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

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.

💡 Research Summary

In this work the authors present a comprehensive investigation of spin‑phonon coupling (SPC) and possible hidden structural transitions in the spin‑driven ferroelectric compounds Mn4Nb2O9 (MNO) and Mn4Ta2O9 (MTO), members of the A4B2O9 family. High‑purity polycrystalline samples were prepared by conventional solid‑state synthesis and confirmed to crystallize in the trigonal P‑3c1 space group through Rietveld refinement of X‑ray diffraction data. Compared with the Co‑based analogues (Co4Nb2O9 and Co4Ta2O9), the Mn‑based materials display larger lattice parameters, reflecting the larger ionic radius of Mn2+, and a modest increase in unit‑cell volume when Nb5+ is replaced by the larger Ta5+ ion.

Raman spectroscopy was carried out from 77 K to 300 K using a low‑power 532 nm laser to avoid heating. Factor‑group analysis predicts 22 Raman‑active modes (7 A1g + 15 Eg) for the P‑3c1 structure; the experiments detect 17 modes in MNO and 16 in MTO, whereas only 11–15 modes are observed for the Co analogues. This richer mode spectrum indicates a higher degree of local structural distortion in the Mn compounds. The observed modes span low‑frequency Mn–O network vibrations (≈60–300 cm⁻¹) and high‑frequency octahedral vibrations involving NbO6/TaO6 units (≈400–800 cm⁻¹).

Temperature‑dependent Raman data reveal two distinct regimes of SPC. Below the short‑range magnetic ordering temperature Tsro ≈ 223 K, many phonons experience a pronounced hardening (blue shift) and a narrowing of linewidths, signalling strong coupling between emerging magnetic correlations and lattice dynamics. Upon further cooling, long‑range antiferromagnetic ordering occurs at TN = 120 K for MNO and 110 K for MTO. At these temperatures additional renormalization of several phonon frequencies, linewidths, and integrated intensities is observed. Most strikingly, new Raman modes appear in the intermediate temperature range (Tsro < T < TN), particularly modes associated with the Nb/Ta octahedra. Their emergence suggests a subtle symmetry lowering—potentially a transition from the high‑symmetry trigonal phase to a lower‑symmetry monoclinic or other distorted structure—that is not captured by conventional diffraction but is evident in a local probe such as Raman scattering. The effect is more pronounced in MNO, whereas in MTO the new modes are more strongly coupled to the magnetic order, reflecting the influence of the B‑site cation.

Magnetic susceptibility measurements (ZFC/FC) corroborate the Raman findings: both compounds display anomalies at Tsro and TN, with MTO showing larger hysteresis and a more pronounced change in susceptibility, consistent with stronger magnetic exchange striction. The authors attribute the differences between Nb and Ta to the distinct electronic characters of the B‑site ions. Nb5+ possesses 4d electrons with relatively weak spin‑orbit coupling (SOC), leading to more ionic Nb–O bonds. Ta5+ contains 5d electrons, which experience stronger SOC and more covalent Ta–O bonding due to enhanced d‑p hybridization. This increased SOC in Ta5+ amplifies the magneto‑elastic interaction, thereby strengthening SPC in MTO relative to MNO.

To further probe the local environment, 93Nb nuclear magnetic resonance (NMR) was performed on MNO and on the Co analogue CNO. The NMR spectra of MNO are broader and exhibit shorter spin‑lattice (T1) and spin‑spin (T2) relaxation times, indicating a more heterogeneous electronic environment around Nb nuclei and a higher degree of local distortion compared with the Co‑based material. Diffuse reflectance spectroscopy (DRS) was employed to assess the electronic structure. After Kubelka‑Munk conversion, both Mn‑based compounds show an optical band gap of ~2.8 eV, comparable to the Co analogues, but the absorption edges differ in shape and intensity, reflecting distinct band structures arising from Nb 4d versus Ta 5d contributions.

The paper draws several key conclusions: (i) robust SPC exists over a wide temperature range, from the onset of short‑range magnetic correlations down to the antiferromagnetic transition; (ii) the B‑site ion controls the strength and temperature dependence of SPC through its SOC and orbital hybridization, with Ta5+ enhancing the coupling; (iii) Raman spectroscopy uncovers a hidden low‑symmetry structural transition that is invisible to conventional diffraction, highlighting the importance of local probes for detecting subtle lattice changes in multiferroic materials; (iv) Mn‑based A4B2O9 compounds are intrinsically more locally distorted than their Co counterparts, which influences both magnetic and ferroelectric properties.

These insights are valuable for the design of magnetoelectric and spintronic devices. By exploiting the tunable SPC via B‑site substitution, external pressure, strain, or magnetic fields, one can engineer the coupling between spin, lattice, and orbital degrees of freedom to achieve higher operating temperatures, larger magnetoelectric coefficients, or controlled ferroelectric switching. The work thus provides a detailed experimental foundation for future theoretical modeling and for the development of functional multiferroic materials based on the versatile A4B2O9 framework.