Anomalous Scattering Rod in Pr₂NiO_4.25

We report on a resonant scattering study of Pr₂NiO_4.25. Materials with the K₂NiF₄ structure (214) have attracted substantial attention over recent years. Cuprate high temperature superconductors with the 214 structure such as La_2-xSr_xCuO₄ have seen the most investigations, but similar compounds of other transition metals also have interesting properties. Nickelates of type Ln₂NiO_4+y (Ln = lanthanide) have unusual magnetic behavior and often charge ordering similar to cuprates. These materials can be hole-doped by both Ln site substitution and the incorporation of excess oxygen, with smaller Ln atoms leading to distortions that allow more oxygen. Samples with Ln=Pr naturally incorporate significant excess oxygen; our studies use Pr₂NiO_4.25. This compound is interesting both for its connections to the cuprate superconductors and for use in low temperature solid oxide fuel cells due to its high oxygen ion conductivity. Resonant scattering studies show several resonant-only peaks. Some are likely associated with complex charge order. In addition, the nominally disallowed [001] peak appears on resonance with different behavior associated with different atomic resonances. Our newest work finds a scattering rod develops over the time along (001) direction despite a surface 20° off that direction and no apparent faceting in AFM. We will discuss the resonant and non-resonant behavior of this scattering rod and its possible origins.