⇦Chinese Physics Letters, 2019, Vol. 36, No. 8, Article code 086401Express Letter Reexploration of Structural Changes in Element Bromine through Pressure-Induced Decomposition of Solid HBr * Ming-Kun Liu (刘明坤), De-Fang Duan (段德芳), Yan-Ping Huang (黄艳萍), Yong-Fu Liang (梁永福), Xiao-Li Huang (黄晓丽)**, Tian Cui (崔田)** Affiliations State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012 Received 1 July 2019, online 22 July 2019 *Supported by the National Natural Science Foundation of China under Grant Nos 51572108, 51632002, 11504127, 11674122, 11574112, 11474127 and 11634004, the 111 Project (No B12011), Program for Changjiang Scholars and Innovative Research Team in University (No IRT-15R23), and the National Fund for Fostering Talents of Basic Science (No J1103202).
**Corresponding authors. Email: huangxiaoli@jlu.edu.cn; cuitian@jlu.edu.cn Citation Text: Liu M K, Duan D F, Huang Y P, Liang Y F and Huang X L et al 2019 Chin. Phys. Lett. 36 086401    Abstract Simple molecular solids have been an important subject in condensed matter physics, particularly for research of pressure-induced molecular dissociation. We re-explore the structural changes of element bromine through pressure-induced decomposition of solid HBr. The phase changes in HBr are investigated by Raman spectroscopy and synchrotron x-ray diffraction up to 125 GPa at room temperature. By applying pressure, HBr decomposes into solid bromine in the pressure range of 18.7–38 GPa. The solid bromine changes from molecular phase to incommensurate phase at 81 GPa, and finally to monatomic phase at 91 GPa. During the process of pressure-induced molecular dissociation, the intermediate incommensurate phase of element bromine is confirmed for the first time from the x-ray diffraction studies. The decomposition of HBr is irreversible since HBr cannot form again upon pressure decompression. DOI:10.1088/0256-307X/36/8/086401 PACS:64.60.-i, 62.50.-p, 64.70.Rh, 91.60.Hg © 2019 Chinese Physics Society Article Text In high-pressure and condensed-matter physics, a substantial amount of research was motivated by the tantalizing idea of metallizing hydrogen. The metallization of hydrogen is regarded as the holy-grail of high-pressure research and has become a compelling subject of interest for many scientists. However, the metallization of solid hydrogen is still challenging despite of the great advancements recently. In 2004, Ashcroft suggested that hydrogen-rich compounds could be as an alternative route because the chemical precompression can reduce the tremendous pressure required for pure hydrogen.[1] This proposal has been proved by the recent two record hydrogen-rich high-temperature superconductors: H$_{3}$S and LaH$_{10}$.[2–5] The discovery of high-temperature superconductivity in compressed sulfur hydride creates a significant breakthrough in exploring the high-temperature superconductors.[2–4] Superconducting transition temperature ($T_{\rm c}$) of $Im\bar{3}m$-H$_{3}$S tops out at 203 K at 200 GPa, which is related to the decomposition product H$_{3}$S instead of H$_{2}$S under pressure. Therefore, the stability and superconductivity of hydrides at high pressure have attracted extensive attention. Another example can be traced to phosphorus hydrides, and its superconductivity has been discovered with $T_{\rm c}$ up to 100 K by using high pressure resistance measurements.[6] From our recent published work, we found that P$_{2}$H$_{4}$ and other decomposed products of compressed PH$_{3}$ were considered as main products of the high-temperature superconducting phase reported in experiment.[7] These results form a physical picture that the conventional hydrides with the common stoichiometric ratio will decompose into new unconventional hydrides, which may produce high-$T_{\rm c}$ superconductivity. Systematic studies on hydrogen halides are greatly helpful for understanding of structural phase transitions, metallization and even superconductivity at high pressure.[8–15] For hydrogen bromide (HBr), recent theoretical studies show that it is unstable above 64 GPa and decomposes into new hydrides, H$_{2}$Br and Br$_{2}$.[16] Theoretical calculations proposed that the resultant H$_{2}$Br is a superconductor with $T_{\rm c} $ = 12.1 K.[16] Experimentally, the studies on HBr were limited to below 50 GPa with spectroscopic methods.