Single-crystal neutron diffraction studies on Ni-based metal-pnictide superconductor BaNi2As2

We report the results of single-crystal neutron diffraction studies of the superconductor BaNi2As2. The experiments were performed on a tiny crystal of mass 0.8 mg at several temperatures between 20 and 200 K using the Single Crystal Diffractometer, SCD, at the Los Alamos Neutron Science Center. Above 130 K, BaNi2As2 crystallizes in the tetragonal ThCr2Si2 structure. Our neutron diffraction data corroborate a first-order structural transition around 130 K with a relatively large hysteresis of about 10 K, in agreement with observations from bulk studies. The anisotropic thermal displacement coefficients are enhanced along c-axis approaching the transition, and a splitting is observed for in-plane type reflections below the transition, which is evidence for a change in crystal structure.