Multiferroic MnWO₄ wolframite under compression

The PXRD experiments were performed with a monochromatic X-ray beam (λ = 0.4246 Å). A pellet of micron-size powder obtained from a ground single crystal¹ was placed into the sample chamber of a diamond-anvil cell (DAC). To guarantee hydrostatic conditions neon (Ne) was employed as pressuretransmitting medium. Pressure was measured using the ruby-fluorescence technique. The PXRD patterns were recorded with a MAR345 image plate.

Figure 1: Le Bail refinement (lines) of X-ray Diffraction patterns. All the reflections at both pressures are indexed to the P2/c low-pressure monoclinic phase of MnWO₄. Ticks indicate the positions of Bragg reflections.

Le Bail refinements of two diffractograms measured at 5.4 GPa and 19.9 GPa (Fig. 1) show that the sample does not undergo any phase transition at these pressures. Up to 19.9 GPa all the Bragg reflections are indexed to the low-pressure monoclinic phase. Previous Raman Spectroscopy experiments performed in less-hydrostatic conditions² had proposed the occurrence of a structural phase transition in MnWO₄ at 17 GPa. However, other data obtained³ in hydrostatic conditions had extended the structural stability of the monoclinic low-pressure phase of MnWO₄ to 25 GPa. In this context, the present PXRD results confirm the influence that non-hydrostaticity has in the behaviour of MnWO₄ under pressure, remarking the importance of performing high pressure experiments in MnWO₄ within the hydrostatic limits. Previous single-crystal XRD experiments⁴ performed within the hydrostatic limit of the methanol-ethanol mixture (up to 9 GPa) had shown that the pressure dependence of the unit cell volume of MnWO₄ follows a second-order Birch- Murnaghan equation of estate (EOS) with a bulk modulus of B₀ = 131(2) GPa (B₀’ = 4 and V₀ = 138.8(1) ų). However, the present experiment has indicated (Fig. 2a) that a third-order EOS is needed to properly describe the pressure dependence of the unit-cell volume. We found fitting both data sets together that the first pressure derivative of the bulk modulus B₀’ deviates from 4 by 7% (B₀’ = 4.3(2) and V₀ = 138.8(2) ų) resulting in a bulk modulus of B₀ = 145(3) GPa.

Figure 2: a) Pressure-dependence of the unit-cell volume of the low-pressure monoclinic phase of MnWO₄. The continuous line is the fit of a third order Birch-Murnaghan equation of state to the data; b) Pressure-dependence of the monoclinic angle b angle; c ) Pressure-dependence of the unit-cell lattice parameters. The red symbols represent data obtained by Macavei and Schultz⁴ and the black squares are from the present data.

Although no phase transition is observed up to 19.9 GPa in the PXRD experiment, it is worth noting that the monoclinic angle b increases by 0.2 degrees (Fig. 2b) from ambient pressure to 19.9 GPa. We also found that the b-axis is more compressible (Fig. 2c) than the other axes of the unit cell. Both facts indicate that pressure induces a distortion of the crystalline lattice that might lead eventually to a pressure-induced structural phase transition as Raman experiments had indicated³. Indeed, the transition into a lower symmetry structure has been observed in a subsequent single-crystal diffraction experiment at high pressure where a (010) oriented single crystal of MnWO₄ was embedded into Ne as pressure-transmitting medium to ensure hydrostatic conditions. The appearance of new reflections like those next to the (100) and (200) reflections of the low-pressure monoclinic phase (Fig. 3a,b) indicate the occurrence of a structural phase transition at 23.6 GPa, in good agreement with Raman Spectroscopy (Fig. 3d). The indexation of those extra reflections is made to a triclinic lattice. As pressure increases extra domains of the triclinic high-pressure phase emerge. At 23.6 GPa an additional triclinic domain appears. At this pressure the monoclinic and two triclinic domains are clearly visible in a projection along the α*-axis of the reciprocal space (Fig. 3c). At higher pressures the emergence of additional triclinic domains deteriorates the crystal.

In summary, from X-ray Diffraction experiments on the multiferroic MnWO₄ we obtained detailed information about the pressure-induced structural transition into a triclinic high-pressure phase at 23.6 GPa, and the onset of the phase transition at 20.1 GPa. These results rule out the possibility of stabilisation of a magnetic phase of MnWO₄ under compression at ambient temperature.

Figure 3: Single crystal diffraction image of MnWO₄ showing the (100) and (200) reflections of a) the low-pressure phase and b) the high-pressure triclinic phase (red); c) Projection along the α*-axis of the reciprocal space showing the location of the measured reflections at 23.6 GPa. The reflections corresponding to the monoclinic phase are shown in black and those corresponding to two triclinic domains of the high-pressure phase are shown in blue and red; d) Section of the Raman spectra showing the appearance of one of the new peaks of the high-pressure phase as a result of the phase transition.

References:

1. Iliev, M. N., Gospodinov, M. M. & Litvinchuk, A. P. Raman spectroscopy of MnWO₄. Physical Review B 80, doi:10.1103/PhysRevB.80.212302 (2009).
2. Dai, R. C., Ding, X., Wang, Z. P. & Zhang, Z. M. Pressure and temperature dependence of Raman scattering of MnWO₄. Chemical Physics Letters 586, 76-80, doi:10.1016/j.cplett.2013.09.035 (2013).
3. Ruiz-Fuertes, J., Errandonea, D., Gomis, O., Friedrich, A. & Manjon, F. J. Room-temperature vibrational properties of multiferroic MnWO₄ under quasi-hydrostatic compression up to 39 GPa. Journal of Applied Physics 115, doi:10.1063/1.4863236 (2014).
4. Macavei, J. & Schulz, H. The crystal structure of wolframite type tungstates under pressure. Zeitschrift für Kristallographie 207, 193-208, doi:10.1524/ zkri.1993.207.Part-2.193 (1993).

Funding acknowledgements:
We acknowledge the Spanish government MINECO under Grants No. MAT2013-46649-C4-1/2-P and MAT2015-71070- REDC, and Generalitat Valenciana under Grants No. ACOMP-2013-1012 and No. ACOMP-2014-243. JRF thanks the Alexander von Humboldt Foundation for a postdoctoral fellowship and the Spanish MINECO for the Juan de La Cierva program (IJCI-2014-20513).

Corresponding authors:
Dr Javier Ruiz-Fuertes, Dr Daniel Errandonea, Universitat de València, [email protected].