High-pressure structural phase transition in ${\mathrm{MnWO}}_4$

The pressure-induced phase transition of the multiferroic manganese tungstate ${\mathrm{MnWO}}_4$ is studied on single crystals using synchrotron x-ray diffraction and Raman spectroscopy. We observe the monoclinic $P2/c$ to triclinic $P\overline{1}$ phase transition at 20.1 GPa and get insight on the phase transition mechanism from the appearance of tilted triclinic domains. Selective Raman spectroscopy experiments with single crystals have shown that the onset of the phase transition occurs 5 GPa below the previously reported pressure obtained from experiments performed with powder samples.

Figure 1

Projection of the structure of the ${\mathrm{MnWO}}_4$ wolframite along [010]. It shows the alternating chains of ${\mathrm{WO}}_6$ and ${\mathrm{MnO}}_6$ octahedra. The black (white) spheres represent the W(Mn) atoms.

Figure 2

${\mathrm{MnWO}}_4$ pressurized crystal used in the SXRD experiment at different pressures from ambient pressure to 31.0 GPa.

Figure 3

Selection of five diffraction patterns of ${\mathrm{MnWO}}_4$ measured in run2 at different pressures. Vertical bars at 3.4 GPa denote the positions of the Bragg reflections for the $P2/c$ low-pressure phase. The asterisks denote the (111) and (200) reflections of solid Ne. The insets show enlarged areas of the diffraction patterns at 20.2 GPa to illustrate the appearance of weak reflections attributed to the high-pressure phase.

Figure 4

(a) The PXRD diffractogram of ${\mathrm{MnWO}}_4$ at 20.2 GPa (circles) is shown together with a LeBail refinement considering both the LP monoclinic and the HP triclinic phases. The black ticks correspond to the positions of the reflections of the monoclinic phase and the red ones to those of the triclinic phase. The two triclinic reflections observed at this pressure are pointed out by arrows. The asterisks indicate the positions of the Ne reflections. The inset shows the triclinic ${\left(010\right)}_{\mathrm{T}}$ reflection. (b) Pressure evolution of the SXRD measured (100)${}_{\mathrm{M}}$ and (200)${}_{\mathrm{M}}$ monoclinic Bragg reflections showing the appearance of the (100)${}_{\mathrm{T}}$ and (200)${}_{\mathrm{T}}$ reflections of the HP triclinic phase up to the complete phase transformation.

Figure 5

Section of a SXRD frame of ${\mathrm{MnWO}}_4$ measured at different pressures (left) showing the emergence of the reflections of two HP triclinic domains (T1, T2) along the monoclinic (M) one. The corresponding projection of the reciprocal space on the $\left(b^{*},c^{*}\right)$ plane (right) is shown at the three selected pressures. The dots represent the location of the measured reflections projected along the $a^{*}$ axis. The axes of the unit cells are shown as dashed lines. The monoclinic reflections are in black while the triclinic reflections of the two domains are in blue (T1) and red (T2).

Figure 6

Pressure dependence of the (a) unit-cell lattice parameters (b) monoclinic $\beta$ angle and (c) volume of ${\mathrm{MnWO}}_4$ in the stability range of the low pressure monoclinic phase. Empty circles represent Macavei and Schulz [13] data while full circles and triangles represent our PXRD data obtained from runs 1 and 2, respectively. The crosses are from our SXRD experiment. The continuous red lines represent the fits to the third-order Birch-Murnaghan equation of state using all available data. The dashed black line represents the equation of state obtained by Macavei and Schulz [13].

Figure 7

Raman spectra of ${\mathrm{MnWO}}_4$ recorded at selected pressures. The ticks indicate the 8 $A_g$ modes assignation accessed in (010) oriented wolframites in backscattering configuration. At 20.5 GPa, the emergence of an additional Raman active mode at 866 ${\mathrm{cm}}^{-1}$ indicates the onset of the phase transition to the triclinic phase. The region of interest is highlighted with the red box.