragonal structure.