Influence of oxygen nonstoichiometry on structure and magnetism inPr2NiO4+δ

The structure and magnetism of ${\mathrm{Pr}}_2$ ${\mathrm{NiO}}_{4+\mathrm{\delta }}$ crystals with δ=0.060 and δ=0.020 were investigated by neutron scattering. ${\mathrm{Pr}}_2$ ${\mathrm{NiO}}_{4.060}$ is single-phase tetragonal (P$4_2$/ncm) from 5 to 300 K with the onset of antiferromagnetic order appearing in the nickel sublattice at ${\mathit{T}}_{\mathit{N}}$=116 K, as well as a gradual coupling of praseodymium spins appearing below 80 K. The ordered nickel moment at 10 K is estimated to be 0.62${\mathrm{\mu }}_{\mathit{B}}$. Two coexisting phases are observed in the δ=0.020 sample, apparently resulting from phase separation. Above 118 K, one phase is orthorhombic (Pccn) and the other tetragonal (P$4_2$/ncm), with ${\mathit{T}}_{\mathit{N}}$=329 and 143 K, respectively. At 118 K, the orthorhombic phase undergoes a transition on cooling to tetragonal P$4_2$/ncm symmetry. Spin structures and a magnetic transition are discussed in relation to ${\mathrm{NiO}}_6$ octahedral distortions.