Entropy‑Engineered Octahedral Breathing: Unlocking Synergistic High Permittivity and Low Loss in NiTiNb 2 O 8 Microwave Dielectrics Via Lattice Dynamics Modulation

ABSTRACT Microwave dielectric ceramics face a persistent trade‐off between high relative permittivity () and high quality factor ( Q × f ), particularly in niobate‐based systems with complex octahedral frameworks. Here, we challenge this conventional paradigm by demonstrating an entropy‑driven lattice dynamics strategy that simultaneously boosts and Q × f in NiTiNb 2 O 8 ceramics. Through systematic control of sintering temperature (1100‒1300°C), we achieve exceptional microwave dielectric properties at 1250°C: = 54.78, Q × f = 12 152 GHz, and = +139.99 ppm/°C. Contrary to the widely held view that densification or secondary phases dominate performance, our multi‑scale analysis combining Rietveld refinement, Raman spectroscopy, and microstructure characterization reveals a fundamentally different mechanism—the “octahedral breathing entropy effect.” We demonstrate that increasing sintering temperature drives the system toward a critical state where the vibrational entropy of [NbO 6 ] octahedra is maximized while lattice strain energy is minimized. At 1250°C, the system enters a resonant dynamic state characterized by coherent octahedral breathing vibrations. In this state, the phonon damping coefficient increases without concomitant energy dissipation, leading to an anomalous enhancement of polarizability while maintaining a high quality factor. Excessive sintering (1300°C) breaks the coherence, triggering a transition to incoherent thermal vibrations that deteriorate both and Q × f . Our work identifies vibrational entropy, controlled through sintering temperature, as a processing‑sensitive parameter for tailoring microwave dielectric properties and demonstrates that accessing states of enhanced octahedral dynamical correlation can simultaneously improve permittivity and quality factor in niobate‑based ceramics.