In Situ High-Pressure Raman Spectroscopic, Single-Crystal X-ray Diffraction, and FTIR Investigations of Rutile and TiO 2 II

: In ultrahigh-pressure (UHP) metamorphic rocks, rutile is an important accessory mineral. Its high-pressure polymorph TiO 2 II can be a signiﬁcant indicator of pressure in the diamond stability ﬁeld. In the present study, in situ high-pressure Raman spectroscopic measurements of natural rutile in UHP eclogite from the main hole of the Chinese Continental Scientiﬁc Drilling Project (CCSD) have been conducted up to ~16 GPa. Rutile and recovered TiO 2 II have also been analyzed via single-crystal X-ray diffraction and FTIR spectroscopy. The results indicate that (1) the phase transition from rutile to baddeleyite-type TiO 2 terminates at about 16 GPa under compression at ambient temperature; (2) the metastable TiO 2 II in the exhumated UHP rocks formed during deep continental subduction can be characterized by a highly distorted octahedral site in the crystal structure. X-ray powder diffraction analyses (with Cu K α radiation) at ambient conditions are sufﬁcient for identifying the lamellae of TiO 2 II within natural rutile based on the angles (2 θ ) of two strong peaks at 25.5 ◦ and 31.5 ◦ ; (3) rutile and recovered TiO 2 II in the continental slabs can contain certain amounts of water during deep subduction and exhumation. The estimated water contents of rutile in the present study range from 1590 to 1780 ppm of H 2 O by weight. In the crystal structure of TiO 2 II, hydrogen can be incorporated close to the long O-O edges (>2.5143 Å) of the TiO 6 octahedra. Further studies on the pressure–temperature stability of hydroxyls in rutile and TiO 2 II may help to understand the transportation and release of water in subducted continental slabs.


Introduction
Rutile (ideal formula: TiO 2 ) is an important accessory mineral in ultrahigh-pressure metamorphic rocks formed during deep continental subduction [1,2].According to previous investigations of water in nominally anhydrous minerals (NAMs), rutile can hold up to 0.28 wt% of H 2 O in the form of structural OH [3,4].Therefore, it is also expected to be a potential host phase of water in subducting continental slabs.
Rutile has a tetragonal structure with space group of P4 2 /mnm.It is made up of only one octahedral site that is commonly occupied by Ti 4+ (Figure 1).Previous in situ X-ray diffraction experiments at ambient temperature at a pressure of up to 60 GPa [5,6] showed that rutile begins to transform to its high-pressure polymorph with the monoclinic baddeleyite (ZrO 2 ) structure (space group P2 1 /c) at about 12 GPa under compression.Upon the release of pressure, baddeleyite-type TiO 2 converts at 7 GPa into TiO 2 II (space group Pbcn), which is another high-pressure polymorph of rutile with the orthorhombic α-PbO 2 structure (Figure 1) and is metastable at a normal pressure.The baddeleyitetype TiO 2 is 11.3(9)% denser than rutile and TiO 2 II is 2.1(3)% denser than rutile.The Minerals 2023, 13, 703 2 of 11 estimated zero-pressure bulk moduli are 230 (20), 260(30), and 290 (20) for rutile, TiO 2 II and baddeleyite-type TiO 2 , respectively [6].
In situ high-pressure spectroscopic studies [7][8][9] indicated that the phase transition from rutile to baddeleyite-type TiO2 begins at 13 GPa and terminates at 21 to 23 GPa at room temperature.Upon decompression, the baddeleyite-type TiO2 to TiO2Ⅱ transition occurs around 7 GPa and terminates at about 5 GPa.In addition, previous investigations of high-pressure and high-temperature phase equilibria of TiO2 showed that the phase boundary between rutile and TiO2Ⅱ changes from having a negative slope to having a positive slope with increasing temperature at about 6 GPa and 850 °C [10].Therefore, in natural ultrahigh-pressure (UHP) metamorphic rocks, widely observed nano-crystal lamellae of TiO2Ⅱ within (or coexisting with) rutile can be a significant indicator of pressure in the diamond stability field [11][12][13].To better understand the high-pressure behavior and crystal structure of rutile and its high-pressure polymorphs in continental slabs during ultra-deep subduction and exhumation, in situ high-pressure Raman spectroscopic measurements of rutile in UHP eclogite collected from continental subduction environment have been conducted up to ~16 GPa in the present work.Natural rutile and recovered TiO2Ⅱ have also been analyzed via single-crystal X-ray diffraction and FTIR spectroscopy.

