18 / 2026-03-03 14:34:44

Impact of local structure on high pressure phase transitions in A4Nb2O9 (A = Mn, Co, Ni) systems

multiferroic material,phase transition,high pressure,NMR,Raman spectroscopy,Synchrotron XRD

高压物理与材料科学 > 静高压物理与材料科学

The A₄B₂O₉ (A = Co, Fe, Mn, Ni and B = Nb,Ta) family constitutes an important class of magnetoelectric and compensated ferrimagnetic systems, where strong spin–lattice coupling and honeycomb-based magnetic networks enable tunable multifunctional properties. We present a comparative ambient- and high-pressure investigation of Co₄Nb₂O₉, Mn₄Nb₂O₉, and Ni₄Nb₂O₉ using nuclear magnetic resonance (NMR), Raman spectroscopy, and synchrotron x-ray diffraction, focusing on the role of local structure in governing structural instabilities. NMR reveals that, despite its orthorhombic average symmetry, Ni₄Nb₂O₉ possesses a local environment remarkably similar to trigonal Mn₄Nb₂O₉, whereas Co₄Nb₂O₉ exhibits a distinctly different local distortion. This local structural similarity between Mn₄Nb₂O₉ and Ni₄Nb₂O₉ results in closely matching pressure-induced behavior: both systems display multiple low-pressure isostructural transitions followed by symmetry-lowering transformations at comparable critical pressures. In contrast, Co₄Nb₂O₉ follows a different transition sequence, including successive isostructural anomalies prior to trigonal-to-monoclinic symmetry breaking. Pronounced Raman linewidth anomalies and phonon softening under pressure indicate potential spin–phonon coupling and evolving magnetic exchange pathways within the honeycomb lattice. Our results demonstrate that the local octahedral environment—directly probed by NMR—dictates the hierarchy of pressure-driven structural transitions and associated magnetic interactions in A₄Nb₂O₉ magnetoelectric systems.