Experimental Observation of Monoclinic Distortion in the Insulating Regime of SmNiO₃ by Synchrotron X-ray Diffraction

RNiO₃ (R = rare-earth element) perovskite materials are well-known to exhibit characteristic metal-insulator transitions. The structural distortion increases as the R member becomes smaller along the series. For SmNiO₃, a high-hydrostatic-pressure preparation procedure, yielding samples with much enhanced crystalline quality, combined with the extremely high angular resolution of synchrotron X-ray diffraction (XRD) allowed us to identify a monoclinic phase in the insulating regime (below the metal-insulator transition temperature (T_MI) of 127 °C), defined in the space group P2₁/n. This monoclinic symmetry had not been demonstrated directly using nonresonant XRD or neutron diffraction. This has important repercussions on the electronic nature of this material since the monoclinic structure contains two inequivalent Ni positions, implying a charge disproportionation phenomenon. In the metallic regime (above T_MI), the standard orthorhombic Pbnm structure is observed. Therefore, there is a coupled structural and electronic transition, as happens for the very small rare-earth compounds of the RNiO₃ perovskite series. Across T_MI there is a dramatic rearrangement of the lattice parameters, degree of tilting, and distortion of the NiO₆ octahedra, showing the convergence of the Ni-O bond lengths upon entering the metallic phase. Brown's valence analysis of the different elements agrees with other reported values in the literature, matching with bond and charge disproportionation models. By magnetization measurements a Néel temperature (T_N) corresponding to the antiferromagnetic ordering of the Ni moments is identified at T_N= 220 K, whereas Sm moments experience long-range ordering below 36 K.