Spin-orbit induced mixed-spin ground state in $R$NiO$_3$ perovskites   probed by XAS: new insight into the metal to insulator transition
  We report on a Ni L$_{2,3}$ edges x-ray absorption spectroscopy (XAS) study in $R$NiO$_3$ perovskites. These compounds exhibit a metal to insulator ($MI$) transition as temperature decreases. The L$_{3}$ edge presents a clear splitting in the insulating state, associated to a less hybridized ground state. Using charge transfer multiplet calculations, we establish the importance of the crystal field and 3d spin-orbit coupling to create a mixed-spin ground state. We explain the $MI$ transition in $R$NiO$_3$ perovskites in terms of modifications in the Ni$^{3+}$ crystal field splitting that induces a spin transition from an essentially low-spin (LS) to a mixed-spin state. 
C. Piamonteze, F. M. F. de Groot, H. C. N. Tolentino, A. Y. Ramos, N.   E. Massa, J. A. Alonso, M. J. Mart\'inez-Lope