High-pressure Raman spectroscopy and lattice-dynamics calculations on scintillating MgWO4: A comparison with isomorphic compounds

Raman scattering measurements and lattice-dynamics calculations have been performed on magnesium tungstate under high pressure up to 41 GPa. Experiments have been carried out under a selection of different pressure-media. The influence of non-hydrostaticity on the structural properties of MgWO4 and isomorphic compounds is examined. Under quasi-hydrostatic conditions a phase transition has been found at 26 GPa in MgWO4. The high-pressure phase has been tentatively assigned to a triclinic structure similar to that of CuWO4. We also report and discuss the Raman symmetries, frequencies, and pressure coefficients in the low- and high-pressure phases. In addition, the Raman frequencies for different wolframites are compared and the variation of the mode frequency with the reduced mass across the family is investigated. Finally, the accuracy of theoretical calculations is systematically discussed for MgWO4, MnWO4, FeWO4, CoWO4, NiWO4, ZnWO4, and CdWO4.

💡 Research Summary

This work presents a comprehensive high‑pressure investigation of magnesium tungstate (MgWO₄), a member of the wolframite family of divalent‑metal tungstates, combining Raman spectroscopy up to 41 GPa with first‑principles lattice‑dynamics calculations. Five Raman experiments were carried out using four different pressure‑transmitting media (neon, a 4:1 methanol‑ethanol mixture, spectroscopic paraffin, and no medium) in diamond‑anvil cells, allowing the authors to assess the influence of non‑hydrostatic stresses on the vibrational response. Under quasi‑hydrostatic conditions (Ne as PTM) the Raman spectra evolve linearly with pressure up to ~10 GPa, after which all 18 Raman‑active modes (10 A_g + 8 B_g) continue to shift linearly. When the medium is non‑hydrostatic (paraffin or no PTM) the pressure coefficients become noticeably larger above 10 GPa, indicating that uniaxial stress stiffens the lattice.

In the low‑pressure monoclinic phase (space group P2₁/c) the high‑frequency region (≈400–920 cm⁻¹) contains six internal stretching modes of the WO₆ octahedra; these are largely insensitive to the mass of the A‑site cation and are reproduced by the calculations within 10 cm⁻¹. The low‑frequency region (≈100–350 cm⁻¹) comprises external modes involving motions of the WO₆ units against the A‑cation. These external modes display a clear dependence on the reduced mass μ = (1/m_A + 1/m_WO₆)⁻¹, with frequencies roughly proportional to 1/√μ for non‑magnetic wolframites (Mg, Zn, Cd).

A distinct structural transition is observed at 26 GPa under quasi‑hydrostatic conditions: a new Raman band appears just below the strongest high‑frequency mode, while several low‑frequency modes lose intensity. The high‑pressure phase is tentatively assigned to a triclinic structure (space group P‑1) analogous to CuWO₄, which is energetically competitive with the previously proposed β‑fergusonite (C2/c) phase. This assignment is supported by the similarity of the high‑pressure Raman pattern to that of CuWO₄ measured at comparable pressures.

First‑principles calculations were performed with VASP using the GGA‑PBE functional and a plane‑wave cutoff of 520 eV. For the transition‑metal wolframites (Mn, Fe, Co, Ni) a GGA+U approach (U_eff = 3.9–7 eV) was employed to treat the correlated d‑electrons; antiferromagnetic ordering was found to be the ground state. The calculated phonon frequencies, pressure coefficients, and Grüneisen parameters agree well with experiment for all compounds, except for the B_g mode near 405 cm⁻¹ in MgWO₄ where the experimental pressure coefficient is five times smaller than the theoretical value—an anomaly possibly linked to non‑hydrostatic effects. The bulk modulus obtained from the equation of state (B₀ ≈ 161 GPa) matches the experimental value (160 GPa), providing a consistent basis for deriving mode Grüneisen parameters.

Extending the analysis to the full wolframite series, the authors compiled experimental and calculated Raman data for MgWO₄, MnWO₄, FeWO₄, CoWO₄, NiWO₄, ZnWO₄, and CdWO₄. They find that internal high‑frequency modes are almost invariant across the series, whereas external low‑frequency modes split into two systematic trends. Non‑magnetic compounds follow the expected 1/√μ scaling, while magnetic compounds (Mn, Fe, Co, Ni) show an opposite trend: their external mode frequencies increase with the mass of the A‑cation. The authors attribute this deviation to magnetic exchange interactions and second‑order Jahn‑Teller distortions, which can even drive a triclinic distortion as observed in CuWO₄.

In summary, the paper delivers a detailed picture of how pressure, hydrostaticity, cation mass, and magnetic effects govern the lattice dynamics of MgWO₄ and related wolframites. The combined experimental‑theoretical approach validates the assignment of a triclinic high‑pressure phase for MgWO₄ and establishes reliable Raman benchmarks for the whole family, which are valuable for the design of scintillators, laser‑host crystals, and other optical devices operating under extreme conditions.