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Pressure-induced lattice instabilities and phonon softening in the orthorhombically distorted ferrimagnet Ni4Nb2O9

Rajesh Jana, Xinyu Wang, Takeshi Nakagawa, Hirofumi Ishii, Alka Garg, Rekha Rao, Thomas Meier

Abstract

The ambient- and high-pressure behavior of the ferrimagnet Ni4Nb2O9 (orthorhombically distorted honeycomb structure), is investigated using NMR, Raman spectroscopy, and synchrotron XRD. Ambient-pressure NMR measurements reveal, despite its orthorhombic symmetry, the local environment of Ni4Nb2O9 closely resembles that of its trigonal analogue Mn4Nb2O9. In contrast, substantially different paramagnetic shifts observed in the two compounds reflect their distinct average crystal symmetries, governing orbital overlap and magnetic exchange pathways. Under external pressure, Ni4Nb2O9 exhibits pronounced sensitivity to lattice distortions and phonon instabilities. Three isostructural transitions are identified near 2, 6, and 10 GPa, manifested by mode splitting, frequency shifts, line broadenings, intensity anomalies, and slope changes in the evolution of lattice parameters. At higher pressure, around 13 GPa, signatures of an incipient long-range structural transition from orthorhombic Pbcn to monoclinic P2/c symmetry emerge, signaling the onset of a symmetry-lowering transformation. The anomalous softening of the 192 cm^-1 Raman mode, accompanied by multiple linewidth and spectral-weight anomalies, serve as a key fingerprint of these structural instabilities, linking local symmetry breaking at low pressures to the long-range transition into the P2/c phase. Notably, pronounced linewidth anomalies, strongly anisotropic pressure coefficients, together with a marked enhancement of the intensity of the low-frequency branch over the 2-13 GPa range, point toward a pressure-induced regime influenced by coupled spin, orbital, and lattice degrees of freedom. The close correspondence of transition pressures in Ni4Nb2O9 and those reported for Mn4Nb2O9 highlights a common mechanism rooted in their similar local structural environments, as revealed by NMR.

Pressure-induced lattice instabilities and phonon softening in the orthorhombically distorted ferrimagnet Ni4Nb2O9

Abstract

The ambient- and high-pressure behavior of the ferrimagnet Ni4Nb2O9 (orthorhombically distorted honeycomb structure), is investigated using NMR, Raman spectroscopy, and synchrotron XRD. Ambient-pressure NMR measurements reveal, despite its orthorhombic symmetry, the local environment of Ni4Nb2O9 closely resembles that of its trigonal analogue Mn4Nb2O9. In contrast, substantially different paramagnetic shifts observed in the two compounds reflect their distinct average crystal symmetries, governing orbital overlap and magnetic exchange pathways. Under external pressure, Ni4Nb2O9 exhibits pronounced sensitivity to lattice distortions and phonon instabilities. Three isostructural transitions are identified near 2, 6, and 10 GPa, manifested by mode splitting, frequency shifts, line broadenings, intensity anomalies, and slope changes in the evolution of lattice parameters. At higher pressure, around 13 GPa, signatures of an incipient long-range structural transition from orthorhombic Pbcn to monoclinic P2/c symmetry emerge, signaling the onset of a symmetry-lowering transformation. The anomalous softening of the 192 cm^-1 Raman mode, accompanied by multiple linewidth and spectral-weight anomalies, serve as a key fingerprint of these structural instabilities, linking local symmetry breaking at low pressures to the long-range transition into the P2/c phase. Notably, pronounced linewidth anomalies, strongly anisotropic pressure coefficients, together with a marked enhancement of the intensity of the low-frequency branch over the 2-13 GPa range, point toward a pressure-induced regime influenced by coupled spin, orbital, and lattice degrees of freedom. The close correspondence of transition pressures in Ni4Nb2O9 and those reported for Mn4Nb2O9 highlights a common mechanism rooted in their similar local structural environments, as revealed by NMR.
Paper Structure (15 sections, 2 equations, 15 figures, 3 tables)

This paper contains 15 sections, 2 equations, 15 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Experimental and computational details
  3. Sample synthesis and phase characterization
  4. Nuclear magnetic resonance specroscopy
  5. High pressure Raman spectroscopy
  6. High-pressure synchrotron x-ray diffraction
  7. Results and Discussion
  8. Ambient phase characterization
  9. Nuclear magnetic resonance study
  10. High pressure Raman studies on NNO
  11. High pressure synchrotron XRD study
  12. Discussion
  13. Conclusion
  14. Acknowledgments
  15. Supplemental material

Figures (15)

  • Figure 1: (Colour online) (a) Illustration of the unit cell of NNO in orthorhombic Pbcn symmetry at ambient conditions.(b) Honeycomb arrangments of Ni1O$_6$ octahedra in the nearly planar layer viwed through c-axis. (c) Buckled layer made of Ni2O$_6$ ribbons running along the b axis and NbO$_6$ octahedra.
  • Figure 2: (Colour online) (a) $^{93}$Nb NMR spectrum of NNO at ambient conditions. (b) Spin-lattice relaxation constant (T1), measured using saturation recovery method. (c) Spin-spin relaxation constant (T2). Black cicles represent the experimental data, while red curves are the fit using the Eqs. 1 and 2 for saturation recovery and magnetization decay model, respectively.
  • Figure 3: (Colour online) High pressure Raamn spectra of NNO up to 12.6 GPa. Splitting/appearance of Raman modes is highlighted by upward solid arrows, while disappearance of modes is indicated by downward solid arrows. Dashed lines follow the pressure evolution of these modes.
  • Figure 4: (Colour online) Higher pressure Raman spectra of NNO over the pressure range of 11.1-20.5 GPa. Upward and downard arrows marked the appearance and disappearance of modes, respectively, while their chnages with pressure is highlighted by the black dahsed lines.
  • Figure 5: Raman spectra of NNO in the pressure range of 20.5-38.3 GPa for the wavenuber range (a) 60-650 cm-1 and (b) 650-900 cm-1. The appearance/disappearance of Raman modes indicate by the upward/downward arrows. Pressure evolution of each Raman modes is followed by the black dahsed lines. The blue dashed rectangle highlight the wavenumber range where Raman modes heavily renormalized and trnsformed into new modes in the pressre ramge 20.5-23 GPa.
  • ...and 10 more figures