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New Phase Transition of Solid Bromine Under High Pressure

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New Phase Transition of Solid Bromine Under High Pressure PHYSICAL REVIEW LETTERS week ending PRL 99, 015501 (2007) 6 JULY 2007 New Phase Transition of Solid Bromine under High Pressure A. San-Miguel,1 H. Libotte,2 M. Gauthier,3 G. Aquilanti,4 S. Pascarelli,4 and J.-P. Gaspard2 1Universite´ de Lyon, F-69000 Lyon, France and Universite´ de Lyon 1, Laboratoire PMCN, CNRS, UMR 5586, F69622 Villeurbanne Cedex, France 2Physique de la Matie`re Condense´e, Universite´ de Lie`ge, B-4000 Lie`ge, Belgium 3IMPMC, CNRS UMR 7590, Universite´ Pierre et Marie Curie, F-75352, Paris 6, Paris, France 4European Synchrotron Radiation Facility, BP220, F-38043 Grenoble, France (Received 26 March 2007; published 6 July 2007) Solid bromine has been studied by x-ray absorption spectroscopy experiments up to a maximum pressure of 75 GPa. The data analysis of the extended fine structure reveals that the intramolecular distance first increases, reaching its maximum value at 25 5 GPa. From this value the intramolecular distance abruptly begins to decrease evidencing a nonpreviously observed phase transformation taking place at 25 5 GPa. A maximum variation of 0.08 A˚ is observed at 65 5 GPa where again a phase transition occurs. This last transformation could correspond with the recently observed change to an incommensurate modulated phase. We discuss the possible generalization of the observed new phase transition at 25 5 GPa to the case of the other halogens. DOI: 10.1103/PhysRevLett.99.015501 PACS numbers: 61.50.Ks, 31.70.Ks, 61.10.Ht, 62.50.+p We report x-ray absorption spectroscopy experiments ([5], and references therein). A consistent picture of the showing the existence of a phase transition in solid dia- high pressure evolution of halogens (Cl2, Br2 , and I2) was tomic bromine at 25 5 GPa. Previous to this work the developed by Fujihisa et al. [5] following the high pressure consensus has been that under compression of diatomic x-ray diffraction investigation of iodine and bromine [6– molecular solids, the intermolecular distances shrink and 9]. They observed that the pressure evolution of the struc- in a lesser extent the intramolecular distance increases or tural parameters of the three halogens scaled well as a remains constant. Both distances become progressively function of a reduced atomic volume defined as vr 3 comparable leading at a given pressure to molecular dis- vat= 8rs , with rs being the interatomic distance of the sociation and metallization through one or more phase molecule in the solid state. An important fact to keep in transformations [1–4]. At the dissociation pressure both mind is that in this definition rs was considered constant. the intermolecular and intramolecular distances are the It was equally found that the dissociation pressure of same. The high resolution of our experiments shows that bromine and iodine takes place at a limiting value of vr the intramolecular distance of bromine initially increases 1:29, which corresponds well with their respective ob- up to 25 GPa as expected. However, at 25 GPa the intra- served transitions pressures at that time at 21 and molecular distance suddenly begins to decrease up to 65 80 GPa. Further transitions to more compact structures 5 GPa. This sudden change in sign of the pressure induced have been identified at higher pressures [10]. bond distance modification is attributed to a new type of Semimetallization through gap closure was predicted to phase transition, which to our knowledge is observed here take place at vr 1:37 based on a value of 16 GPa for for the first time. This result should modify the way we iodine. This gives a prediction of 60 GPa for the semi- understand dissociation and metallization in diatomic sol- metallization pressure of bromine, which is in good agree- ids at high pressure and could have significant theoretical impact on the investigation of these phenomena in solid H2, or other molecular solids which are of central interest, both from a theoretical point of view as well as for their implications, for example, in astrophysical problems. The heavier halogens (Br2, I2), constitute model systems for the study of simple molecular solids under high pres- sure as the necessary pressures for metallization or disso- ciation are typically a factor of 10 to 100 smaller than for H2. The low pressure solid phase of the halogens is an orthorhombic molecular crystal (D18 or Cmca) made of 2h FIG. 1 (color online). View of the structure of the low pressure layers of molecules (see Fig. 1). Each layer is constituted phase of bromine (Cmca, space group No. 64). The layered of in-plane molecules in a zigzag arrangement. During the structure is emphasized and the unit cell is depicted. The EXAFS first stages of compression, the predominant effect on the measurements here presented have allowed us to follow the structural arrangement is the reduction of the interlayer pressure evolution of the intramolecular distance and the next- distances as observed by x-ray diffraction experiments neighbor distances, r2 and r3. 