Unexplored Signatures of magnetoelastic and Isosymmetric metal-insulator phase transition in a Rare-earth Nickelate via mode crystallography

Rare-earth Nickelates (RNiO3) are known to exhibit a cooperative phenomenon between lattice, electronic and magnetic degrees of freedom, where R could be any member of rare earth series [1-4]. These compounds go through a sharp metal-insulator phase transition accompanied by a symmetry lowering transition from high temperature orthorhombic(metallic) to low temperature monoclinic(insulating) phase at M-I phase transition temperature (TM-I). In addition, all systems order antiferromagnetically at Neel temperature (TN), which is same as TM−I for Pr and Nd but lower than TM−I for smaller cations [3,5]. In this work [6], we have explored the phase transitions in a rare earth nickelate EuNiO3, by carrying out temperature-dependent structural analysis in conjunction with distortion mode analysis. The temperature-dependent powder Synchrotron X-ray diffraction (SXRD) studies have revealed the presence of an orthorhombic Pbnm phase (tilt system ??−??−??+), with an elementary perovskite (pseudo-monoclinic) 000 cell, over the analysed temperature range, i.e., 100-623 K [7-9]. Further, we have observed two distinct anomalies in temperature- dependent evolution of pseudo-monoclinic cell parameters (cp/ap, γ, Vmono) around 463 K and 200 K corresponding to respective Isosymmetric Metal-Insulator transition temperature (TM−I), and Neel temperature (TN) linked with a volume gain at low temperatures dictating a magnetoelastic coupling in the system. We have shown the existence of two distinct pseudo-monoclinic phases, viz., Monometal (T > TM−I) and Monoinsulator (T