[8–11,17] In order to ascertain the behavior of HBr under high pressure, we perform systematic Raman spectroscopy and synchrotron x-ray diffraction (XRD) studies above megarbar pressures. The structural changes of element bromine are investigated through pressure-induced decomposition of solid HBr. Under extreme conditions attained by applying high pressure, the distance between the neighboring molecules becomes comparable to the atomic distance in the molecule. When inter- and intra-molecular distances are identical, the molecules in the solid cannot be identified anymore. Thus, the so-called molecular dissociation is attained, and the monatomic phase is formed. The modulated structure is an intermediate phase between molecular and monatomic phases. It is a transient state during molecular dissociation, and intimately related to the dissociation process of molecules. Moreover, the modulated structure is a special structure with a modulated wave. In our work, we re-explore the structural changes of element Br$_{2}$ and further elucidate its dissociation process. The present study may offer a structural model for theoretical simulations aimed at elucidating pressure-induced molecular dissociation in simple molecular solids especially for solid hydrogen. In our experiment, HBr was a corrosive gas stored in a compressed gas cylinder under ambient conditions. The sample chamber with diameter 60 µm was made by a laser drilling machine on a rhenium metal sheet. A thin layer of gold ring was embedded in the chamber to prevent the sample from corroding the rhenium gasket chamber. The target sample was loaded into the diamond anvil cell (DAC) with 100 µm culet size in a glove box filled with argon gas by liquid nitrogen cooling treatment, which is also reported in our previous work.[7] The first-order Raman shift of the diamond is used for determining the pressure.[18]
cpl-36-8-086401-fig1.png
Fig. 1. Optical micrographs of the sample taken upon compression. At 38 GPa, the sample turns black. The red circle represents the sample chamber. The diameters in (a)–(e) are about 60 µm and in (f)–(j) are about 90 µm.
In situ high-pressure Raman spectra were measured up to 123.5 GPa at room temperature. The Raman scattering measurements were performed in back-scattering configuration with a T64000 Horiba Jobin-Yvon spectrometer, with a 532 nm line as the excitation laser. The excitation laser light having a roughly estimated power of 5 mW was defocused to about 30 µm in diameter on the sample surface in order to avoid possible radiation damage to the sample. In situ synchrotron XRD measurements were carried out at the 4W2 High-Pressure Station of the Beijing Synchrotron Radiation Facility (BSRF). The incident wavelength was 0.6199 Å. A MAR3450 image plate detector was used to collect the diffraction patterns and two-dimensional XRD images were radially integrated using the FIT2D software,[19] yielding patterns of intensity versus diffraction angle 2$\theta$. Prior to measurements, a CeO$_{2}$ standard was used to calibrate the geometric parameters. The average acquisition time for each diffraction pattern to obtain sufficient intensity was 300 s. The refinement of XRD patterns was completed by means of Reflex module in Material Studio Program.[20] After the sample was loaded into the DAC, the sealed pressure was about 3.2\,GPa. The photographs in Fig.\,1 show the changes of the sample appearances with increasing pressure. At the beginning, the sample was transparent and colorless. When pressure increases to 38\,GPa, the sample turns to black and opaque, and then into reflective state at about 79.8\,GPa. This indicates that the sample underwent a large band-gap collapse during the compression process. Based on Fig.\,1, it can be speculated that the sample may become metallic with pressure. The change of sample color in the whole experiment is similar to our previous studies on PH$_{3}$.[7]
cpl-36-8-086401-fig2.png
Fig. 2. (a) Selected Raman spectra of compressed HBr measured up to 123.5\,GPa. The spectra are divided into two parts: 80--780\,cm$^{-1}$ and 1565--4400\,cm$^{-1}$. (b) Pressure dependence of Raman shifts for corresponding HBr and Br$_{2}$ vibrational modes at selected pressures. The hollow symbols represent the previous results,[21] and the solid symbols represent the results from our experiments.
