Study of the crystal structure of non-superconducting doped polycristalline (Nd1-xCax)NiO2 nickelates in the 4K-300K range

Unconventional superconductivity below Tc ~ 15K was discovered recently in thin films of R1-xAxNiO2 nickelates with R = La, Nd, Pr and A=Sr or R,A = La,Ca. Up to now, superconductivity has not been found in any bulk sample with identical composition. In fact, the synthesis process involves a critical topochemical reduction which introduces defects (remaining oxygen atoms, Ni vacancies…), maybe at a higher level in bulk materials, which could be at the origin of the absence of superconductivity. We want to quantify these defects at 300 K by XRD (and neutron diffraction, related proposal actually evaluated) at ID22 beamline of ESRF in our bulk (Nd1-xCax)NiO2 samples, insulating at low T, where doping is introduced by Ca2+ doping. We propose also to measure high resolution XRD patterns in the 5-300 K range to determine the crystal structure at low T and look for possible structural transitions maybe linked to the spin-glass like behavior detected around T* = 6K – 18K in magnetization.