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Reversible Photochemical Transformation of Sulfur and Hydrogen Mixture to (HS)H at High Pressures 2

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Sakun Duwal, and Choong-Shik Yoo J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 27, 2017

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The Journal of Physical Chemistry

Reversible Photochemical Transformation of S and H2 Mixture to (H2S)2H2 at High Pressures

Sakun Duwal and Choong-Shik Yoo* Department of Chemistry and Institute of Shock Physics Washington State University, Pullman, Washington, 99164, USA

ABSTRACT We report the reversible photochemical transformation of sulfur-hydrogen mixture to (H2S)2H2, which occurs through an expanded state of photoactive a-S phase of sulfur at 4 GPa. Upon further compression, the photo-product (H2S)2H2 undergoes a phase transition at 17 GPa, and 40 GPa. The pressure-induced Raman changes indicate that the phase transition from phase I to II at 17 GPa is associated with the proton-ordering process in (H2S)2H2, evident by the profound splitting of S-H and H-H vibrational modes, whereas the transition at 40 GPa is accompanied by the disappearance of all the S-H stretching and bending modes and partial dissociation to sulfur. With increase in pressure, the molar volume of (H2S)2H2 is substantially larger than that of S + H2 mixtures, suggesting the significance of photochemical effect in order to drive the reaction from S + H2 to (H2S)2H2. In addition, we have also provided the thermaland pressure- induced effect in the mixtures using confocal Raman spectroscopy. From our results, it is clear that the effect of pressure and photochemistry can be coupled to drive the reaction at room temperature and lower pressure, rather than having to drive the reaction thermally or mechanically, underscoring the significance of the photochemical effect in understanding the path-dependent transformations of sulfur and sulfur containing materials. 1 ACS Paragon Plus Environment


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INTRODUCTION With recent advances in high-pressure technologies1-3, it is feasible to achieve a large (often up to a several-fold) compression of solid lattice, at which condition the material can be easily forced into a new physical and chemical configuration. The high-pressure thus offers enhanced opportunities to discover new phases, both stable and metastable, and to tune exotic properties in a wide-range of the atomistic length scale, substantially greater than (often several orders of) those achieved by other thermal (varying temperatures) and chemical (varying composition or making alloys) means at ambient pressure. Examples include the record high Tc (200K) superconductor in dense H2S above 150 GPa4 and its predicted novel superconducting state, H3S5. High-pressure chemistry is a thermal process that occurs through compressed states, either the ground states at given densities or near ground states that are kinetically constrained by large activation barriers. In contrast, the photochemical reaction is an athermal process that occurs through an expanded excited state. Therefore, the reaction pathways of the two processes are greatly different, especially at low pressures (< a few GPa), because of relatively large energy differences (a few eV) between the ground and excited electronic states (i.e., the band gap) with respect to relatively small compression energy (typically less than a few tenth of eV). As such, the former results in denser products, whereas the latter results in expanded products for bound electronic potentials or decomposition products for unbound excited states. Nevertheless, the two processes become relevant at high pressures (> 10-30 GPa), where the compression energy (i.e. ∆E = P∆V) becomes comparable to the band gap energy. Therefore, the photochemical process can be used to control high-pressure chemistry and synthesize novel materials that could never have occurred at a ground state due to the relatively high energy 2 ACS Paragon Plus Environment

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barrier. In fact, by coupling the photochemical effect with the pressure effect, one can alter the molecular and electronic structure, which could give rise to new reaction pathways, or alternatively, could lower the reaction threshold pressure as has been observed in systems like furan6 and benzene7. Typically, to tune the chemistry between sulfur and hydrogen, pressures higher than 70 GPa (~∆E = 2eV)8 will require, given the bond energy of sulfur is 347 kJ/mol (3.6 eV)9. Therefore, the goal in this current study is to tune the chemistry between sulfur and hydrogen by coupling both the pressure and photochemical effect in order to substantially lower the reaction pressure. Elemental sulfur exists in a wide range of allotropes (S-I, a-S, S-II, S-III, S-VI, S-IV etc.) that are stable at various pressure-temperature (PT) conditions and can be generated through different experimental conditions10. Among these allotropes, the most stable form of sulfur at ambient condition is S-I (or αS8), which is a molecular crystal with crown shaped S8 puckered rings6. Sulfur has a complex phase diagram, consisting of several polymorphs whose stabilities and transformations strongly vary depending on various measurements reported.8,10-19 Figure 1, for example, summarizes the phase diagram to 20 GPa, determined by the previous Raman studies.8,10 According to this phase diagram, S-I transforms to amorphous sulfur (a-S) above 3 GPa, which then transforms to trigonal S-II above 6 GPa. Above 10 GPa, S-II transforms to rhombohedral S-VI and further to tetragonal S-III with square-shaped chains above 12 GPa8,10. Upon further compression, S-III transforms to body centered orthorhombic (bco) S-IV at 83 GPa8 and further to rhombohedra S-V (β-Po) above 157 GPa, both of which become superconductors at low temperatures.20 Even among the previous Raman studies,8,10-19 the evolution of phases vary depending upon the laser energy and laser power density used in those experiments. This is in part because sulfur becomes photochemically sensitive above 3 GPa10. 3 ACS Paragon Plus Environment