Sample Description and Experimental Methods
Natural rutile (Ti0.988Fe0.011O2) was collected from an UHP eclogite (no.B132R114P1a) in the main hole of the Chinese Continental Scientific Drilling Project (CCSD) in a supracrustal rock slab, which was subducted to a depth of over 100 km and then exhumed to the surface during continental collisional orogeny [14,15].The eclogite is composed of fresh medium-grained garnet, omphacite, phengite, and minor rutile (Figure 2) and has experienced peak metamorphic P-T conditions greater than 3 GPa and 700 °C [15].In situ high-pressure spectroscopic studies [7][8][9] indicated that the phase transition from rutile to baddeleyite-type TiO 2 begins at 13 GPa and terminates at 21 to 23 GPa at room temperature.Upon decompression, the baddeleyite-type TiO 2 to TiO 2 II transition occurs around 7 GPa and terminates at about 5 GPa.In addition, previous investigations of high-pressure and high-temperature phase equilibria of TiO 2 showed that the phase boundary between rutile and TiO 2 II changes from having a negative slope to having a positive slope with increasing temperature at about 6 GPa and 850 • C [10].Therefore, in natural ultrahigh-pressure (UHP) metamorphic rocks, widely observed nano-crystal lamellae of TiO 2 II within (or coexisting with) rutile can be a significant indicator of pressure in the diamond stability field [11][12][13].
To better understand the high-pressure behavior and crystal structure of rutile and its high-pressure polymorphs in continental slabs during ultra-deep subduction and exhumation, in situ high-pressure Raman spectroscopic measurements of rutile in UHP eclogite collected from continental subduction environment have been conducted up to ~16 GPa in the present work.Natural rutile and recovered TiO 2 II have also been analyzed via single-crystal X-ray diffraction and FTIR spectroscopy.

