The high-pressure behavior of two Mn-based honeycomb-structured magnetoelectric materials, Mn4Nb2O9 (MNO) and Mn4Ta2O9 (MTO), has been investigated using Raman spectroscopy, synchrotron x-ray diffraction and density functional theory (DFT) calculations. In MTO, application of a small pressure of only 0.5 GPa induces an isostructural transition driven by local symmetry breaking. With further increase in pressure, three additional isostructural transitions are observed, around 3.2, 6, and 10 GPa, followed by the onset of a long-range structural transition near 14 GPa, where the ambient P-3c1 phase begins to transform to a P2/c phase. These two phases coexist up to 27 GPa. The Nb analogue, MNO, also exhibits similar isostructural transitions at approximately 2, 6.6, and 10 GPa; however, the onset of the mixed P2/c and P-3c1 phases occurs at a slightly lower pressure ( 12.5 GPa), with coexistence extending up to 26.5 GPa. These long-range transitions are corroborated by pressure-dependent enthalpy variations revealed through DFT computations. Rietveld refinement reveals pronounced anisotropic lattice compression (42–49%) between the c and a axes, leading to a notable reduction in the c/a ratio. This anisotropy may enhance interlayer coupling and promote magnetic ordering under compression, consistent with the emergence of Raman modes resembling those reported at low temperatures, along with anomalous variations in Raman mode linewidth and intensity. The pronounced changes in Raman self energy parameters, anomalies in the reduced pressure–Eulerian strain profile, and the onset of local symmetry breaking at much lower pressures in MTO than in MNO highlight the crucial role of differing spin–orbit coupling strength and orbital hybridization effects associated with Nb5+ and Ta5+ cations.

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 (Tsro ∼ 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.

Over the last decade, frequency-domain in-situ high-pressure nuclear magnetic resonance (NMR) spectroscopy in diamond anvil cells (DACs) has been employed as a structural and electronic probe of condensed matter systems at pressures well into the megabar range. However, extensive spin interactions and sample heterogeneities under pressure often lead to significant spectral overlap, inhibiting independent observation of chemically similar spin sub-species in the same sample. In this work, we introduce a time-domain relaxometry framework specifically suited for DAC experiments, named µT2-NMR. Experimental flexibility and operational robustness are benchmarked on three hydrogen-rich molecular solids at pressures up to 72 GPa. We demonstrate that µT2-NMR can resolve individual molecular subunits in relaxation space, paving the way for novel high-pressure, high-resolution NMR applications in molecular solids.

There is a new preprint on Arxiv summarising our recent studies on dense hydrogen phase III between 181 and 208 GPa. This is the first time that we were able to fulyl employ our developed high-resolution techniques at such extreme conditions. See our Publications section for more information.