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Therefore, depending on the laser power density and wavelength, the behavior of sulfur varies upon compression. For instance, if a low power laser is used, S-I bypasses the transitions to S-II and S-VI, and transforms to a-S and then to tetragonal S-III. As a result, the previously reported phase diagrams of sulfur are greatly diverse, depending on the experimental methods.8,10-18 Photochemistry has also significant effects on sulfur-containing compounds. For instance, our previous study21 shows that photochemistry initiates the decomposition of D2S to S above 30 GPa. However, if the photochemistry is prevented, D2S instead undergoes the phase IV-to-V transition at 30 GPa, retarding the decomposition to sulfur at considerably higher pressures. While the photochemistry in H2S can be avoided at relatively low pressures (60-100 GPa) to a mixture of sulfur and superconducting states of H3S with the highest Tc reported.4,5,22-29 On the other hand, it is interesting to note that H2S in H2S+H2 mixtures is not photoactive at high pressures, but forms a novel compound (H2S)2H2, which is stable up to 40 GPa.30 The complex behaviors of sulfur and sulfur-containing materials at high pressures seem to advocate for studies on the path-dependent transformation of sulfur-containing materials, including thermo-mechanical and photochemical processes. Therefore, we have investigated the pressure-, thermal- and photo-induced transformations of S+H2 mixtures to 50 GPa. The results show a stark difference between the pressure- and thermal/photo-induced processes. The mixture undergoes thermal and photochemical reactions to form (H2S)2H2 above 3 GPa, whereas it remains stable to 40 GPa without influence of strong light or heat. Interestingly, the photochemical reaction occurs reversibly upon pressure cycling. Our results signify that the photochemistry in S+H2 mixtures occurs through an expanded excited state of a-S to form a low

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density (H2S)2H2, which partially decomposes to sulfur above 40 GPa, and is reversible upon decompression.

EXPERIMENTAL METHODS Mixtures of hydrogen (99.999% pure from A-L Compressed Gases, Inc.) and sulfur (99.5% pure from Alfa Aesar) are loaded in a Diamond Anvil Cell (DAC) with Type IA diamond anvils featuring 300 µm culets and 200 µm thick rhenium gaskets. The gaskets are first pre-indented to a thickness of 35 µm, and a hole of 120 µm in diameter is drilled using an electric discharge machine. A small piece (~15 µm wide) of solid sulfur is loaded in the gasket hole together with a ruby sphere (~5 µm) for pressure measurement.31 The cell is then gently closed and placed inside our custom designed high-pressure gas loader to load pressurized H2 at 2000 atmosphere. Once the high-pressure gas is loaded in the cell, the cell is then closed and taken out from the highpressure loader for further experiments. The pressure is applied and Raman spectra are obtained. Raman spectra are collected using a home-built micro-Raman system, equipped with a holographic diffractive bandpass filter, a Raman notch filter and a 500 mm focal length monochromator (Princeton Instrument). The Raman light is collected through a 100 µm slit of the monochromator, which is dispersed off of an 1800 lines/mm grating onto a liquid nitrogen cooled charged coupled device (CCD) with a spectral resolution of about ~0.8 cm-1. The spectrograph was calibrated using the emission lines of Ne and Kr pencil-type calibration lamps. The system also contains a confocal arrangement that uses two lenses with 100 mm focal lengths, which are placed 200 mm apart. Exactly at the mid-point between these two lenses, a rectangular aperture is placed, which can be closed so that only the Raman scattered light from 5 ACS Paragon Plus Environment