Sample Description and Experimental Methods
Natural rutile (Ti 0.988 Fe 0.011 O 2 ) was collected from an UHP eclogite (no.B132R114P1a) in the main hole of the Chinese Continental Scientific Drilling Project (CCSD) in a supracrustal rock slab, which was subducted to a depth of over 100 km and then exhumed to the surface during continental collisional orogeny [14,15].The eclogite is composed of fresh mediumgrained garnet, omphacite, phengite, and minor rutile (Figure 2) and has experienced peak metamorphic P-T conditions greater than 3 GPa and 700 • C [15].
In situ high-pressure Raman spectroscopic measurements were conducted using a symmetric-type diamond anvil cell (DAC) with a pair of type IIa diamond anvils (culet of ~0.3 mm in diameter).A rhenium gasket was pre-indented to have a thickness of ~60 µm, and a hole of a diameter of ~0.15 mm was drilled as the sample chamber.A rutile sample with the size of 40 × 40 × 30 µm was selected and loaded into the sample chamber together Minerals 2023, 13, 703 3 of 11 with two ruby spheres.Argon (Ar) was loaded as a pressure-transmitting medium.The potential pressure gradients are estimated to be lower than 0.2 GPa at pressures up to 20 GPa [16].Raman spectra were collected with a HORIBA LabRAM HR Evolution laser Raman spectrometer at Institute of Geology, Chinese Academy of Geological Sciences, in the wavenumber range of 100 to 1200 cm −1 (spectral resolution of 1 cm −1 ), using 10 accumulations and a 5 s exposure time with a 20× microscope objective.All spectra were excited by a 532 nm solid-state laser at a power of 100 mW.Pressure was calculated from the shift of the ruby R1 luminescent line [17].The frequencies (cm −1 ) of the bands in the spectra from in situ high-pressure Raman spectroscopic measurements are listed in Table 1.In situ high-pressure Raman spectroscopic measurements were conducted using a symmetric-type diamond anvil cell (DAC) with a pair of type Ⅱa diamond anvils (culet of ~0.3 mm in diameter).A rhenium gasket was pre-indented to have a thickness of ~60 µm, and a hole of a diameter of ~0.15 mm was drilled as the sample chamber.A rutile sample with the size of 40 × 40 × 30 µm was selected and loaded into the sample chamber together with two ruby spheres.Argon (Ar) was loaded as a pressure-transmitting medium.The potential pressure gradients are estimated to be lower than 0.2 GPa at pressures up to 20 GPa [16].Raman spectra were collected with a HORIBA LabRAM HR Evolution laser Raman spectrometer at Institute of Geology, Chinese Academy of Geological Sciences, in the wavenumber range of 100 to 1200 cm −1 (spectral resolution of 1 cm −1 ), using 10 accumulations and a 5 s exposure time with a 20× microscope objective.All spectra were excited by a 532 nm solid-state laser at a power of 100 mW.Pressure was calculated from the shift of the ruby R1 luminescent line [17].The frequencies (cm −1 ) of the bands in the spectra from in situ high-pressure Raman spectroscopic measurements are listed in Table 1.
Single crystals of natural rutile (about 60 × 40 × 30 µm in size) and recovered TiO2Ⅱ (about 40 × 40 × 30 µm in size) (Figure 3) were mounted on glass fibers for single-crystal X-ray diffraction measurements, respectively.Intensity data were collected with a Bruker D8 Venture diffractometer at Center for High Pressure Science and Technology Advanced Research, Beijing, using APEX4 software.An X-ray (λ = 0.71073 Å) was conducted with a IµS 3.0 generator using a rotating Mo anode.Crystal structures were refined from the intensity data via SHELXL-2018 [18] in the package WINGX [19].The refinements were based on the scattering factors and absorption coefficients for Ti 4+ and O 2− from the International Tables for Crystallography, Volume C [20].The refined unit cell parameters and atom positions of rutile and TiO2Ⅱ are given in Table S1.We calculated X-ray powder diffraction patterns for the Cu Kα radiation (λ = 1.5405Å) of rutile and TiO2Ⅱ from the crystal structure data collected via single-crystal X-ray diffraction, using the program Crys-talDiffract.These are compared in Figure 4.
Unpolarized infrared spectra of three natural rutile samples and recovered TiO2Ⅱ were collected at ambient conditions in the wavenumber range of 3000 to 3800 cm −1 , using a Bruker INVENIO-R FTIR spectrometer with a 15× objective on the HYPERION 1000 microscope at Institute of Geology and Geophysics, Chinese Academy of Sciences.Each spectrum was accumulated over 128 scans with a 2 cm −1 resolution.The calculation of the  Single crystals of natural rutile (about 60 × 40 × 30 µm in size) and recovered TiO 2 II (about 40 × 40 × 30 µm in size) (Figure 3) were mounted on glass fibers for single-crystal X-ray diffraction measurements, respectively.Intensity data were collected with a Bruker D8 Venture diffractometer at Center for High Pressure Science and Technology Advanced Research, Beijing, using APEX4 software.An X-ray (λ = 0.71073 Å) was conducted with a IµS 3.0 generator using a rotating Mo anode.Crystal structures were refined from the intensity data via SHELXL-2018 [18] in the package WINGX [19].The refinements were based on the scattering factors and absorption coefficients for Ti 4+ and O 2− from the International Tables for Crystallography, Volume C [20].The refined unit cell parameters and atom positions of rutile and TiO 2 II are given in Table S1.We calculated X-ray powder diffraction patterns for the Cu Kα radiation (λ = 1.5405Å) of rutile and TiO 2 II from the crystal structure data collected via single-crystal X-ray diffraction, using the program CrystalDiffract.These are compared in Figure 4.
Unpolarized infrared spectra of three natural rutile samples and recovered TiO 2 II were collected at ambient conditions in the wavenumber range of 3000 to 3800 cm −1 , using a Bruker INVENIO-R FTIR spectrometer with a 15× objective on the HYPERION 1000 microscope at Institute of Geology and Geophysics, Chinese Academy of Sciences.Each spectrum was accumulated over 128 scans with a 2 cm −1 resolution.The calculation of the water contents referred to [21], using Lambert-Beer's Law in the form c = a/ε, wherein the absorption coefficient a = A/t, and where c is the water concentration (mol•L −1 ), ε is the integrated molar absorption coefficient (L•mol −1 •cm −1 ), A is the measured absorbance, and t is the thickness (cm) of the measured crystal plate.The unpolarized absorbance of randomly oriented grains in this study is estimated to be one-third of the sum of the three principal absorbances [22].In the Raman spectra collected in the in situ high-pressure experiment (Figure 6 and Table 1), three strong bands at 235, 445, and 610 cm −1 can be observed, while the bands initially located at 135, 325, 360, and 830 cm −1 can hardly be detected due to their relatively low intensity.At elevated pressures up to 10.8 GPa, the bands at 445 and 610 cm −1 linearly shift to higher frequencies without discontinuity (Figure 7a), indicating the continuous and steady compression of the TiO 6 octahedra.In accordance with previous observations [7,8], the intensity of the 235 cm −1 band decreases with increasing pressure (Figure 6a).This decrease can be explained by the increased ordering of the crystal structure under compression [8].At pressures above 13.2GPa, a number of new, sharp bands instantaneously appear in the spectrum (Figure 6a), implying the beginning of the rutile-tobaddeleyite-type TiO 2 transition.At about 16 GPa, the Raman spectrum displays 12 bands at 159, 221, 264, 323, 350, 408, 440, 492, 634, 660, 702, and 833 cm −1 (Figure 6a and Table 1).

