Magnetostructural Transition in Spin Frustrated Halide Double Perovskites

Geometrical frustration in the face-centered-cubic (fcc) lattice presents a fundamental challenge in determining antiferromagnetic order, as the ground state is highly sensitive to subtle differences in competing magnetic interactions and structural symmetry. Here, we explore the magnetostructural interplay in two halide double perovskites, Cs₂NaFeCl₆ and Cs₂AgFeCl₆. Although both materials have a cubic structure at room temperature, neutron diffraction shows that they adopt different antiferromagnetic structures upon cooling. Cs₂NaFeCl₆ experiences a transition to an AFM-III order below 2.6 K, governed by J ₁ and J ₂ (first and second nearest-neighbor) magnetic exchange interactions. Cs₂AgFeCl₆, however, adopts an AFM-I order below 17 K, accompanied by a significant tetragonal distortion confirmed from both neutron diffraction and polarized Raman spectroscopy. Thermal expansion measurements reveal anomalous lattice expansion at the magnetic transitions in both compounds but are substantially stronger in Cs₂AgFeCl₆. Combining these findings with density functional theory (DFT) studies, we conclude that the strength of magnetoelastic coupling dictates the magnetic ground state. A strong J ₁ in Cs₂AgFeCl₆ induces a large tetragonal lattice distortion, relieving magnetic frustration and stabilizing the AFM-I phase. In contrast, weaker magnetoelastic coupling in Cs₂NaFeCl₆ causes minimal distortion, favoring the AFM-III phase via the J ₁-J ₂ mechanism. Our findings show that magnetic interactions can be a primary driving force for structural phase transitions in these materials, while the strong structural distortion could determine the selection of magnetic ground-state ordering.