Neutron diffraction studies of the crystal and magnetic structure of Nd_1.5Sr_0.5NiO₄

The crystal structure and magnetic ordering of Nd_1.5Sr_0.5NiO₄ which shows a susceptibility maximum at 17 K have been investigated by neutron diffraction. A tetragonal K₂NiF₄-type structure is confirmed at room temperature with a = 3.7756(3) Å and c = 12.485(1) Å. A mean NiO distance of 1.986(5) Å reflects the presence of equal concentrations of Ni²⁺ and Ni³⁺. Low temperature neutron diffraction data to 11 K reveal features not apparent in the bulk susceptibility. Below 50 K a broad, diffuse reflection at Q = 0.835 Å^-1 is found which can be indexed as ( 1 200) relative to the chemical cell. The intensity of this peak has an unusual temperature dependence with abrupt increases between 25 and 21 K and between 14 and 11 K. The correlation length derived from the peak width decreases from a constant value of 20(4) Å between 14 and 21 K to 12(2) Å at 11 K. Also at 11 K a resolution-limited reflection appears which can be indexed as (101) on a magnetic cell with a_M = √2a_CH which is typical for long-range antiferromagnetic order in K₂NiF₄ structure compounds. The diffuse ( 1 200) reflection is ascribed to short-range magnetic order on the Ni sublattice while the long-range order below 11 K involves most likely NdNi intersublattice coupling. Both forms of order coexist at 11 K.