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

In the present work, we explored the interplay between the lattice and magnetic degrees of freedom in a rare-earth nickelate ${\text{EuNiO}}₃$, 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 ${a}₀^{\ensuremath{-}}{a}₀^{\ensuremath{-}}{c}₀⁺$), 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 $({c}_p/{a}_p,$ $\ensuremath{\gamma},$ ${V}_{\text{mono}})$ around 463 K and 200 K corresponding to respective isosymmetric metal-insulator transition temperature $({T}_{M\text{\ensuremath{-}}I})$, and Neel temperature $({T}_N)$ 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., ${\mathrm{Mono}}_{\mathrm{metal}}$ (T > ${T}_{M\text{\ensuremath{-}}I}$) and ${\mathrm{Mono}}_{\mathrm{insulator}}$ (T ${T}_{M\text{\ensuremath{-}}I}$), where the latter is more distorted than the former. The transition from ${\mathrm{Mono}}_{\mathrm{metal}}$ to ${\mathrm{Mono}}_{\mathrm{insulator}}$ at ${T}_{M\text{\ensuremath{-}}I}$ is reminiscent of a phase transition from orthorhombic (metallic) to monoclinic (insulating) phase, observed in other members of the rare-earth family. In addition, ${T}_{M\text{\ensuremath{-}}I}$ and ${T}_N$ 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., ${X}₅⁺(q=0,1/2,0)$ and ${R}₅⁺(q=1/2,1/2,1/2)$ as a function of temperature.