In situ High-Pressure Raman Spectroscopy
Under ambient condition, as shown in Figure 5, the Raman spectrum of rutile in this study exhibits seven bands at 135, 235, 325, 360, 445, 610 and 830 cm −1 .The broad band at 235 cm −1 is ascribed to multi-phonon scattering, which indicates the disordered crystal structure of rutile at 1 atm [1,8,23].The two strong bands at 445 and 610 cm −1 are attributed to the O-Ti-O bending vibration and asymmetric Ti-O stretching vibration, respectively.The two weak bands at 135 and 830 cm −1 are due to the rotation of the TiO6 octahedra and symmetric Ti-O stretching vibration, respectively [1,7,9,24].In the Raman spectra collected in the in situ high-pressure experiment (Figure 6 and Table 1), three strong bands at 235, 445, and 610 cm −1 can be observed, while the bands initially located at 135, 325, 360, and 830 cm −1 can hardly be detected due to their relatively low intensity.At elevated pressures up to 10.8 GPa, the bands at 445 and 610 cm −1 linearly shift to higher frequencies without discontinuity (Figure 7a), indicating the continuous and steady compression of the TiO6 octahedra.In accordance with previous observations [7,8], the intensity of the 235 cm −1 band decreases with increasing pressure (Figure 6a).This decrease can be explained by the increased ordering of the crystal structure under compression [8].At pressures above 13.2GPa, a number of new, sharp bands instantaneously appear in the spectrum (Figure 6a), implying the beginning of the rutile-to-baddeleyite-type TiO2 transition.At about 16 GPa, the Raman spectrum displays 12 bands at 159, 221, 264, 323, 350, 408, 440, 492, 634, 660, 702, and 833 cm −1 (Figure 6a and Table 1).These bands are consistent with the previously reported vibrational modes of baddeleyite-type TiO2 [7,9,25], indicating the termination of the transition.
As shown in Figures 6b and 7b, Raman bands of baddeleyite-type TiO2 linearly shift to lower frequencies with decreasing pressure down to 10 GPa without notable discontinuity and band broadening, revealing that the crystal structure of baddeleyite-type TiO2 is still stable during decompression.In addition, there is no observable increase or decrease in the intensities of the bands upon the release of pressure.In the pressure range of about 9 to 6 GPa, the bands exhibit a highly non-linear shift with an abrupt change in frequency due to the phase transition from baddeleyite-type TiO2 to TiO2Ⅱ.The intensities of the bands change anomalously with the appearance of a number of new bands (Figures 6 and 7).In accordance with the reported vibrational modes of TiO2Ⅱ [7][8][9], the Raman spectrum shows eight bands at 178, 288, 318, 374, 440, 555, 592, and 624 cm −1 at about 5 GPa (Figure 6b and Table 1), demonstrating the termination of the phase transformation.All these bands are still visible in the Raman spectrum of the recovered TiO2Ⅱ at 1 atm.However, the two strong bands at about 178 and 440 cm −1 decrease in intensity after the pressure is released, implying that their intensities can be pressure-dependent (Figure 6b).The present work shows that the pressure interval (13 to 16 GPa) of the rutile to baddeleyite-type TiO2 transition at ambient temperature is significantly narrower than that (12 to 21 GPa) reported in previous studies [7,9].In addition, a transition in rutile can also be observed to begin at about 7 GPa [8], implying a wider pressure interval.Therefore, more work is needed to determine the potential influence of kinetic factors (such as minor impurities, grain size, pressure medium and length of time under pressure) on this phase transition.In addition, at room temperature, the transformation may reflect a slow rate of abrupt transition [8].Further high-pressure and high-temperature experiments are expected to reveal the equilibrium phase boundary among these rutile polymorphs.

