A preprint reports a series of pressure-associated changes in the Raman spectrum of the hexagonal material Na2Co2TeO6. The clearest proposed transition markers appeared above 13.8 GPa, where the researchers highlighted the emergence of a new feature, the disappearance of multiple modes, and a reversal in the intensity maxima of two polarized Raman signals, P8 and P9. The authors present these observations as possible indicators of a structural phase transition with changed symmetry, but the measurements do not directly confirm that interpretation.
The reported spectrum did not follow one simple pattern as pressure rose. Initially, all observed Raman modes shifted to higher frequencies. The researchers divided the evolution qualitatively into low-, medium-, and high-pressure regions, while noting that the boundaries were blurred and differed between crystals.
What was measured under pressure
The work compared two independently synthesized crystal types, both in the hexagonal phase. High-pressure measurements were performed on Crystal A and Crystal B in a Boehler–Almax diamond anvil cell. The diamond culet was 400 µm across. Raman spectra were collected while pressure was applied up to 16.3 GPa, with additional Crystal B data noted at pressures up to 20 GPa. Pressure was calibrated using ruby fluorescence, and the Raman measurements used a commercial Renishaw inVia microscope.
At ambient pressure, the researchers fitted nine distinct Lorentzian peak functions across the 50 to 1000 cm−1 range. Other modes were not resolved at room temperature. Each mode’s pressure dependence was fitted with a linear relation, allowing the team to track how the fitted peaks moved as the load changed.
The polarized Raman series used five pressure values: 2.5, 6, 9.5, 12.5, and 16.3 GPa.
New peaks and split features in the middle range
In the medium-pressure region, P1 became indistinguishable from the background, while the pressure coefficients of P2 and P3 changed. A1 appeared at around 9 GPa and softened through 11.5 GPa. Features in the 650 to 780 cm−1 range split into three to five peaks. P10 and A3 appeared at 2.5 and 6 GPa, respectively, and A2 became prominent.
Around 12.5 GPa, the researchers marked the beginning of the high-pressure region. L1 and L2 appeared near 170 and 550 cm−1, respectively. P2 and P3 became indistinguishable, the pressure coefficients of P4 and P5 changed, and P7 disappeared. The group of features between 650 and 780 cm−1 resolved into four peaks.
The direction of the signal also changed
Polarization measurements added a separate sign of pressure-associated change. At 2.5 and 6 GPa, P8 and P9 showed well-defined two-fold patterns, with intensity maxima near 120° and 300° and minima near 20° and 200°. Above 12.5 GPa, the lobes of those patterns broadened and the maxima rotated.
The high-pressure behavior became most prominent in the authors’ conclusion above 13.8 GPa. They pointed to the emergence of L1, the disappearance of multiple modes, and the reversal of the P8 and P9 intensity maxima as possible signs of a structural phase transition with changed symmetry. These remain spectroscopic indicators, rather than direct measurements of the crystal structure.
A signal of change, not a structural verdict
Crystal B showed the same reported high-pressure trend as Crystal A, with data extending to 20 GPa. That comparison supports qualitative consistency across the two tested crystals. The reported region boundaries were blurred and crystal-dependent, so the exact pressure of a change remains less certain.
The authors suggest that changes involving P6, A1, and the polarization behavior could reflect an analogous change in the CoO6 octahedral environment, the arrangement of oxygen around cobalt. They also acknowledge that Raman data alone cannot establish this structural explanation. Direct high-pressure single-crystal X-ray diffraction is identified as the way to test it.
There are also limits to reading precise intensities from the fitted spectra. Low-intensity neighboring peaks overlapped, which could affect the intensities extracted from the fits. That matters for interpreting changes in the P8 and P9 polarization patterns. The reported evidence therefore points toward pressure-associated structural changes, but it does not determine what the high-pressure phases look like.
The document is arXiv:2608.25076v1, dated 25 Aug 2026. Its central result is a pressure-linked change in Raman signatures across the tested crystals, with the strongest proposed transition markers above 13.8 GPa. Whether those markers correspond to changed structure or symmetry remains an open question for direct structural measurements.
Paper data and sources
Original title: High-pressure phase transitions in the quantum spin liquid candidate Na2Co2TeO6 probed by Raman spectroscopy
Authors: Ihsan Ahmed Kolasseri, Maria Mei Ravnebæk, Subhadip Das et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-25
DOI: Not available
Original paper · Full text