5:15 PM - 6:30 PM
[SMP26-P01] Pressure-induced phase transitions in wülfingite (Zn(OH)2): Raman spectroscopic and First-principles study
Keywords:wulfingite, Zn(OH)2, pressure-induced phase transition, Raman spectroscopy, First-principles calculation
Zn(OH)2 has several polymorphs at ambient pressure, and one of which, epsilon phase, is known as the mineral wülfingite. Wülfingite has a cristobalite-like structure, and pressure-induced phase transitions were found at 1.1 and 2.1 GPa using in-situ X-ray diffraction study by Kusaba et al.1 Since the coordination number of Zn is 4 in most of those polymorphs known so far, which is very different from that of brucite (Mg(OH)2), the hydrogen bonding behavior is expected to be different. In this study, we investigated structural changes and the associated changes in hydrogen bonding using in-situ Raman spectroscopy and first-principles calculations.
Wülfingite was synthesized as follows. Zinc sulfate 7-hydrate was dissolved in water and sodium hydroxide solution was added to it to form a white precipitate. This precipitate was ZnO. When the filtered solution of the precipitate was left at room temperature for several weeks, large, transparent crystals grew, but small white crystals adhered to the surface. The clear crystals were identified as wülfingite and the white crystals as ZnO from powder X-ray diffraction and Raman spectroscopy. Clear crystal without ZnO was selected for DAC study. The pressure medium was a methanol-ethanol mixture, and ruby fluorescence method was used for pressure measurements.
In the high-pressure in situ Raman measurements, several new peaks appeared at about 1 GPa in addition to the previous Raman peak of wülfingite. According to Kusaba et al.2, the transition at 1 GPa is reported to be a single-crystal-to-single-crystal transition from orthorhombic wülfingite to a tetragonal structure. This single-crystal-to-single-crystal transition was also confirmed by observation using a Raman microscopy. Wülfingite shows OH stretching peaks at 3190 and 3300 cm-1 at ambient pressure, but after the transition, the peaks shift to around 3410 cm-1 (with shoulders at 3350 and 3500) and the hydrogen bonding becomes weaker. As the pressure was further increased, the sharp Raman peaks disappeared at around 2 GPa and were replaced by broad peaks, which is consistent with previous X-ray studies showing a change to a broad X-ray pattern at 2.1 GPa. The OH stretching Raman peaks are observed at 3600 and 3650 cm-1. The position of the 3650 cm-1 peak at is close to that of Mg(OH)2, suggesting that the coordination number of Zn may have changed to 6. The Raman spectrum does not show a pattern characteristic of amorphous, and the OH peak is not particularly broad, suggesting poor crystallinity of the high-pressure phase rather than amorphization.
First-principles calculations were performed using Quantum-Espresso to optimize the known structures under pressure and to see the pressure changes in volume and enthalpy. Wülfingite, the tetragonal phase found at 1~2 GPa, and the brucite phase were performed. The phase found above 2 GPa could not be calculated due to its unknown structure. The enthalpies at 0 K predict a transition from wülfingite to the tetragonal phase at 4.5 GPa and from the tetragonal phase to the brucite phase at 9 GPa. The brucite phase has been reported to be synthesized at 11~14 GPa, but requires a certain temperature (~400 oC). In the compression of wülfingite, the transition to the tetragonal phase could not be reproduced, but a transition to a different structure was observed. In this structure, the coordination number of Zn was 5, which increased to 6 at further higher pressures. During decompression of this structure, another 4-coordinated structure was obtained. However, these structures could not explain the X-ray diffraction patterns observed above 2 GPa, and their enthalpies did not seem to indicate that they could be stable phases. The hydrogen bond distances (O-H...O) of the tetragonal phase is longer than that of wülfingite, which is in agreement with the Raman results.
