Structural and magnetic phase diagram of Pr2-xCaxNiO4+δ (δ >> 0 and δ ≈ 0) - a prototype system studying electronic orderings in the superconducting nickelates

Certain Ruddlesden-Popper nickelates of the form Rn+1NinO3n+1 (n=2 where R is a rare-earth element, e.g. La3Ni2O7 [1]) have recently been discovered to host unconventional superconductivity (SC), giving a nearby chemical analogue to the famous cuprates which are well-known for their high critical temperatures and unsolved pairing mechanism. Surprisingly however, there has been little recent attention on the n = 1 family member, that is directly structurally analogous to the cuprate La2-xBaxCuO4. Here we propose a details neutron powder diffraction study of the phase diagram of Pr2-xCaxNiO4, that at x = 0.5 is isoelectronic with high pressure superconducting La3Ni2O7. We envisage that the phase diagrams detailing the magnetic and structural orderings at ambient pressure will be a vital point of reference for future studies of high-pressure SC in these and related materials.