Single-Crystal X-ray Diffraction Analyses
According to single-crystal X-ray diffraction measurements in this study (Table 2), the average Ti-O distance of the octahedral site (1.9574Å) in recovered TiO2Ⅱ is similar to that in rutile (1.9599 Å), whereas TiO2Ⅱ has smaller TiO6 octahedra, compared to rutile (Figure 8).As shown in Figure 8 and Table 2, the central Ti 4+ in the octahedral site in rutile is bonded to four oxygens at an identical distance (1.9467 Å), which is slightly shorter than the lengths of other two Ti-O bonds (1.9863 Å).However, in the crystal structure of TiO2Ⅱ, two Ti-O bonds (2.0368 Å) are much longer than the other two (1.8874Å) in a plane.As a result, the estimated mean octahedral quadratic elongation [26] and octahedral angle variance of the octahedral site in TiO2Ⅱ are significantly larger than those in rutile (Figure 8 and Table 2), indicating that the recovered TiO2Ⅱ has more distorted TiO6 octahedra than rutile does under ambient conditions [27].As shown in Figure 4, X-ray powder diffraction analyses (with Cu Kα radiation) are expected to identify the lamellae of TiO2Ⅱ within natural rutile under ambient conditions based on the angles (2θ) of two strong peaks at 25.5° and 31.5°.As shown in Figures 6b and 7b, Raman bands of baddeleyite-type TiO 2 linearly shift to lower frequencies with decreasing pressure down to 10 GPa without notable discontinuity and band broadening, revealing that the crystal structure of baddeleyite-type TiO 2 is still stable during decompression.In addition, there is no observable increase or decrease in the intensities of the bands upon the release of pressure.In the pressure range of about 9 to 6 GPa, the bands exhibit a highly non-linear shift with an abrupt change in frequency due to the phase transition from baddeleyite-type TiO 2 to TiO 2 II.The intensities of the bands change anomalously with the appearance of a number of new bands (Figures 6 and 7).In accordance with the reported vibrational modes of TiO 2 II [7][8][9], the Raman spectrum shows eight bands at 178, 288, 318, 374, 440, 555, 592, and 624 cm −1 at about 5 GPa (Figure 6b and Table 1), demonstrating the termination of the phase transformation.All these bands are still visible in the Raman spectrum of the recovered TiO 2 II at 1 atm.However, the two strong bands at about 178 and 440 cm −1 decrease in intensity after the pressure is released, implying that their intensities can be pressure-dependent (Figure 6b).
The present work shows that the pressure interval (13 to 16 GPa) of the rutile to baddeleyite-type TiO 2 transition at ambient temperature is significantly narrower than that (12 to 21 GPa) reported in previous studies [7,9].In addition, a transition in rutile can also be observed to begin at about 7 GPa [8], implying a wider pressure interval.Therefore, more work is needed to determine the potential influence of kinetic factors (such as minor impurities, grain size, pressure medium and length of time under pressure) on this phase transition.In addition, at room temperature, the transformation may reflect a slow rate of abrupt transition [8].Further high-pressure and high-temperature experiments are expected to reveal the equilibrium phase boundary among these rutile polymorphs.