We also collect the Raman spectra of compressed HBr up to 123.5\,GPa at room temperature, as shown in Fig.\,2(a). In the present study, liquid HBr transforms to a cubic structure ($Fm3m$) at 0.5\,GPa. The HBr molecule itself has only one broad characteristic Raman peak around 2298\,cm$^{-1}$, which is assigned as the stretching mode of HBr in Fig.\,2(a). When pressure increases to 13.1\,GPa, the stretching mode splits into a doublet mode, including symmetric and anti-symmetric stretching modes (see Fig.\,2(a)). The splitting of the stretching peak is attributed to the phase transition: cubic phase I ($Fm3m$) transforms to the orthorhombic phase I$\!$I ($Cmc2_{1}$). Up to 18.7\,GPa, several peaks appear in the low wavenumber region. These peaks around 146\,cm$^{-1}$, 164\,cm$^{-1}$, 283\,cm$^{-1}$ and 300\,cm$^{-1}$ agree well with the vibration modes of molecular orthorhombic $Cmca$-Br$_{2}$ phase proposed by in the previous work.[17,23] The former two belong to the librational modes of element bromine (Br$_{2}$), while the latter two belong to the stretching modes of Br$_{2}$.[17,23] The emergence of Br$_{2}$ signal indicates that HBr begins to decompose. The signal of Br$_{2}$ is observed at 18.7\,GPa, it is different from the results of Katoh \it et al.[9] that HBr molecules dissociate to form Br$_{2}$ at 43\,GPa. Actually, the Raman signal of Br$_{2}$ has been observed at 13.1\,GPa by Katoh \it et al.,[9] but it was assigned as impurity in HBr. Up to 32\,GPa, the stretching modes of HBr disappear completely, indicating that the original sample decomposes completely. The detailed assignments of vibrational modes in HBr and Br$_{2}$ are clearly listed in Table 1. \footnotesize \bf Table 1. The assignment of Raman modes of HBr and Br$_{2}$ compounds at pressures. RS: Raman shift in units of cm$^{-1}$. \tabcolsep 4pt \centerline\footnotesize \begin{tabular}{llc} \hline Compounds & Vibration modes & RS\\ \hline HBr ($Cmc2_{1}$)& Symmetric stretching vibration & 1784 \\ (at 18.7\,GPa)& Anti-symmetric stretching vibration & 2066 \\ & Lattice vibration & 178 \\\hline & Symmetric librational vibration & 176 \\ Br$_{2}$ ($Cmca$) & Anti-symmetric librational vibration & 204 \\ (at 25\,GPa) & Symmetric stretching vibration & 299 \\ & Anti-symmetric stretching vibration & 302 \\ \hline \end{tabular}} With the pressure uploading, the vibration modes of Br$_{2}$ starts to shift gradually to higher wave numbers, and no other changes can be seen up to 64.2\,GPa. At 64.2\,GPa, a new peak around 181\,cm$^{-1}$ was observed. This peak is close to the librational modes of Br$_{2}$, but originated neither from molecular phase nor from monatomic phase of Br$_{2}$. In fact, the new peak is belong to the incommensurate structure of Br$_{2}$, proposed by Kume \it et al.[21] Upon further increase of pressure, all the peaks decrease or almost vanish above 92\,GPa, confirming the metallic character of the sample. Upon decompression, monatomic phase of Br$_{2}$ transforms into molecular phase, but the characteristic Raman peaks of HBr around 2300\,cm$^{-1}$ are not observed. This indicates that the original HBr does not form again during pressure downloading. Figure 2(b) shows the pressure dependence of Raman shift for corresponding vibrational modes of HBr and Br$_{2}$ at room temperature. An obvious softening behavior is observed in the H--Br symmetric stretching mode of HBr with increasing pressure. The same behavior has also been observed in some simple hydrogen-rich molecules, such as NH$_{3}$, N$_{2}$H$_{4}$ and H$_{2}$O.[22–24] The pressure-induced changes in hydrogen bond play a key role in the vibrational changes of these hydrogen-rich materials. In the HBr molecule, bromine and hydrogen atoms are connected by covalent bonds, while the neighboring HBr molecules are contacted by hydrogen bonding. As shown in Fig.