Unveiling Robust Spin-Phonon Coupling and Magnetic-Order-Driven Phonon Softening in Honeycomb Mn₃NiTa₂O₉

Observation and interpretation of the temperature dependence of the prominent Raman modes of Mn₃NiTa₂O₉(MNTO) from T = 8 K to 300 K is reported and compared with those reported in the isostructural Mn₄Ta₂O₉(MTO) with honeycomb lattice and trigonal symmetry. The systematic blue shift of all Raman modes of MNTO relative to those in MTO is interpreted to result from the reduced lattice parameters of MNTO due to Ni substitution. The smaller magnetic moment of Ni²⁺relative to that of Mn²⁺is the likely source of a few % reduction in the observed magnetic transitions (T_N∼ 97 K and T_S∼ 46 K) of MNTO. The phonon frequencies (ω) and linewidths (Γ) were obtained by fitting each Raman mode with a single Lorentzian profile. For T > T_N, the phonon frequencies decrease monotonically while the linewidths broaden with increasing temperatures, these results are well described by a three-phonon anharmonic decay model. With decreasing temperatures below T_N, pronounced softening of nearly all Raman modes in MNTO is observed, becoming particularly significant below T_Sin contrast to MTO, where phonon hardening was reported. These anomalous phonon shifts are attributed to strong spin-phonon coupling (λ), which is positive in MNTO but negative in MTO. The reversal in the sign of λ in MNTO highlights a fundamental modification of the spin-lattice interaction induced by Ni substitution demonstrating that subtle structural changes can profoundly alter the coupling between lattice vibrations and magnetic order in the honeycomb magnetoelectric tantalates.