Evidence for Isostructural Magnetoelastic Coupling across the Magnetic Transition in Disordered NdSrCoFeO6–δ

Disordered double perovskites offer a rich interplay between chemical disorder, magnetic frustration, and lattice degrees of freedom, yet the role of the average crystal structure in mediating magnetic transitions remains poorly understood. Here, we report a magnetoelastic lattice-strain anomaly in the B-site-disordered double perovskite NdSrCoFeO6–δ, investigated by temperature-dependent high-resolution synchrotron X-ray diffraction and heat-capacity measurements. The average crystal structure remains orthorhombic Pnma from 300 to 10 K, with no symmetry-lowering structural transition. However, the lattice metrics display a clear anomaly in the temperature range of the ferromagnetic-like transition, marked by an inflection in the unit-cell volume and an anisotropic change in the thermal expansion behavior. The strongest response occurs along the orthorhombic b-axis, indicating that magnetic correlations are coupled to the octahedral-tilting degree of freedom rather than producing a crystallographic symmetry change. A volumetric strain analysis further reveals a contraction of the unit cell relative to a smooth high-temperature lattice baseline near the heat-capacity anomaly at TC ≈ 142 K. Notably, weaker lattice anomalies persist up to 235 K, within the previously reported Griffiths-like phase regime, suggesting that the lattice may sense magnetic heterogeneity before long-range order fully develops. These results demonstrate that the average lattice in NdSrCoFeO6–δ is not a passive host but an active component of the magnetic phase evolution, with implications for understanding magnetoelastic coupling in chemically disordered cobaltite-ferrite perovskites.