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the plane of interest can be collected in a back scattering geometry. This is an important feature, especially for mixture samples, to obtain Raman spectra from spatially selected sample areas with a minimum level of diamond backgrounds. An Ar+ ion laser with a wavelength of 514.5 nm is used as an excitation source and is focused through a 20x apochromatic, infinity-corrected objective lens. The photochemistry of S+H2 mixture is induced by illuminating sulfur with 514.5 nm Ar+ ion laser light of ~10 mW (30 µm in diameter) for several hours at 3 GPa, which slowly converts opaque sulfur to transparent rod-like crystals. This gives the laser power density of ~3 kW/cm2, which can heat the sample by the laser absorption of sulfur. Nevertheless, we found that the heating effect is rather small (less than 10 degrees) owing to high thermal diffusivity of hydrogen. Raman spectra are collected at different regions of the sample at each pressure to 50 GPa, using a minimum level of laser exposure, typically less than 100 mW for 5 minutes. The pressure is slowly increased at steps of 0.5 GPa over an hour. The results present in this study have been reproduced for more than a half dozen experiments. In addition, to study the thermal effect in the S+H2 mixtures, the sample was externally heated to 100ºC using a band heater wrapped around the membrane-DAC. The temperature measurements were taken using a K-type thermocouple in contact with the diamond anvils.

RESULTS A.

Thermo-Mechanical Transformations of S+H2 Mixtures


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Figure 2 shows the Raman spectra in S+H2 mixtures to 50 GPa, taken from a sulfur-rich region (dark area in the inset) in the spectral regions of sulfur vibrations between 200-700 cm-1 and hydrogen vibrations above 4100 cm-1. The pressure-induced spectral changes show no apparent chemical reactions between sulfur and hydrogen to 50 GPa, but the phase transitions in sulfur from S-I to amorphous sulfur (a-S) at ~5.5 GPa and then to S-III at ~20 GPa. Importantly, there is no apparent Raman feature that can be associated with photo-induced S-II8,10 at pressures between 5 and 10 GPa, or metastable S-VI formed from S-II above 10 GPa10 (see Figure 1), underscoring the absence of photochemistry. Raman spectra of hydrogen are also typical for pure hydrogen in terms of its pressure dependence and spectral sharpening at ~6 GPa upon solidification (as shown in Figure 2), as well as the presence of a series of libration modes, such as those that appeared in the Raman spectra taken from hydrogen-rich regions (not shown in Figure 2). There is no sign of the peak splitting, asymmetric spectral distortion, or new peaks that can be attributed to the pressure-induced chemical reactions between S and H2 or the presence of interstitial filled hydrogen. Therefore, we conclude that the S+H2 samples remain as a heterogeneous mixture to 50 GPa. B.

Photochemical Reactions of S + H2 to (H2S)2H2 The photochemistry between S and H2 can be induced by a long exposure of intense 514.5

nm laser light at ~3 GPa, where hydrogen remains as dense fluid. Figure 3 illustrates the associated Raman changes and the visual appearance (in the inset) of the sample before (1.1 GPa) and after (4.2 and 17.4 GPa) the photochemistry. At 1.1 GPa, the spectrum shows pure H2 behavior with H2 rotons at 362.36 cm-1, 591.57 cm-1, 820.78 cm-1, and 1044.10 cm-1, as well as an H2 vibron at 4172.61 cm-1. The microphotograph of the sample at 1.1 GPa (in the inset) shows



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solid sulfur and pure hydrogen (the transparent region). At around 3 GPa, sulfur starts to become photosensitive as it transforms from orthorhombic S-I phase to a-S10. Once the photochemistry is triggered, sulfur reacts with hydrogen fluid to form a rod-shape transparent single crystal (see the inset at 4.2 GPa) grown over several hours. The Raman spectrum taken from the product (the red spectra at 4.2 GPa in Figure 3) shows the emergence of several new peaks at 1175 cm-1, 2541 cm-1, 2582 cm-1, 2695 cm-1 and 4139 cm-1, signifying the formation of S-H and H-H bonds. The 1175 cm-1 mode, for example, is likely from the bending mode ν2 of H2S, which compares well with the 1165 cm-1 of pure H2S phase I at 6 GPa32. Similarly, the peaks at ~2541 cm-1 are reasonably well compared to the stretching mode, ν1, of pure H2S observed at 2511 cm-1 at 6 GPa32, considering a small blue shift due to the repulsive interaction between H2S and H2.30 The doublet at 4139 cm-1 and 4200 cm-1 is then associated with the hydrogen vibrons, νH-H, from the photo-reacted product and unreacted pure H2, respectively. Also shown are a series of the rotons of hydrogen below 1000 cm-1, consistent with those in stronger intensities obtained from a hydrogen-rich region (the black spectrum). Upon compression to 17.4 GPa, the photochemical product develops a set of lattice modes below 500 cm-1, while the ν1 and νH-H modes further split into multiple peaks. These spectral changes are accompanied by darkening and lattice distortion of the photochemical product (the inset at 17.4 GPa), indicating an occurrence of structural phase transition. These pressure-induced spectral changes are shown in more details in Figures 4 and 5. It is important to note that the Raman spectra of the product at 4.2 GPa and 17.4 GPa are similar to those of (H2S)2H2 formed by compressing H2S and H2 mixtures reported by Strobel et al30. Based on this similarity, we attribute the photochemical product formed at 4 GPa in the present study to (H2S)2H2.


