Raman scattering investigation across the magnetic and metal-insulator transition in rare earth nickelate RNiO3 (R=Sm, Nd) thin films

We report a temperature-dependent Raman scattering investigation of thin-film rare earth nickelates SmNiO3, NdNiO3, and Sm0.60Nd0.40NiO3 which present a metal-to-insulator (MI) transition at T-MI and an antiferromagnetic-paramagnetic Neel transition at T-N. Our results provide evidence that all investigated samples present a structural phase transition at T-MI but the Raman signature across T-MI is significantly different for NdNiO3 (T-MI=T-N) compared to SmNiO3 and Sm0.60Nd0.40NiO3 (T-MI not equal T-N). It is namely observed that the paramagnetic-insulator phase (T-N<T<T-MI) in SmNiO3 and Sm0.60Nd0.40NiO3 is characterized by a pronounced softening of one particular phonon band around 420 cm(-1). This signature is unusual and points to an important and continuous change in the distortion of NiO6 octahedra (thus the Ni-O bonding) which stabilizes upon cooling at the magnetic transition. The observed behavior might well be a general feature for all rare earth nickelates with T-MI not equal T-N and illustrates intriguing coupling mechanism in the T-N<T<T-MI regime.