EXAFS evidence for the spin-phonon coupling in the monoclinic PrNiO3 nickelate perovskite

An understanding of the electronic and structural changes across the temperature-induced phase transition in nickelates with a perovskite structure (RNiO3, with R being Y, Tl, rare-earths) is of key importance to shape these materials as devices for industrial applications in several fields, ranging from sensors, catalysts, and non-volatile memory devices. Particularly, PrNiO3 has received special attention because structural, electronic, and magnetic transitions coincide in this compound at temperatures of 125-130 K, which occur under differing pressure and temperature conditions in other nickelates. To draw a refined picture of the origin of these transitions, we investigated the structural changes taking place at a short-range order (local level) in PrNiO3 around the Ni atoms by means of X-ray absorption spectroscopy and at temperatures between 10 and 300 K. Ni K-edge extended X-ray-absorption fine structure (EXAFS) data below T-N approximate to 130 K confirm the monoclinic phase (P2(1)/n). At higher temperatures, we observed the convergence of the pair-unit distances Ni-Pr, suggesting the stabilization of the orthorhombic lattice (Pbnm). We derived the Einstein temperatures from the temperature-dependent EXAFS data, which provided an estimate of the Ni-O bond stiffness. We found an anomalous behaviour of the Debye-Waller factors sigma(2)(Gamma) of the Ni-O bond below the structural transition at 130 K. The anomalous temperature evolution of sigma(2)(Gamma) was modelled using a molecular field approximation for the scalar spin correlation function for the pair-bond Ni-O. This model suggests strong spin-phonon coupling and the softening of the lattice vibrations below T-N approximate to 130 K in agreement with the magnetic and vibrational properties of this structure. The present results demonstrate that EXAFS is not only a powerful technique for depicting structural changes, but also for exploring the coupling behaviour between the spin configuration and phonons. The present approach provides new opportunities for such types of studies in related materials.