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Figure 4 shows the pressure-induced Raman changes of the photochemical product obtained during compression and decompression. It shows a series of phase/chemical transformations that occur reversibly upon pressure cycling. Upon the photochemical reaction, the S+H2 mixture forms either H2S as evident from the ν1 at ~2524 cm-1 at 3.7 GPa, or (H2S)2H2 as evident from the characteristic νH-H peak at 4140 cm-1 near the νH-H from untransformed H2 at 4200 cm-1 at 4.2 GPa. Interestingly, both the ν1 and νH-H peaks of (H2S)2H2 (marked as phase I in blue) rapidly broaden as pressure increases to 15 GPa, which may reflect an enhancement of proton (or hydrogen bond) disorder. While remaining symmetric to 15 GPa, these ν1 and νH-H peaks abruptly split into several peaks at 17.4 GPa, indicating a structural phase transition of (H2S)2H2 (phase II in red). Upon further compression, (H2S)2H2 slowly loses the intensity of all vibrational modes, and partially decomposes to sulfur above 40 GPa, evident by the emergence of νS-S of S-III (at 230, 254, 332, 529, 535, and 568 cm-1). Upon decompression, all abovedescribed spectral changes occur reversibly with a little hysteresis in pressure. Figure 5 plots (a) the pressure-induced peak shifts, (b) the spectral evolution of the νH-H to multiple peaks and (c) the spectral width of selected νH-H peaks. It shows the onsets of pressure-induced phase/chemical transformations in H2+S mixture and its photo-product (H2S)2H2, as signified by the vertical-dotted lines. In additions, several significant spectral changes are noted in Figure 5: First, the νH-H modes of (H2S)2H2 (the peaks 1 through 6) appear at considerably lower frequencies than that of pure H233 (peak 7), indicating the presence of attractive interactions between H2 and H2S and thereby a substantial level of chemical bonding between them. Similar softening of the νH-H has also been observed in the mixture of SiH4 + H2.34 Second, the ν1 and νH-H split into multiplets, especially in phase II above 17 GPa, indicating



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the presence of a large number of crystal force fields (or interstitial sites) in a distorted lowsymmetry crystal structure. Third, a relatively simple vibrational spectrum above 40 GPa, consisting of two νH-H bands at 4155 cm-1 and 4148 cm-1, and poorly resolved lattice modes at 147 cm-1, 188 cm-1 and 262 cm-1 at 43 GPa. Fourth, a strong broadening of the νH-H mode of phase I (Figure 5c), again, indicating the pressure-induced proton (or hydrogen-bond) disorder arising as the intermolecular distance between H2 and H2S decreases. Fifth, a strong “negative” pressure shift of the ν1 peak, signifying the strengthening of hydrogen bonding in (H2S)2H2. Note that the observed ∆ν/∆P slope (-25.53 cm-1/GPa) of (H2S)2H2 is almost equal to that of the νO-H in H2O (-22.5 cm-1/GPa)35. In turn, the positive slope of the νH-H in phase I may reflect considerably weaker hydrogen bonding of interstitial filled H2 molecules to H2S at low pressures. Sixth, the appearance of sharp sulfur peaks below 560 cm-1 at ~ 40 GPa, signifying the decomposition of (H2S)2H2 to sulfur and hydrogen. Recall that Strobel30 et al. have reported the formation of (H2S)2H2 by compressing H2S+H2 mixtures to 30 GPa, but no structural phase transition based on their x-ray data. Nevertheless, it is important to note that the previously reported Raman spectra at 7 and 17.8 GPa30 are strikingly similar to those obtained for phase I at 4.2 GPa and phase II at 17.4 GPa in the present study. Therefore, we attribute this difference to the subtle structural change primarily associated with hydrogen and hydrogen bonding. In fact, the theoretical work by Duan et al24 on (H2S)2H2 has suggested a P1 structure for the phase at 20 GPa. The phase transition above 17.4 GPa can be understood in terms of the P1 structure. Duan et al24 have further simulated the Raman spectrum of the P1 structure, and the results agree qualitatively well with the observed Raman spectra at 17.4 GPa in the current study.


