Observation of robust spin-phonon coupling and indication of hidden structural transition in the spin-driven ferroelectrics Mn4B_2O_9 (B= Nb, Ta)

TL;DR

Mn$_4$Nb$_2$O$_9$ and Mn$_4$Ta$_2$O$_9$ are explored as spin-driven ferroelectrics to quantify spin–phonon coupling (SPC) via Raman spectroscopy, NMR, diffuse reflectance, and magnetic susceptibility. The work identifies strong SPC developing below the short-range ordering temperature $T_{sro} oughly223$ K, with substantial phonon renormalization and the emergence of additional octahedral modes between $T_{sro}$ and the long-range order temperature $T_N$ (≈120 K for MNO and 110 K for MTO). Evidence for a possible low-symmetry structural transition, particularly in Mn-based systems, is inferred from mode activation and linewidth anomalies, and is closely linked to magnetic ordering, especially in MTO. The results show that the nonmagnetic $B$-site cation (Nb vs Ta) and the associated spin–orbit coupling and orbital hybridization significantly tune SPC and magnetostructural behavior, providing a comprehensive view of spin–lattice coupling in Mn- and Co-based $A_4B_2O_9$ magnetoelectrics and highlighting orbital degrees of freedom as a crucial control parameter. Overall, the study advances understanding of how spin, lattice, and orbital degrees of freedom intertwine to shape magnetoelectric phenomena in this family of materials.

Abstract

We report detailed Raman spectroscopic and magnetic susceptibility studies on the spin-driven ferroelectric compounds Mn4Nb2O9 (MNO) and Mn4Ta2O9 (MTO). Both systems exhibit strong spin-phonon coupling below the short-range magnetic ordering temperature (T(sro)=223 K), followed by further renormalization of several Raman modes at the long-range magnetic ordering temperatures (TN = 120 K for MNO and 110 K for MTO). Pronounced anomalies in Raman mode frequencies and linewidths, along with the emergence of octahedral modes between Tsro and TN, indicate a possible low-symmetry structural transition, more evident in MNO and closely linked to magnetic ordering in MTO. Distinct low-temperature evolutions of Raman mode shift, linewidth, and integrated intensity in MNO and MTO highlight the role of the nonmagnetic B-site cation in tuning spin-lattice coupling, driven by differences in spin-orbit coupling and orbital hybridization between Nb5+ (4d) and Ta5+ (5d). By combining Raman spectroscopy with nuclear magnetic resonance, and diffuse reflectance spectroscopy, we further show that Mn-based systems possess a more distorted local structure than their Co analogues, while their electronic structures differ despite comparable band gaps. These results provide a comprehensive understanding of spin-lattice coupling in Mn- and Co-based A4B2O9 magnetoelectric systems.

Figures (14)

• Figure 1: Rietveld-refined X-ray diffraction patterns of (a) MNO and (b) MTO at ambient pressure. Experimental data are shown as red circles, and the solid black lines represent the calculated profiles from the refinement. The difference between the observed and calculated patterns is displayed as blue curves at the bottom of each panel. Vertical green tick marks indicate the allowed Bragg reflection positions for the trigonal phase. (c) Schematic illustration of the trigonal P-3c1 unit cell of MNO/MTO. (d) Planar honeycomb layer (L1) formed by edge-sharing Mn1O$_6$ octahedra when viewed along the c axis. (e) Top view of the buckled honeycomb layer (L2), highlighting the alternating connectivity of magnetic Mn2O$_6$ and nonmagnetic octahedra Nb/TaO$_6$.
• Figure 2: Raman spectra of MNO and MTO compared with their Co-based analogues in the ranges (a) 50–680 cm$^{-1}$ and (b) 680–900 cm$^{-1}$. Black dashed lines indicate the systematic evolution of corresponding Raman modes across the four compounds.
• Figure 3: Tauc plots of the four systems derived from diffuse reflectance spectra using Eq. (1): (a) MNO, (b) MTO, (c) CNO, and (d) CTO. Red solid lines represent the linear fits according to Eq. (2), and their extrapolation to the energy axis yields the corresponding optical band gaps.
• Figure 4: Frequency-sweep NMR spectra of MNO and CNO measured under an applied magnetic field of 9.3 T. The black and red arrows indicate the paramagnetic shifts (f-f$_0$) of MNO and CNO, respectively, determined relative to the nonmagnetic reference K[NbCl$_6$].
• Figure 5: Saturation-recovery measurements for (a) CNO and; (b) MNO. Magnetization-decay measurements for (c) CNO; and (d) MNO. Black circles represent the experimental data, while red lines denote fits using Eqs. (4) and (5), from which the spin–lattice (T1 ) and spin–spin (T2) relaxation constants are determined.