Unexplored signatures of magnetoelastic and isosymmetric metal-insulator phase transition in a rare-earth nickelate via mode crystallography

Abstract

In the present work, we explored the interplay between the lattice and magnetic degrees of freedom 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 revealed the presence of an orthorhombic Pbnm phase (tilt system a0-a0-c0+), with an elementary perovskite (pseudomonoclinic) cell, over the analyzed temperature range, i.e., 100-623 K. Further, we observed two distinct anomalies in the temperature-dependent evolution of pseudomonoclinic 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 show the existence of two distinct pseudomonoclinic phases, viz., Monometal (T > TM-I) and Monoinsulator (T < TM-I), where the latter is more distorted than the former. The transition from Monometal to Monoinsulator at TM-I is reminiscent of a phase transition from orthorhombic (metallic) to monoclinic (insulating) phase, observed in other members of the rare-earth family. In addition, TM-I and TN are clearly evident by the nonanalytical behavior of the condensed soft phonon modes amplitude corresponding to the zone boundary of the cubic Brillouin zone, viz., X5+(q=0,1/2,0) and R5+(q=1/2,1/2,1/2) as a function of temperature. © 2022 American Physical Society.