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C.

Thermal Reactions of S + H2 to (H2S)2H2 Figure 6 shows the Raman spectra of S+H2 sample heated to 100ºC. The black spectrum

corresponds to the Raman spectra at 5.3 GPa, taken at 53 ºC at H2 rich region, before the occurrence of the reaction. All of these Raman features can be attributed to the pure hydrogen. The blue spectrum corresponds to the one taken at 5.7 GPa at 100 ºC, after sulfur melts. The Raman spectrum shows an appearance of a buried S-H stretching mode inside the diamond vibron at 2535 cm-1 and a downshifted H-H vibron at 4140 cm-1 suggesting an onset of reaction between sulfur and hydrogen. The red spectrum is taken at 5.4 GPa, after the sample has cooled down to room temperature from 100 ºC. As seen in Figure 6, the spectrum consists of S-H bending mode at 1180 cm-1, S-H stretching mode at 2534 cm-1 and a downshifted H-H mode at 4140 cm-1 suggesting the formation of (H2S)2H2. Upon compressing to higher pressure, similar phase transitions as in Figure 4 are observed. The insets in Figure 6 show the visual changes of the sample as the reaction progresses. The formation of (H2S)2H2 upon heating suggests that a reaction between sulfur and hydrogen can be triggered through a thermal pathway, as well, which leads to the same product, as achieved by the photochemical reaction.

DISCUSSION The present study presents the spectral evidence for the formation of (H2S)2H2 in S+H2 mixtures, analogous to the same formed in the previous H2S+H2 mixture30. However, unlike the previous H2S+H2 mixture, the application of pressure alone is not sufficient to produce (H2S)2H2 in the present S + H2 mixture. It requires the photochemical excitation of a-S above 3 GPa. In S8 molecules, the HOMO orbitals are the π* orbitals, whereas the LUMO orbitals are the σ* orbitals. 11 ACS Paragon Plus Environment


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The S8 molecules in the orthorhombic structure of S-I are held together by weak van der Waals interaction. So, as pressure is applied, the gap between the HOMO and LUMO orbitals decreases. Above 3 GPa, as S-I transforms to photoactive a-S, the decreased HOMO-LUMO gap makes the electronic excitation from the π* orbitals to σ* orbitals much easier upon the absorption of light (S + hν S*). This photo-expanded excited state of sulfur, S*, then interacts with the σ orbitals of the ground state of fluid hydrogen at ~3.5 GPa and forms a molecular alloy (H2S)2H2. To confirm that this current reaction is the sole result of photochemical reaction, not thermal, we have (i) exposed the sulfur rich region to the laser for a similar amount of time starting from 0 GPa, 1 GPa and 2 GPa. And, upon doing that, we observed no reaction at all below 3 GPa, which suggests that this reaction is the sole result of the photochemical process, which starts only after sulfur starts to become photosensitive as α-S8 transforms to a-S. This photo-induced amorphization of α-S8 has been correlated to the red shift of the absorption edge, which is caused by the broadening of the valence and conduction bands, which results in a narrowing of the energy gap upon the application of pressure10. (ii) We have also calculated the temperature at different pressures before and during exposure to laser using the stokes and antistokes shift of the sulfur modes (Raman spectrum shown in Figure S1). The calculated temperature is 293 K at 2.4 GPa, before exposure to the laser, and 298 K during exposure to the laser for several hours. This suggests a ΔT of 5 K, which is not enough temperature to drive this reaction thermally, as we have shown that it requires 100 ºC to thermally drive the reaction in Figure 6. The small temperature increase of 5 K is likely due to high thermal diffusivity of hydrogen, which quickly diffuses the heat away from sulfur to diamond anvils also very high thermal conductors.


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Once formed, (H2S)2H2 is no longer photoactive until it partially decomposes to S above 40 GPa. This is interesting because both S and H2S are photoactive above 3 and 30 GPa, respectively. The substantially higher photoactive pressure in H2S is likely due to the larger bandgap in H2S-IV than that of a-S, resulting from the formation of σ S-H bonds in H2S. To a similar extent, the bandgap of (H2S)2H2 with additional H2S..H2 bond is expected to be even larger than that of H2S, which would push the photochemistry well above the decomposition pressure of 40 GPa. It is important to note that the photochemical reaction in S+H2 mixture occurs reversibly upon the pressure cycle. This can be explained in terms of the molar volume change as illustrated in Figure 7 where the pure component volume of (3H2+2S) is compared with that of (H2S)2H2. The EOS data for S and H2 have been taken from refs. 8 and 33 respectively and the data for (H2S)2H2 and (H2S+H2) have been reproduced from ref. 30. The figure clearly shows that the molar volume of (H2S)2H2 is considerably larger than that of (3H2+2S) at all pressures, suggesting that the application of pressure alone is not sufficient to drive the reaction to (H2S)2H2 from sulfur and hydrogen. Nevertheless, the molar volume difference at low pressures is very small; as such, the photochemistry can be turned through an expanded excited state of S*+H2 (labeled as state ‘2’) to form H2S or (H2S)2H2. Once H2S is formed, it reacts with excess H2 and converts to (H2S)2H2 above 4-5 GPa, as also found in the previous study30. The photochemical/pressure-induced chemical change of H2+S mixture is then summarized as:

