Crystal structures of highly hole-doped layered perovskite nickelate Pr_2−xSr_xNiO₄ studied by neutron diffraction

A high-resolution time-of-flight powder neutron diffraction study of the layered nickel oxide Pr₂−_xSr_xNiO₄ with x = 0.7 and 0.9 was performed to characterize the crystal structures of these highly hole-doped nickelates. For the sample with x = 0.7, the Ni–O bond lengths decrease uniformly with decreasing temperature, and the atomic displacement parameters are similar to those for x = 1=3. In contrast, for the sample with x = 0.9, the out-of-plane Ni–O bond length shows a sharp thermal contraction in the high-temperature region, which is suggestive of changes in the orbital occupation accompanied by the development of checkerboard-type charge correlations. Furthermore, the x = 0.9 sample is characterized by a large atomic displacement parameter for the apical O atoms along the out-of-plane direction, which is interpreted as the existence of two types of Ni³⁺ sites with different orbital occupancies. The distinct difference between the x = 0.9 sample and the lower-concentration compounds should be related to the development of checkerboard-type charge ordering in the metallic matrix and possible orbital ordering at the Ni³⁺ sites at x ∼ 1 in the hole-doped layered nickel oxides.