This study investigates the low-temperature vibrational and magnetic properties, along with the high-pressure structural and vibrational behavior, of the honeycomb magnetoelectric ${\mathrm{Co}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$. Raman spectroscopy and magnetic susceptibility measurements reveal pronounced spin-phonon coupling up to $\sim 250$ K (${\mathrm{T}}_{\mathrm{sro}}$), accompanied by local symmetry breaking linked to magnetic short-range order. High-pressure Raman spectroscopy and synchrotron radiation x-ray diffraction identify three isostructural phase transitions at $\sim 3, \sim 5.5$, and $\sim 7.7$ GPa, followed by two long-range crystal symmetry transitions at $\sim 13$ and $\sim 18.4$ GPa. The first symmetry transition distorts the P-3c1 phase partially into P2/c symmetry, while the second transition introduces an additional monoclinic C2/c phase. These structural transitions align well with our density functional theory based calculations. Partial density of states and Bader charge analyses further reveal charge redistribution between Co and O atoms as a key factor driving the onset of long-range structural transition under high pressure. The stronger renormalization of Raman modes at low temperatures, combined with the lower-pressure structural phase transitions and distinct symmetry sequence compared to the related compound ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$, reflects the enhanced spin-orbit coupling introduced by Ta substitution. Notably, anomalies in Raman mode linewidths and the appearance of new modes $[\omega$(1) and $\omega$(2)] in the 3–7.7 GPa pressure range resemble the low-temperature magnetic behavior, indicating persistence of potential short-range interactions at high pressures. These findings highlight the impact of heavier $5d$-Ta compared to $4d$-Nb on magnetolattice coupling and pressure-induced phenomena in $A_4{B}_2{\mathrm{O}}_9$ ($A$ = Fe, Co, Mn and $B$ = Nb, Ta) systems, offering valuable insights for future research on these materials.

Close

PrevNext

View figure in article

• Crystal structure
• Magnetic phase transitions
• Phase transitions
• Pressure effects
• Spin-phonon coupling
• Multiferroics
• Strongly correlated systems
• Density functional theory
• Raman spectroscopy
• Susceptibility measurements
• X-ray diffraction