Crystal Structure of Tetragonal Form of La₂NiO_4+<i>x</i>

Abstract

The crystal structure of the title oxide was studied by means of the X-ray and neutron single crystal diffraction measurements. At room temperature, the tetragonal crystal structure is P4 2 /ncm-type (No. 138), which is one of the subgroup of the space group I4/mmm. The lattice parameters of a sample annealed and slowly cooled in oxygen atmosphere from 673 K are a = b =5.4640(1) Å and c =12.6719(2) Å, while the oxygen content, x =0.10(4), was determined from obtained neutron data. The title oxide undergoes a tetragonal (P4 2 /ncm)/tetragonal (I4/mmm) phase transition at about 560 K. The transition temperature is almost identical both in the annealed and as-grown crystals.