Spin-lattice coupling and bond-angle locking across the magnetic transition in B -site ordered double perovskite Sr 2 NiWO 6

The interplay between magnetic order and lattice distortion in B -site ordered double perovskites offers a fertile ground for exploring spin-lattice coupling phenomena. Here we report a temperature-dependent neutron powder diffraction study of Sr 2 NiWO 6 (SNWO), which orders magnetically below T N = 56 K with a type-II antiferromagnetic (AFM) structure. Detailed Rietveld refinements across 5-300 K reveal that the in-plane Ni-O(1)-W bond angle, which governs the dominant next-nearest-neighbor superexchange path, decreases monotonically from 167.4(1)° at 300 K down to 165.1(1)° at T N , and then remains locked upon further cooling. This bond-angle locking is accompanied by strongly suppressed thermal variations of the in-plane lattice parameter a, the O(1) coordinates, and the W-O(1) bond length below T N , alongside a reduced expansion rate of the anomalously expanding c axis below T N . These structural anomalies, together with an approximately linear relationship between the anomalous volume contraction and the squared ordered Ni moment, provide quantitative evidence consistent with spin-lattice coupling in SNWO. Within a possible magnetoelastic feedback picture, the establishment of AFM order may modify the balance between magnetic exchange and lattice distortion, thereby stabilizing the local superexchange geometry.