Structural Transition in Compressed Amorphous Sulfur Chryste `le Sanloup * School of Geosciences and Center for Science at Extreme Conditions, University of Edinburgh, Mayfield Road, Edinburgh, EH9 3JZ, United Kingdom Eugene Gregoryanz and Olga Degtyareva SUPA, School of Physics and Center for Science at Extreme Conditions, University of Edinburgh, Mayfield Road, Edinburgh, EH9 3JZ, United Kingdom Michael Hanfland European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France (Received 14 November 2007; revised manuscript received 13 December 2007; published 20 February 2008) Properties of amorphous sulfur (a-S) were investigated by synchrotron x-ray diffraction up to 100 GPa between 40 and 175 K. Measurements of the structure factor yielded the radial distribution function and the densities of two amorphous forms. a-S undergoes a structural transition above 65 GPa, accompanied by density discontinuity of 7%. These results indicate the amorphous-amorphous transition, from a low- density to a high-density form, and open up the possibility for the direct measurements of density of liquid-amorphous materials at extreme conditions. DOI: 10.1103/PhysRevLett.100.075701 PACS numbers: 64.70.p, 61.05.C, 61.43.Er, 62.50.p Polyamorphism between low- and high-density amor- phous (LDA/HDA) forms has been extensively studied in archetypal compounds such as silica [1], ice [2], and GeO 2 [3]. Among the elements, Si [4,5] and Ge [4,6] are known to exhibit pressure-induced polyamorphism which, as in the case of SiO 2 and H 2 O, is characterized by transforma- tion within the tetrahedral framework, the HDA form hav- ing a higher coordination number [7]. Polyamorphism has also attracted attention due to the controversies on the nature of the transition, i.e., first vs second order. This has been illustrated by the cases of amorphous water [8 – 10] and silica [11–13]. To resolve these issues, density measurements are the most complementary method to the observation of structural changes as a first-order transition can be detected by a density change across the transition. In the case of simple liquids, density can be indirectly ex- tracted from structure factor measurements, either using a hard-sphere model or empirical relationships [14]. Nevertheless, direct density measurements on noncrystal- line samples under static high-pressure conditions still present a technical challenge, limiting the accessible pres- sure range to 10 GPa [2,15 –18]. With the arrival of the third generation synchrotron sources and better detector techniques, it became possible to observe very weak dif- fraction signals from amorphous and liquid states in diamond-anvil cells (DACs) at pressures up to 100 GPa [18 – 20]. Quantitatively accurate structure factors, allow- ing diffraction-based determination of the density, were first measured in a DAC on liquid argon and liquid water [18]. Although the pressure in these experiments was limited to 1.1 GPa, this pioneer work demonstrated the feasibility of density measurements in DAC experiments, even on low Z materials. Recently, pressure-induced amorphization (PIA) of sul- fur was observed at low temperatures, and the P-T field of the existence of a-S was mapped out [21]. In this Letter, we report in situ x-ray diffraction data on a-S between 50 and 100 GPa and between 40 and 175 K, and an implemented method to extract the information at such extreme condi- tions. Diffraction-based direct density measurements yield the structure factor, SQ, the radial distribution functions, gr, and density data up to 100 GPa, extending the pres- sure range of such direct measurements in the DACs by a factor of 100 [18]. Our measurements show that two re- gions of different structures and densities exist within the field of a-S. The method to extract the density of noncrystalline materials from diffraction data was first derived for liquids at ambient pressure [22] and later adapted to DACs [18]. In the latter case, the diffraction signal needs to be corrected for the background signal from the anvils, and for the limited angular transmission by the cell. The method used here for the data analysis is an adaptation of this method to much weaker signals due to the higher pressure conditions. In the diffraction-based method, density is extracted by minimizing the oscillations on gr for r lower than the first interatomic distance. These oscillations are due to errors in processing the scattered intensity into SQ. Termination effects due to the limited experimental Q range might also generate artificial ripples on gr but at larger r ( 2–5  A range [22]) and are therefore less critical for density measurements than for precise structural inter- pretations. In this work, a crystalline sample reference spectrum was taken from the closest P-T point at which a-S was observed. It should be noted that the spectra observed upon recrystallization of a-S were single-crys- PRL 100, 075701 (2008) PHYSICAL REVIEW LETTERS week ending 22 FEBRUARY 2008 0031-9007= 08=100(7)=075701(4) 075701-1  2008 The American Physical Society