Room-temperature vibrational properties of multiferroic MnWO4 under quasi-hydrostatic compression up to 39 GPa

Abstract

The multiferroic manganese tungstate (MnWO 4 ) has been studied by high-pressure Raman spectroscopy at room temperature under quasi-hydrostatic conditions up to 39.3 GPa. The low-pressure wolframite phase undergoes a phase transition at 25.7 GPa, a pressure around 8 GPa higher than that found in previous works, which used less hydrostatic pressure-transmitting media. The pressure dependence of the Raman active modes of both the low-and high-pressure phases is reported and discussed comparing with the results available in the literature for MnWO 4 and related wolframites. A gradual pressure-induced phase transition from the low-to the high-pressure phase is suggested on the basis of the linear intensity decrease of the Raman mode with the lowest frequency up to the end of the phase transition. V C 2014 AIP Publishing LLC.

Brief Review of the Effects of Pressure on Wolframite-Type Oxides

“…The larger number of modes observed in the HP phase of CdWO4 in comparison with MnWO4 clearly indicates that the crystal structures of the two compounds are different. In spite of this fact, in the post-wolframite phase of CdWO4, there is a substantial drop of the frequency of the highest frequency mode (the most intense one), as observed in MnWO4 [20]. In the case of CdWO4, a clear correlation can be stablished between the frequency drop of this W-O stretching mode and the increase of the tungsten-oxygen coordination number [13].…”

“…However, there are small discrepancies that suggest that, despite being all the HP phase structurally related to wolframite, there are some differences in the their HP phases. These can be clearly seen when comparing the Raman spectra published for the HP phases of CdWO4 [18] and MnWO4 [20]. In CdWO4 the phase transition to the post-wolframite structure leads to a doubling of the unit-cell (hence, the formula unit increases from 2 to 4) [14], with the consequent increase in the number of Raman modes.…”

“…Table III gives the frequencies of the Raman-active modes of the HP phase of MnWO4 and its pressure coefficients. A detailed discussion of the Raman spectrum of the HP phase can be found elsewhere [20] and is beyond the scope of this article. Here, we will just mention the most relevant features of it.…”

“…6 a selection of Raman spectra measured in MnWO4 at different pressures. The experiments were carried out in a diamond-anvil cell using neon as pressure medium to guarantee quasi-hydrostatic conditions [20]. Clear changes in the Raman spectrum are detected at 25.7 GPa.…”

Pressure effects on magnetic ground states in cobalt-doped multiferroicMn1−xCoxWO4

“…Besides, Co 2+ also introduces a different chemical pressure due to its smaller ionic size, which is known as an important tuning parameter in magnetically frustrated systems. Earlier pressure measurements on pure MnWO 4 have revealed the evolution of the crystal structure and a phase transition to triclinic symmetry around 18-25 GPa [42][43][44][45][46]. But the pressure effect on magnetic structures remains unclear.…”