XAFS STUDY ON REDUCTION PROCESS OF PAULI-PARAMAGNETIC LANIO3 TO ANTIFERROMAGNETIC LA2NI2O5

Structural variation in a H-2 reduction process of Pauli-paramagnetic LaNiO3 to antiferromagnetic La2Ni2O5 was investigated by means of XAFS spectroscopy. XANES spectra of LaNiO3-x (0 less-than-or-equal-to x le ss-than-or-equal-to 0.5) indicate that not the pyramidal coordination but the square-planar one is mainly formed even in an early step on th e reduction. The Ni-O distance gradually decreased with an increase of the reducing durations, although the ionic radius of Ni ion should ex pand with the reduction. This phenomenon may be related to the lattice distortion of the octahedral NiO6 and planar NiO4 with cooperative Ja hn-Teller ions of d7-Ni3+ in the low spin state and d9-Ni+, lying in t he La2Ni2O5.