Ruiz-Fuertes J, Friedrich A, Gomis O, Errandonea D, Morgenroth W, Sans JA, Santamaria-Perez D. High-pressure structural phase transition in MnWO4. Physical Review B 91, 104109, doi:10.1103/PhysRevB.91.104109 (2015).
Multiferroic; High-pressure; X-ray diffraction; Raman spectroscopy; Phase transition.
Multiferroic substances have great potential for the development of faster and more efficient data storage devices but this potential will only be realised if their Néel temperatures can be increased to ambient temperature. Above the Néel temperature, materials change from being antiferromagnetic (magnetic moments aligned in a regular alternating pattern) to paramagnetic (moments aligned with an applied magnetic field). Manganese tungstate, MnWO4, is a member of the wolframite mineral group and has a multiferroic nature, i.e. it exhibits multiple antiferromagnetic states below its Néel temperature of 13.7 K and one of them is also ferroelectric. Previous studies showed that the Néel temperature for MnWO4 increases with applied pressure, in the absence of a phase transition. However, a structural phase transition, at which the MnWO4 crystal structure switches from monoclinic to triclinic, has also been discovered. This transition has been detected at 25 GPa and at 17 GPa under hydrostatic and non-hydrostatic conditions, respectively. The exact onset of this phase transition is therefore disputed and needs to be determined precisely.
Powder X-ray Diffraction (PXRD) experiments on MnWO4 samples were conducted under hydrostatic conditions at Diamond Light Source’s Extreme Conditions beamline (I15). These experiments showed that in these conditions MnWO4 remains in its low-pressure phase up to 19.9 GPa at least. These results stimulated subsequent X-ray Diffraction and Raman Spectroscopy experiments on single-crystal samples which clearly indicated that the onset of the phase transition occurs at 20.1 GPa, therefore ruling out the presence of a stable magnetic phase of MnWO4 at ambient temperature.
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) Å3). 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) Å3) 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.
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).
Dr Javier Ruiz-Fuertes, Dr Daniel Errandonea, Universitat de València, firstname.lastname@example.org.
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