Pressure-driven vibrational and structural peculiarities in the honeycomb layered magnetoelectrics Mn4(B)2O9 (B= Nb, Ta)

Abstract

The high-pressure behavior of two Mn-based honeycomb-structured magnetoelectric materials, Mn4Nb2O9 (MNO) and Mn4Ta2O9 (MTO), was investigated using Raman spectroscopy, synchrotron x-ray diffraction, and density functional theory (DFT) calculations. In MTO, the 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 at about 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 into a P2/c phase. These two phases coexist up to 27 GPa. The Nb analogue, MNO, also exhibits similar isostructural transitions at about 2, 6.6, and 10 GPa. However, the onset of the mixed P2/c and P-3c1 phases occurs at a slightly lower pressure of 12.5 GPa, with phase coexistence extending up to 26.5 GPa. These long-range transitions are supported by pressure-dependent enthalpy changes obtained from DFT calculations. Rietveld refinement reveals pronounced anisotropic lattice compression, with a 42 to 49 percent difference between the c and a axes, leading to a notable reduction in the c/a ratio. This anisotropy may strengthen interlayer coupling and promote magnetic ordering under compression, consistent with the appearance of Raman modes similar to those reported at low temperatures, together with anomalous changes in Raman mode linewidth and intensity. The marked 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 important role of differences in spin-orbit coupling strength and orbital hybridization associated with Nb5+ and Ta5+ cations.

Figures (22)

• Figure 1: (a) Unit-cell structure of MNO/MTO crystallizing in the trigonal P-3c1 symmetry at ambient pressure. (b) Planar honeycomb layer (L1) consisting of edge-shared Mn1O$_6$ octahedra, viewed along the $c$ axis. (c) Buckled layer (L2) composed of two honeycomb sublayers formed by edge-shared Mn2O$_6$ and Nb/TaO$_6$ octahedra. (d) Interlayer connectivity between the planar and buckled layers, where Mn1O$_6$ and Mn2O$_6$ octahedra from the respective layers are linked via face sharing, while Nb/TaO$_6$ octahedra from the buckled layer are connected to Mn1O$_6$ octahedra in the planar layer through corner sharing. (e) Three-dimensional view of the buckled layer, illustrating the buckling of corner-shared Mn2O$_6$ octahedra forming two sublayers, with alternating Nb/TaO$_6$ octahedra from adjacent sublayers connected via face sharing.
• Figure 2: High-pressure Raman spectra of MNO up to 15 GPa in the wavenumber range 105–700 cm$^{-1}$. The emergence of new Raman modes is indicated by upward arrows, while the disappearance of modes is marked by downward arrows. The pressure evolution of individual modes is tracked by dashed black lines.
• Figure 3: High-pressure Raman spectra of MNO over the pressure range 15–27 GPa in the wavenumber ranges (a) 90–810 cm$^{-1}$ and (b) 630–840 cm$^{-1}$. The appearance and disappearance of Raman modes are indicated by upward and downward arrows, respectively. The pressure evolution of individual modes is traced by black dashed lines.
• Figure 4: (a)-(i) Pressure evolution of the Raman mode frequencies and linewidths of low-frequency modes in MNO. Dashed black lines indicate the isostructural transition pressures, while blue dashed lines mark the onset of long-range structural transitions or major structural rearrangements.
• Figure 5: High-pressure Raman spectra of MTO up to 15 GPa over the frequency range 75–775 cm$^{-1}$. The appearance/separation and disappearance of Raman modes are indicated by upward and downward arrows, respectively. Black dashed lines trace the pressure evolution of the individual modes.