Magnetostructural Coupling at the Néel Point in YNiO₃ Single Crystals

The recent discovery of superconductivity in infinite layer thin films and bulk Ruddlesden-Popper nickelates has stimulated the investigation of other predicted properties of these materials. Among them, the existence of magnetism-driven ferroelectricity in the parent compounds RNiO₃ (R = 4f lanthanide and Y) at the onset of the Néel order, T_N, has remained particularly elusive. Using diffraction techniques, we reveal here the existence of magnetostriction at T_N in bulk YNiO₃ single crystals. Interestingly, the associated lattice anomalies are much more pronounced along the b crystal axis, which coincides with the electric polarization direction expected from symmetry arguments. This axis undergoes an abrupt contraction below T_N that reaches Δb/b ∼ −0.01%, a value comparable to those found in some magnetoresistive manganites and much larger than those reported for magnetism-driven multiferroics. This observation suggests a strong spin-lattice coupling in these materials, consistent with theoretical predictions. Using the symmetry-adapted distortion mode formalism, we identify the main ionic displacements contributing to the lattice anomalies and discuss the most likely polar displacements below T_N. Furthermore, our data support symmetric superexchange as the most likely mechanism responsible for the magnetoelastic coupling. These results, that may be common to the full RNiO₃ family, provide new experimental evidence supporting the predicted existence of magnetism-driven ferroelectricity in RNiO₃ perovskites.