Single-Crystal X-ray Diffraction Analyses
According to single-crystal X-ray diffraction measurements in this study (Table 2), the average Ti-O distance of the octahedral site (1.9574Å) in recovered TiO 2 II is similar to that in rutile (1.9599 Å), whereas TiO 2 II has smaller TiO 6 octahedra, compared to rutile (Figure 8).As shown in Figure 8 and Table 2, the central Ti 4+ in the octahedral site in rutile is bonded to four oxygens at an identical distance (1.9467 Å), which is slightly shorter than the lengths of other two Ti-O bonds (1.9863 Å).However, in the crystal structure of TiO 2 II, two Ti-O bonds (2.0368 Å) are much longer than the other two (1.8874Å) in a plane.As a result, the estimated mean octahedral quadratic elongation [26] and octahedral angle variance of the octahedral site in TiO 2 II are significantly larger than those in rutile (Figure 8 and Table 2), indicating that the recovered TiO 2 II has more distorted TiO 6 octahedra than rutile does under ambient conditions [27].As shown in Figure 4, X-ray powder diffraction analyses (with Cu Kα radiation) are expected to identify the lamellae of TiO 2 II within natural rutile under ambient conditions based on the angles (2θ) of two strong peaks at 25.5 • and 31.5 • .

FTIR Spectroscopy
The representative mid-infrared spectrum of natural rutile in this study displays three absorption bands at 3280, 3295, and 3320 cm −1 in the wavenumber range of 3000 to 3800 cm −1 (Figure 9a).All these bands are ascribed to the O-H stretching vibration [2][3][4].The main O-H band at 3280 cm −1 can be observed in the infrared spectra of both natural and synthetic rutile [1,2,4,28].This band is attributed to H incorporated close to the shared O-O edge (the shortest edge of the TiO 6 octahedron) and is not associated with any substitutional defects in the crystal structure [29,30].However, the two weak O-H bands at 3320 and 3295 cm −1 are, respectively, related to Ti 3+ and Fe 3+ impurities at the octahedral site [2,29,31].The water contents of three rutile samples are estimated to be 1590, 1620, and 1780 ppm of H 2 O by weight, respectively.The maximum water content of rutile generally increases with increasing inferred temperature and pressure [3].Therefore, rutile from the UHP eclogite in this study has a relatively high water solubility compared to that of other crustal rocks.For instance, the maximum reported water concentration of rutile in pegmatite is only 820 ppm of H 2 O by weight [3,21].
O-O edge (the shortest edge of the TiO6 octahedron) and is not associated with any substitutional defects in the crystal structure [29,30].However, the two weak O-H bands at 3320 and 3295 cm −1 are, respectively, related to Ti 3+ and Fe 3+ impurities at the octahedral site [2,29,31].The water contents of three rutile samples are estimated to be 1590, 1620, and 1780 ppm of H2O by weight, respectively.The maximum water content of rutile generally increases with increasing inferred temperature and pressure [3].Therefore, rutile from the UHP eclogite in this study has a relatively high water solubility compared to that of other crustal rocks.For instance, the maximum reported water concentration of rutile in pegmatite is only 820 ppm of H2O by weight [3,21].
In the IR spectrum of recovered TiO2Ⅱ under ambient conditions, seven bands at 3286, 3312, 3354, 3434, 3510, 3555, and 3745 cm −1 can be observed (Figure 9b).All these bands are consistent with the reported bands of TiO2Ⅱ in the range of 3000 to 3800 cm −1 [7], implying that TiO2Ⅱ in the continental slabs can also contain certain amounts of water.The bands at 3354, 3434, 3510, 3555, and 3745 cm −1 are probably associated with longer O-O edges or O-O distances (>2.5143Å) (Table 2), indicating the relatively weak effects of hydrogen bonding (O-H•••O) on O-H stretching [32][33][34].Since the broad bands around 3400 to 3450 cm −1 can also be due to liquid water (or sub-microscopic fluid inclusions) in NAMs [3], the possibility that water molecules are taken up by TiO2Ⅱ after a phase transition occurs cannot be excluded.Since several hundred parts per million of water in NAMs can trigger original partial melting of HP and UHP rocks [35], further studies on the pressure-temperature stability of hydroxyls in rutile and TiO2Ⅱ may also help to understand the transportation and release of water in continental slabs during deep subduction and exhumation.In the IR spectrum of recovered TiO 2 II under ambient conditions, seven bands at 3286, 3312, 3354, 3434, 3510, 3555, and 3745 cm −1 can be observed (Figure 9b).All these bands are consistent with the reported bands of TiO 2 II in the range of 3000 to 3800 cm −1 [7], implying that TiO 2 II in the continental slabs can also contain certain amounts of water.The bands at 3354, 3434, 3510, 3555, and 3745 cm −1 are probably associated with longer O-O edges or O-O distances (>2.5143Å) (Table 2), indicating the relatively weak effects of hydrogen bonding (O-H•••O) on O-H stretching [32][33][34].Since the broad bands around 3400 to 3450 cm −1 can also be due to liquid water (or sub-microscopic fluid inclusions) in NAMs [3], the possibility that water molecules are taken up by TiO 2 II after a phase transition occurs cannot be excluded.Since several hundred parts per million of water