Both Raman and first-principles calculations show that the hydrogen bonding tends to weaken with pressure. The relatively stronger hydrogen bonding in the 4-coordinate compared to the 6-coordinate cannot be explained by bond strength or bond valence, but is due to the stronger covalent nature of Zn-O in the 4-coordinate. Since Raman spectra of the phases appearing above 2 GPa are very broad, we are planning to obtain good crystallinity data by using an external DAC with higher temperature.
1) Kusaba, K. et al., https://doi.org/10.1088/1742-6596/215/1/012001
2) Kusaba, K. et al., https://doi.org/10.1016/j.cplett.2007.02.009
Wülfingite was synthesized as follows. Zinc sulfate 7-hydrate was dissolved in water and sodium hydroxide solution was added to it to form a white precipitate. This precipitate was ZnO. When the filtered solution of the precipitate was left at room temperature for several weeks, large, transparent crystals grew, but small white crystals adhered to the surface. The clear crystals were identified as wülfingite and the white crystals as ZnO from powder X-ray diffraction and Raman spectroscopy. Clear crystal without ZnO was selected for DAC study. The pressure medium was a methanol-ethanol mixture, and ruby fluorescence method was used for pressure measurements.
In the high-pressure in situ Raman measurements, several new peaks appeared at about 1 GPa in addition to the previous Raman peak of wülfingite. According to Kusaba et al.2, the transition at 1 GPa is reported to be a single-crystal-to-single-crystal transition from orthorhombic wülfingite to a tetragonal structure. This single-crystal-to-single-crystal transition was also confirmed by observation using a Raman microscopy. Wülfingite shows OH stretching peaks at 3190 and 3300 cm-1 at ambient pressure, but after the transition, the peaks shift to around 3410 cm-1 (with shoulders at 3350 and 3500) and the hydrogen bonding becomes weaker. As the pressure was further increased, the sharp Raman peaks disappeared at around 2 GPa and were replaced by broad peaks, which is consistent with previous X-ray studies showing a change to a broad X-ray pattern at 2.1 GPa. The OH stretching Raman peaks are observed at 3600 and 3650 cm-1. The position of the 3650 cm-1 peak at is close to that of Mg(OH)2, suggesting that the coordination number of Zn may have changed to 6. The Raman spectrum does not show a pattern characteristic of amorphous, and the OH peak is not particularly broad, suggesting poor crystallinity of the high-pressure phase rather than amorphization.
First-principles calculations were performed using Quantum-Espresso to optimize the known structures under pressure and to see the pressure changes in volume and enthalpy. Wülfingite, the tetragonal phase found at 1~2 GPa, and the brucite phase were performed. The phase found above 2 GPa could not be calculated due to its unknown structure. The enthalpies at 0 K predict a transition from wülfingite to the tetragonal phase at 4.5 GPa and from the tetragonal phase to the brucite phase at 9 GPa. The brucite phase has been reported to be synthesized at 11~14 GPa, but requires a certain temperature (~400 oC). In the compression of wülfingite, the transition to the tetragonal phase could not be reproduced, but a transition to a different structure was observed. In this structure, the coordination number of Zn was 5, which increased to 6 at further higher pressures. During decompression of this structure, another 4-coordinated structure was obtained. However, these structures could not explain the X-ray diffraction patterns observed above 2 GPa, and their enthalpies did not seem to indicate that they could be stable phases. The hydrogen bond distances (O-H...O) of the tetragonal phase is longer than that of wülfingite, which is in agreement with the Raman results.
Both Raman and first-principles calculations show that the hydrogen bonding tends to weaken with pressure. The relatively stronger hydrogen bonding in the 4-coordinate compared to the 6-coordinate cannot be explained by bond strength or bond valence, but is due to the stronger covalent nature of Zn-O in the 4-coordinate. Since Raman spectra of the phases appearing above 2 GPa are very broad, we are planning to obtain good crystallinity data by using an external DAC with higher temperature.
1) Kusaba, K. et al., https://doi.org/10.1088/1742-6596/215/1/012001
2) Kusaba, K. et al., https://doi.org/10.1016/j.cplett.2007.02.009