Structural Studies on the Phase Transition of La₃Ni₂O_6.92 at about 550 K

Structural studies of La₃Ni₂O_6.92 with a double layered perovskite structure have been performed by the X-ray Rietveld analyses between 300K and 650K. The interatomic distance between Ni and the apical oxygen (on the outside of NiO₂ double layer) rapidly increases around the phase-transition temperature TB ∼ 550K with decreasing temperature. Then, the energy of the (3z²-r²) band is considered to shift downwards with respect to that of the (x²-y²) band with decreasing temperature and the lower Hubbard band of the former becomes to be (almost) fully occupied at the transition temperature, which induces the Mott transition (delocalized-localized transition) of the (3z²-r²) electrons.