\,2(b), with increasing pressure, the stretching modes of HBr shift to low wavenumber. The red-shift behavior of vibration mode under pressure is attributed to the hydrogen bond, for which the increasing H-Br covalent bond length leads to a negative pressure shift of the H--Br stretching mode. Upon further compression, H--Br stretching modes start to widen, and vanish at 32\,GPa. This means that the vibrational modes of HBr are gradually damaged and HBr undergo a pressure-induced decomposition. Only the Raman signals of Br$_{2}$ vibrational modes are left. Up to 92.7\,GPa (Fig.\,2(b)), molecular phase I ($Cmca$) begins to transition into monatomic phase I$\!$I ($Immm$) in Br$_{2}$, and no Raman signal is observed, in agreement with the results of Kume \it et al.[21] During this process, an incommensurate structure between phases I and I$\!$I is observed. Pressure dependence of Raman shifts for Br$_{2}$ is also shown in Fig.\,2(b). The solid symbols represent the results from our experiments and the hollow symbols represent the results reported previously.[21] These two data are matched well, further proving that the dissociation product is Br$_{2}$. As for the decomposition of HBr, the decomposition products may be predicted as Br$_{2}$ and H$_{2}$. However, the Raman signals of hydrogen are not observed all along the experiments. It is once attributed to the solid Br$_{2}$ that is opaque against the incident laser light, preventing the incident light from going inside the decomposed solid Br$_{2}$, so the hydrogen signal is not obtained. Based on the unknown new Raman peaks occurred with the appearance of the Br$_{2}$ and considering the works of Duan \it et al.,[16] it is also possible that the decomposition products are not Br$_{2}$ or H$_{2}$, but the Br$_{2}$ and other bromine hydrides, such as H$_{2}$Br. We have compared the calculated and experimental Raman modes of H$_{2}$Br at 25\,GPa, and they are not coincident with each other. In order to further explore the structural behavior and decomposition product of HBr under high pressure, we have performed the synchrotron XRD measurements up to 98.7\,GPa.
cpl-36-8-086401-fig3.png
Fig. 3. (a) The collected synchrotron XRD spectra of compressed HBr up to 98.7\,GPa. The incident wavelength is 0.6199\,Å. (b) Full profile Rietveld refinement of HBr and Br$_{2}$ at different pressures. The red solid line, circle points and blue sticks represent the calculated data, experimental data and Bragg peaks, respectively.
The synchrotron XRD patterns of compressed HBr have been measured up to 98.7\,GPa, as shown in Fig.\,3. Under pressure, the diffraction peaks shift towards higher angle due to the compression of the inter-atomic bonds. At 7.6\,GPa, the sample is identified to be the cubic structure ($Fm3m$). With increasing pressure, cubic-HBr transforms into orthorhombic structure ($Cmc2_{1}$) at 18\,GPa. Upon compression to 39.3\,GPa, the orthorhombic structure ($Cmca$) of Br$_{2}$ is observed. The corresponding crystal structures are shown in Fig.\,4. At 81\,GPa, new Bragg peaks appear, indicating a structural phase transition. For Br$_{2}$, a modulated structure between molecular and monatomic phases has been proposed by Duan \it et al.[25] Our experimental results match well with their calculated results, and the modulated structure of Br$_{2}$ is firstly observed through the present XRD studies. The modulated structure is an intermediate phase between the molecular and monatomic phases of Br$_{2}$. The Rietveld refinement on XRD patterns of HBr and Br$_{2}$ confirms the corresponding crystal structures and reveals the presence of the modulated structure in Br$_{2}$, which can be clearly seen from Fig.\,3(b).
cpl-36-8-086401-fig4.png
Fig. 4. The sequence of phase changes in compressed HBr at different pressures.
