Structural study across TN of the spin-driven multiferroic phase in rare earth nickelates

RNiO3 nickelates are referent materials to study the evolution of the physical properties at the crossover from localized to itinerant behavior in highly correlated electron systems. The subtle competition between bandwidth (related to Ni-O-Ni distortion), on-site electron-electron repulsion (U), and charge transfer energy between O and Ni sites produces multiple entangled order parameters in RNiO3. Both, the exact mechanism that drives their unusual MIT as well as the multiferroic properties of the insulating magnetic phase are still under debate. In RNiO3 (R=Yb, Y, Dy and Nd, varying the Ni-O-Ni distortion and the charge disproportionation degree) we aim to investigate their low-temperature structures. Very recent Raman studies have revealed new modes across TN unveiling subtle structural changes induced by spin ordering. They have great importance for the magnetic and multiferroic properties of RNiO3 compounds and heterostructures.