New Phase Diagram of Oxygen at High Pressures and Temperatures Mario Santoro, 1,2 Eugene Gregoryanz, 1 Ho-kwang Mao, 1 and Russell J. Hemley 1 1 Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road N.W., Washington, D.C. 20015 USA 2 LENS, European Laboratory for Non-linear Spectroscopy and INFM, Via Carrara 1, I-50019 Sesto Fiorentino, Firenze, Italy (Received 21 April 2004; published 20 December 2004) In situ high P-T Raman measurements and optical observations of solid and fluid oxygen up to 1250 K between 8 and 25 GPa reveal the existence of a new molecular phase  and strikingly unusual behavior of the melting curve. Three triple points were also identified in the P-T domain of the new phase. The data give a direct measure of the melting curve that greatly extends previous optical investigations. We find the melting temperature is significantly higher than that indicated by the existing phase diagram (e.g., 400 K higher at 25 GPa). Raman measurements in low and high frequency regions reveal the extent of orientational order disorder and persistence of strong intermolecular interactions in the high P-T phases. DOI: 10.1103/PhysRevLett.93.265701 PACS numbers: 64.70.Dv, 64.70.Kb, 78.30.–j The profound changes in simple molecular systems induced at high pressures and temperatures are fundamen- tal to a broad range of problems in physics, chemistry, and planetary science [1,2]. Among these molecular materials, oxygen is particularly important. Oxygen has unique fea- tures by virtue of its spin (S  1), and the resultant spin- spin interactions affect its numerous P-T phases, making the system a critical test of condensed-matter theory [3,4]. Oxygen is also the third most abundant element in the solar system, and its behavior under extreme P-T conditions provides important insight into the corresponding transi- tions in other systems such as hydrogen. Furthermore, under high P-T conditions oxygen is a highly reactive material (particularly in the fluid state) and plays a key role in chemical reactions that occur in planetary interiors. Despite the importance of oxygen under extreme condi- tions, there is a lack of direct experimental information on the system. High P-T shock-wave experiments on oxygen have been limited, and the comparison with theory con- troversial [5–7]. Static compression experiments, in prin- ciple, provide essential information on the equilibrium phase diagram and a range of other physical properties under the required conditions. However, technical prob- lems are thought to preclude in situ measurements of oxy- gen under static pressures and temperatures [8]. Here we report direct measurements of the phase diagram of oxygen over a broad range using new high P-T techniques. We find an unexpected steep increase in the melting temperature with pressure arising from the existence of an altogether new high P-T phase. The significantly higher melting temperature and new phase of oxygen have important implications for the interpretation of dynamic compression experiments as well as for planetary science and first- principles theory. Oxygen exhibits a rich polymorphism and unusual physical properties [3,4]. Three different solid phases are encountered from 5.5 to 96 GPa, which are all character- ized by a layered structure with oxygen molecules perpen- dicular to a crystal plane and parallel within the layer [9]. In order of increasing pressure, these are the  (R 3m) and  (Fmmm) [10], and the  (A2=m) [11] phases. The  phase appears to have a broad pressure domain of stabil- ity (up to 86 GPa) and a deep red color arising from strong charge transfer [12]; it was suggested that this phase is made of diamagnetic O 4 molecular units [13–15]. At higher pressure ( phase, above 96 GPa) the material is a molecular metal and superconductor [10,16 –19]. The ex- tent to which these properties persist at high pressures and temperatures is currently of major interest. Shock-wave studies performed in the 18–200 GPa and 3000–6000 K range have been confined to the fluid state [5,6]. Within this range, a nonmetal-metal transition was recently reported at 4500 K and 120 GPa and was inter- preted as a Mott transition where density-driven band closure and disorder play a fundamental role [5]. First- principles calculations predict a molecular dissociation within the metallic phase and indicate a key role of spin fluctuations in determining the structure of the fluid [7]. On the other hand, shock-wave measurements are limited because pressure and temperature are coupled in a transient experiment, and for oxygen the only probes so far have been electrical conductivity, which does not provide in- sight about structural and dynamical properties; also, there are concerns about possible chemical reactions of the electrodes used. Static high P-T studies of solid oxygen have been performed in diamond anvil cells only up to 650 K and 16.7 GPa [8,20]. The melting line, solid-solid phase transitions, and the -- triple point were deter- mined up to 460 K from shifts in the Raman vibron [20]; at higher temperatures only visual observations were em- ployed and on the basis of these observations the existence of the fluid-- triple point was inferred. Extending these studies to more extreme conditions has been problematic. Indeed, it has been argued that it is virtually impossible to confine oxygen in diamond cells at temperatures above 650 K due to reactions with the gasket and anvils, leading to the mechanical instability of the container and conse- quent loss of sample [8]. PRL 93, 265701 (2004) PHYSICAL REVIEW LETTERS week ending 31 DECEMBER 2004 0031-9007= 04=93(26)=265701(4)$22.50 265701-1  2004 The American Physical Society