Figure 4 shows the phase change sequences of compressed HBr at different pressures. It can be seen that cubic structure ($Fm3m$) of HBr transforms into an orthorhombic structure ($Cmc2_{1}$) at 18.7\,GPa. HBr decomposes under pressure, and molecular Br$_{2}$ is observed. The discovered molecular Br$_{2}$ transforms to incommensurate structure and finally into monatomic structure. Similar to element iodine, the molecular dissociation of Br$_{2}$ follows the molecular-incommensurate-monatomic phase transformations, consistent with the previous Raman results.[21] According to the work of Duan \it et al.,[16] the decomposed products are deemed to be Br$_{2}$ and some other bromine hydrides such as H$_{2}$Br and H$_{4}$Br. However, in our synchrotron XRD measurements, the signal of bromine hydrides is not observed. Furthermore, we compare the calculated and experimental Raman modes of H$_{2}$Br at 25\,GPa. Our experimental results cannot reproduce the calculated vibrational modes of H$_{2}$Br. This further excludes the existence of predicted H$_{2}$Br. Another possible situation is that the decomposition products are Br$_{2}$ and hydrogen. However, the signal of H$_{2}$ is not observed by Raman spectrum experiments. It is considered that solid Br$_{2}$ is opaque against the incident laser light, preventing the incident light from going inside the decomposed solid, as mentioned above, and the effective volume may be too small to provide detectable Raman signal of H$_{2}$ molecules. There is also a possibility that H$_{2}$ reacts with metal gasket to produce metal hydride. We do not have the direct proof for the reaction of metal gasket with hydrogen, maybe further experimental studies are needed to give more information about the structural behavior of HBr and Br$_{2}$. In summary, the behavior of HBr under high pressure has been investigated by optical micrographs, Raman scattering spectra and synchrotron XRD measurements. By applying pressure, HBr starts to decompose into Br$_{2}$ at 18\,GPa. The molecular-incommensurate-monatomic phase transition of Br$_{2}$ is observed for the first time from the current high-pressure synchrotron XRD patterns. The decomposition of HBr is irreversible since HBr cannot form again upon decompression. The present result offers new reference for other halide hydrides and provides novel understanding on the phase changes in simple molecular hydrides. References Hydrogen Dominant Metallic Alloys: High Temperature Superconductors?Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride systemPressure-induced metallization of dense (H2S)2H2 with high-Tc superconductivityHigh-temperature superconductivity in sulfur hydride evidenced by alternating-current magnetic susceptibilitySuperconductivity at 250 K in lanthanum hydride under high pressuresSuperconductivity above 100 K in PH3 at high pressuresUnravelling decomposition products of phosphine under high pressurePressure Effects on Hydrogen Bonding in the Disordered Phase of Solid HBrRaman and infrared study of phase transitions in solid HBr under pressurePhase study of solid HBr by Raman spectroscopyPressure-induced structural changes of HBrPhotodissociation of hydrogen chloride and hydrogen bromideSuperconducting state in bromium halide at high pressureSuperconducting High-Pressure Phases Composed of Hydrogen and IodineObservation of Metal-insulator and Metal-Metal Transitions in Hydrogen Iodide under PressureDecomposition of solid hydrogen bromide at high pressureRaman study of solid HBr under high pressure and low temperatureMegabar high-pressure cells for Raman measurementsTwo-dimensional detector software: From real detector to idealised image or two-theta scanBook Reviews - The Rietveld Method, R. A. Young, Editor, IUCr Monographs in Crystallography, 5, International Union of Crystallography, Oxford University Press, New York, NY, pp. 298. - The Rietveld Method, R. A. Young, Editor, Oxford University Press, Oxford, England1993. ISBN 0198555776, x + 298 pp., 87 figures, 32 tables - Metal & Alloys Indexes, International Centre for Diffraction Data, Swarthmore, PA, $5.00.High Pressure Raman Study of Bromine and Iodine: Soft Phonon in the Incommensurate PhaseHydrogen Bond in Compressed Solid HydrazineElastic properties study of single crystal NH3 up to 26 GPaFast and Slow Dynamics of Hydrogen Bonds in Liquid WaterAb initio studies of solid bromine under high pressure
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