Figure 1 .
Figure 1.Projection of crystal structures (consisting of TiO6 octahedra) of rutile and TiO2Ⅱ based on the single-crystal X-ray diffraction analyses in this study.

Figure 1 .
Figure 1.Projection of crystal structures (consisting of TiO 6 octahedra) of rutile and TiO 2 II based on the single-crystal X-ray diffraction analyses in this study.

Figure 3 .
Figure 3. Photomicrographs of natural rutile and recovered TiO 2 II in this study.

Figure 3 .
Figure 3. Photomicrographs of natural rutile and recovered TiO2 Ⅱ in this study.

3 .
Result and Discussion 3.1.In Situ High-Pressure Raman Spectroscopy Under ambient condition, as shown in Figure 5, the Raman spectrum of rutile in this study exhibits seven bands at 135, 235, 325, 360, 445, 610 and 830 cm −1 .The broad band at 235 cm −1 is ascribed to multi-phonon scattering, which indicates the disordered crystal structure of rutile at 1 atm [1,8,23].The two strong bands at 445 and 610 cm −1 are attributed to the O-Ti-O bending vibration and asymmetric Ti-O stretching vibration, respectively.The two weak bands at 135 and 830 cm −1 are due to the rotation of the TiO 6 octahedra and symmetric Ti-O stretching vibration, respectively [1,7,9,24].
consistent with the previously reported vibrational modes of baddeleyitetype TiO 2[7,9,25], indicating the termination of the transition.Minerals 2023, 13, x FOR PEER REVIEW 6 of 12

Figure 5 .
Figure 5.The Raman spectrum of rutile in the wavenumber range of 100 to 1200 cm −1 under ambient conditions.

Figure 5 .
Figure 5.The Raman spectrum of rutile in the wavenumber range of 100 to 1200 cm −1 under ambient conditions.

Figure 6 .
Figure 6.Representative Raman spectra of rutile with varying pressure under (a) compression and (b) decompression.

Figure 6 .
Figure 6.Representative Raman spectra of rutile with varying pressure under (a) compression and (b) decompression.

Figure 7 .
Figure 7. Variations in band frequencies in the Raman spectra with varying pressure under (a) compression and (b) decompression.

Figure 7 .
Figure 7. Variations in band frequencies in the Raman spectra with varying pressure under (a) compression and (b) decompression.

Figure 8 .
Figure 8. Structural representations and geometry parameters of octahedral sites in (a) rutile and (b) TiO2 Ⅱ.

Figure 8 .
Figure 8. Structural representations and geometry parameters of octahedral sites in rutile and TiO 2 II.

Figure 9 .
Figure 9. Representative mid-infrared absorption spectra of (a) natural rutile and (b) recovered TiO 2 II under ambient conditions.The spectrum of rutile was normalized to 30 µm thickness but was vertically offset.

Table 1 .
Frequencies (cm −1 ) of the bands in the spectra from in situ high-pressure Raman spectroscopic measurements.

Table 2 .
Site geometry and occupancy parameters for rutile and TiO 2 II.
Note: the bond length and atom distance are measured in Å. Minerals 2023, 13, x FOR PEER REVIEW 9 of 12

Table 2 .
Site geometry and occupancy parameters for rutile and TiO2Ⅱ.