3 + 2 3 + 2 ∗ → (2 + ) → ( ) The inset in Figure 7 shows a schematic of electronic structure changes along the abovedescribed reaction coordination. The numbers signify the photochemical excitation from S+H2 13 ACS Paragon Plus Environment


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(‘1’) to an expanded excited state of S*+H2 (‘2’) at 3 GPa. Note that the photochemical reactions to H2S (the grey vertical arrow) and (H2S)2H2 (the red vertical arrow) can occur through either the excited states or the ground states, assuming relatively low activation barrier between H2S and (H2S)2H2 based on the previously observed pressure-induced transformation of H2S+H2 to (H2S)2H2.27 Note that the large activation barrier in the forward reaction (∆Efa) prevents the pressure-induced reaction of H2+S mixture to H2S or (H2S)2H2 in the ground state. On the other hand, as pressure is increased the activation barrier for the backward reaction (∆Eba) could decrease significantly, and, as a result, partial decomposition to sulfur could be favored. The reversible nature of this sample upon decompression confirms the partial dissociation to sulfur, because, if the sample decomposes entirely to S and H2 at higher pressures, it is very unlikely to go back to (H2S)2H2 upon decompression. The prominent spectral changes associated with the ν1 and νH-H in Figures 4 and 5 seem to indicate that the structural phase I to II transition is driven by ordering of hydrogen bonding in (H2S)2H2. The red shift of the ν1 (or S-H stretching) mode above 4.2 GPa indicates the presence of weakly interacting hydrogen bonds at low pressure. And as pressure is increased, the S...H hydrogen bond strengthens; as a result, the S-H covalent bond weakens. Since there are two different covalent S-H bonds present (as suggested by Duan et al24), implying two different bond strengths, the resulting S-H vibrational mode (or ν1) is rather broad, as seen in Figure 4 at 15.4 GPa. As the neighboring H2 molecules start to participate in the formation of new H2S..H2 bonds, it starts to perturb H-H covalent bonds and, thereby, broaden the νH-H mode, as also seen in Figure 4 at 15.4 GPa. With further increase in pressure, all the disordered hydrogen atoms start to become ordered and result in the prominently sharp S-H (ν1) bands and H-H (νH-H) vibrons at 17.4 GPa, indicating a transition from proton-disordered phase I with proton-ordered phase II. 14 ACS Paragon Plus Environment

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Splitting of the νH-H mode in (H2S)2H2 above 17 GPa (Figures 4 and 5a) seems to signify the proton exchange reaction in phase II. As mentioned above, the strength of hydrogen bonding substantially increases in phase II in comparison with that in phase I. As such, the hydrogen bonding in phase I is primarily between neighboring H2S molecules with dS..H =1.80 Å, whereas H2 molecules are far apart from nearest H2S with dS..H = 2.02 Å at 5 GPa30. Thus, H2 molecules contribute to hydrogen bonding (i.e., H2S…H2) only weakly so to enhance the disorders in H2S…H2S hydrogen bonding in H2S and also in H-H covalent bond in H2 itself. The rapid broadening of the ν1 and νH-H peaks as pressure increases (Figure 5c) then reflects the pressure enhancement of such a disorder in phase I. However, in phase II or above 17 GPa, the nearest intermolecular distances between H2 and H2S collapses to 1.82 Å, comparable to those of hydrogen bonded S…H distances between neighboring H2S molecules (1.79 Å)30. Therefore, it stimulates the proton exchange between H2 and H2S molecules in phase II. Such a proton exchange reaction could result in a wide range of S-H and S…H bond distances and thereby the frustration of the ν1 and νH-H modes as observed in the multiple splittings in Figure 5a. Duan et al24 have suggested that (H2S)2H2 (or H3S with H:S stoichiometric ratio of 3:1) can be synthesized by either using H2S + H2 or S + H2 as precursor materials. Indeed, the first scenario has been confirmed by Strobel et al30. However, the present study shows that the mechanical energy alone is not sufficient to drive this reaction in S+H2 mixtures (see Figures 2 and 7). For comparison, Duan et al24 has proposed the P1 structure to be stable below 37 GPa and the Cccm structure to be stable between 37 and 111 GPa for (H2S)2H2. The disappearance of the entire lattice, bending and stretching modes with just one H-H mode (labeled “3”) present at 40 GPa upon the transition from phase II to III could be attributed to the Cccm phase. In fact, Duan22-24 et al have suggested that this Cccm structure is energetically favored with partial 15 ACS Paragon Plus Environment