0031-9007=07=99(1)=015501(4) 015501-1 © 2007 The American Physical Society PHYSICAL REVIEW LETTERS week ending PRL 99, 015501 (2007) 6 JULY 2007 ment with electrical conductivity experiments [11]. In fact, the preedge peak (see Fig. 2), the intensity of which rapidly electrical conduction experiments in iodine [12,13] show decreases with contamination. In contrast with our pre- that the electrical resistivity drops down abruptly from 0 to vious work on bromine [14], both the x-ray-absorption 13 GPa in the direction perpendicular to the I2 layers and near-edge structure (XANES) and EXAFS part of the from 0 to 18 GPa in the layers’ directions as it has been spectra were obtained. discussed in [14]. Different calculations [5,13–15] indicate The x-ray absorption Br K edge preedge strong peak that the interlayer interaction is responsible for the pro- (see inset of Fig. 2) corresponds to the transition of the gressive gap closure through orbital resonance. photoelectron to the first nonoccupied states in the elec- Recent investigations have shown the presence of an tronic density of states. These states derive from the intermediate incommensurate modulated phase [2,16] antibonding energy level of the Br2 molecule [22]. Figure 2 leading to a revision of the halogens’ phase diagram. shows the evolution of the FWHM of this peak as a This intermediate phase has been well characterized in function of pressure. The already observed discontinuity the case of iodine [2], showing nearest interatomic dis- in the slope of this feature at 25 GPa [14] is then confirmed tances continuously distributed between the bond length of by both experiments and is even better defined. We observe I2 in the molecular crystal and the nearest interatomic in Fig. 2 a further change in the evolution of the FWHM at distance in the fully dissociated monatomic crystal. The 65 5 GPa. The x-ray absorption preedge peak is fol- transition to the incommensurate modulated phase takes lowed at higher photoelectron energies in the spectra by place at about 23 GPa for iodine and 84 GPa for bromine. the Br K edge absorption jump. We have followed the The dissociation pressures are pushed further to pressures position of the Br K edge at the inflexion point of the of about 30 and 115 GPa for iodine and bromine, respec- absorption jump. Its evolution with pressure is linear tively [16]. with a value at ambient pressure of 13 471 eV and a slope The observation of an evolution of the intramolecular of 0:02 eV GPaÿ1. distance in iodine at the first stages of compression [17] The EXAFS oscillations were extracted by usual tech- will modify the values of the key parameter of reduced nics and are shown in Fig. 3(a) for some selected data. The atomic volume. This, added to the discovery of the inter- most important contribution to the signal is due to the mediate incommensurate phase and the shift of the differ- single scattering path of the photoelectron inside the mole- ent transition pressures, calls for a revision of our image of cule. With increasing pressure, contributions from second the high pressure evolution of halogens. In this Letter we and higher neighboring shells become progressively im- show that the bromine intramolecular distance not only portant. Our data analysis includes the ab initio calculation significantly evolves with pressure, but also its variation of the atomic scattering amplitude and phase shift for each shows a sharp discontinuity at 25 GPa, which evidences a scattering path using the FEFF [23] code and the fit with the phase transition. The consequence on the case of iodine structural model of bromine using the FEFFIT code [24]. In will be discussed. addition, we have constrained our data analysis so that the Our experiments were performed at the ID24 energy atomic volume at the measured pressure corresponds, dispersive extended x-ray-absorption fine structure spec- within the error bars, to the one measured by x-ray diffrac- troscopy (EXAFS) beam line [18,19] of the European tion [5]. The obtained fits, shown in Fig. 3(a), are remark- Synchrotron Radiation Facility (ESRF, Grenoble, France) ably satisfactory up to 68 GPa. The interatomic distances using a specially designed optics [20,21]. The x-ray beam with an energy window around the bromine K edge, was focused both in the horizontal and vertical planes to reach a 10 10 m2 spot. Pressure was applied using a diamond anvil cell at room temperature. The solid sample was loaded in the cell at liquid nitrogen temperature in an argon atmosphere to avoid sample contamination. Two different experiments were performed. In the first one, pressure was calibrated from the bromine equation of state [5] monitor- ing the (111) and (112) diffraction peaks.
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