Reduction processes of rare-earth nickelate perovskites LnNiO3 (Ln=La, Pr, Nd)

Neutron diffraction technique was applied to PrNiO2.34 and the unit cell of PrNiO2.34 was reinvestigated. The diffraction peaks could be assigned by the monoclinic cell of a=8.656(6) Å, b=3.655(2) Å, c=5.442(4) Å and β=71.50(7)°. The cell can be expressed as 5×1×2 of the fundamental perovskite. This suggests PrNiO2.34 should be the compound Pr3Ni3O8. The existence of Lnm+n[Ni3+O6/2]m[Ni+O4/2]n homologous series can be proposed, where m sheets of octahedral layers alternate with n sheets of square planar layers along the [110] direction of the fundamental perovskite-type lattice. All the compounds appeared in the reduction processes of the perovskite-type LnNiO3 (Ln=La, Pr, Nd), La3Ni3O8, La2Ni2O5 and Ln3Ni3O7 (Ln=Pr, Nd) belong to the homologous series.