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hydrogen bond symmetrization. And processes like symmetrization, for instance, in the case of H2O36, HCl37 and HBr38 are accompanied with disappearance of the bending and stretching modes, with only one collective mode left. However, in our current case, signatures of sulfur modes in Raman are so strong that the collective mode that corresponds to the Cccm could be buried underneath the sulfur peaks. The Cccm structure then transforms to a denser R3m structure above 111 GPa, and then to Im-3m structure above 180 GPa. This Im-3m structure is believed to have a Tc of 191-204 K at 200 GPa22,24. Nevertheless, despite this difference in the structural stability, the predicted sharp bandgap change from 3.3 eV of the P1 phase to 1.3 eV of the Cccm phase at 37 GPa seems consistent with the sharp color change observed in (H2S)2H2 from initially transparent at ~4 GPa and then to black at ~40 GPa. Understanding the exact mechanism of the photochemical reaction at high pressure is nontrivial matter, which requires the detailed information of electronic structures, intermediate chemical/electronic states and their pressure-dependent changes, and many body effects on both radiative and non-radiative processes. Thus, we consider it beyond the scope of the present paper. Nevertheless, the present paper provides evidences for the photochemical transformation in S+H2 at high pressures, underscoring the significance of photochemical effects on sulfur containing materials, including H3S showing the record high Tc (200 K above 150 GPa)4. The present results also demonstrate the photochemical pathway of S+H2 to (H2S)2H2 – different from the previously reported thermo-mechanical pathway of H2S+S to the same product.30 Such a path-dependent transformation is also important for the synthesis of H3S, as reported in two recent papers.39,40 Furthermore, note that the Raman spectra of H3S reported in Ref. 40 is


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different from that of (H2S)H2 in the present study, especially in the lattice region. This clearly suggests that (H2S)2H2 and H3S are indeed chemically distinctive despite the same stoichiometry.

CONCLUSION We have presented the photochemistry of S+H2 mixtures to (H2S)2H2, which occurs through an expanded state of photoactive a-S phase at ~4 GPa. Interestingly, this photochemical reaction occurs reversibly upon pressure cycling; that is, the product (H2S)2H2 undergoes transformation to phase II above 17 GPa, and III above 40 GPa, and upon unloading, it goes back to phase I below 15 GPa. Under dark conditions, no pressure-induced chemical change was observed in S+H2 mixture to 50 GPa – the maximum pressure studied, whereas if given sufficient thermal energy, H2+S could form (H2S)2H2. This sharp contrast between the pressureand photo-induced processes in S+H2 mixtures clearly provides strong constraints to the complex behaviors previously observed in dense D2S21 and superconducting H3S22,24 and advocates for the significance of photochemical effects in understanding high-pressure behaviors of sulfur and sulfur-containing materials. In fact, as has been pointed out by our previous work21 and one of the recent work41, photochemical instability in H2S could be the reason for the discrepancies in the results42, 43 regarding the transition to the superconducting H3S.



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ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS publications website for evaluation of thermal vs photo effects on S+H2 Mixtures. AUTHOR INFORMATION Corresponding Author: Correspondence and requests for materials should be addressed to C. S. Yoo at [email protected], (509) 335-2712 Notes: The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The present study has been performed in support of National Science Foundation, Division of Materials Research under (Grant No. 1203834), Department of Energy - National Nuclear Security Administration (DOE-NA0003342), American Chemical Society- Petroleum Research Fund (No. 54806-ND10), and Sloan Foundation through the Deep Carbon Observatory – Extreme Physics and Chemistry. One of author, Sakun Duwal, appreciates the support of the Carnegie DOE Alliance Center. The authors would also like to acknowledge the insightful contributions of one anonymous reviewer.


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FIGURE CAPTIONS Figure 1 A phase diagram of sulfur, modified from the previous Raman studies in ref. 6. The insets show the structures of sulfur in different phases. The structure parameters used were taken from Ref. 8. The grey area in the S-I and a-S phase boundary signifies a hysteresis of the transition, whereas the dotted phase boundaries signify the path dependent phase II and VI. Upon further compression, S-III transforms to S-IV (bco) at 83 GPa and further to S-V (β-Po) above 150 GPa, both of which become high Tc superconductors at low temperatures.20 Figure 2 Raman spectra of S + H2 mixture, taken at a sulfur rich region, showing the pressureinduced phase transitions of sulfur and H2. It shows no pressure-induced chemical changes between sulfur and hydrogen. The inset is a microphotograph of S + H2 taken at 50 GPa. The dark region corresponds to sulfur and the transparent region corresponds to hydrogen. Figure 3 Raman spectra of S + H2 showing the spectral evidence of photochemical reaction of S + H2 mixture at 4.2 GPa. The insets show the microphotographs of S-I in fluid H2 at 1.1 GPa; a-S and fluid H2 together with a rod-shaped transparent crystal photo-chemically formed from aS at 4.2 GPa; and the photochemical product and unreacted solid hydrogen at 17.4 GPa. The black spectra are taken at the hydrogen rich regions, whereas the red spectra are taken at the photochemical product regions. At 4.2 GPa, a bending mode, ν2, stretching mode, ν1, and additional downshifted H2 vibron are observed from the photo-product, signifying the Raman characteristic of (H2S)2H2. At 17.4 GPa, the crystalline compound darkens, and several new modes appear for the lattice modes, S-H stretchings, and H2 vibrons, suggesting a phase transition of the photo-product (H2S)2H2.


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Figure 4 Pressure-induced Raman changes of the photo-chemical products in H2+S mixtures, showing the spectral evidences for H2S (black), (H2S)2H2-I (blue), (H2S)2H2-II (red), and S+H2 (green) observed, reversibly, as pressure increases and decreases. Figure 5 (a) Pressure-dependent shifts of Raman peaks to 60 GPa, showing the spectral changes associated with the photochemistry of S+H2 mixture at 3 GPa, the phase transition of photo-product (H2S)2H2 at 17 GPa and its decomposition at 40 GPa. The symbols correspond to the experimental data points, whereas the line represents the polynomial fits. The grey dotted lines correspond to the pure H2 vibron taken from ref. 33, (b) Spectral deconvolution of the peaks observed in the H2 vibron (νH-H) region, showing the emergence of multiple peaks with increase in pressure. The deconvoluted peaks are labeled as ‘1’ to ‘7’ for clarification. (c) The pressure-induced changes of the spectral widths of selected νH-H peaks, showing the abrupt spectral changes at 17 GPa and 40 GPa and a rapid increase of the ‘3’ peak width. The symbols correspond to the experimental data points, whereas the lines correspond to the polynomial fits. Figure 6 Thermal induced reaction of S+H2 mixture at 5 GPa and 100°C. The black spectrum corresponds to the one taken at 76°C, before the reaction occurred. The blue spectrum corresponds to the one taken at 100°C, as sulfur melted. It shows the evolution of S-H stretching mode and an appearance of downshifted H-H vibron. The red spectrum is taken after the sample is cooled back to room temperature. The appearance of the S-H bending mode at bending mode at 1180 cm-1, S-H stretching mode at 2534 cm-1 and a downshifted H-H mode at 4140 cm-1 suggesting the formation of (H2S)2H2. The insets in Figure 6 show the visual changes of the sample as the reaction progresses.



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Figure 7 The pressure-dependent molar volume changes of (H2S)2H2 in red, 2H2S + H2 in black and 3H2 + 2S reproduced for comparison from refs. 30, 8 and 33, respectively. The numbers in yellow circles correspond to various states during the reaction scheme. The state ‘1’ corresponds to the ground state of S + H2. The state ‘2’ corresponds to the photo-expanded state of sulfur that reacts with the hydrogen. (Inset) Schematic of the reaction coordinate diagram of the ground (black at 3 GPa and red at 40 GPa) and excited (blue at 3 GPa) states. The numbers signify the photochemical excitation from S+H2 (‘1’) to an expanded excited state of S*+H2 (‘2’) at 3 GPa. Note that the photochemical reactions can occur to H2S (grey vertical arrow) and (H2S)2H2 (red vertical arrow) through either the excited states or the ground states. Note that the large activation barrier in the forward reaction (∆Efa) prevents the pressure-induced reaction to H2S or (H2S)2H2 in the ground state.


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S-I

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Figure 7

Reversible Photochemical Transformation of S and ... - ACS Publications

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