Phase diagram of solid oxygen at low temperature and high pressure

1. Phase diagram of solid oxygen at low temperature and high pressure ReferenceF. Gorelli, M. Santoro, L. Ulivi, M. Hanfland, Phys. Rev. B 65, 172106 (2002) SHIMIZUGroup MIZOBATAShigeki

2. Contents • Introduction・ Character of oxygen・ Many phases of solid oxygen・ Phase diagram by Raman scattering • Experimental results and discussion • Summary

3. oxygen magneticelementary molecule important role molecular arrangement in crystal, compressibility of crystal solid oxygen liquid oxygen boiling point・・・90 K light blue and paramagnetic liquid freezing point・・・54 Ksuperconductivity(at 120GPa Tc=0.6 K) http://www.webelements.com/ Oxygen

4. Solid oxygen • at ambient pressure 23.5 90 K 43.5 54 T(K) cubic rhombohedral monoclinic γ β α liquid short-range AFM order antiferromagnetism (AFM) paramagnetism • at room temperature 5.5 GPa 9.5 10 96 P(GPa) orthorhombic monoclinic rhombohedral monoclinic ε ζ β δ molecular insulator molecular metal

5. ? P-T phase diagram by Raman scattering α: monoclinic δ: orthorhombicexistence of newphases (α´ and δ´) ? S. Desgreniers, Y.K. Vohra, and A.L. Ruoff, J. Phys. Chem. 94, 1117 (1990) • At low temperature and high pressurethephase boundaries are uncertain.

6. Problems • At low temperature and high pressurethephase boundaries are uncertain. clarify the phase transitions by x-ray diffraction experiment extreme condition (at low temperatureand highpressure) Oxygen is low-Zelement and gas. technical difficulty

7. sample ruby gasket Experiment 1 • sample loading for oxygen cool sample loading gas liquid • gas-loading cell (membrane cell) http://www.diacellproducts.com/index.html The gas-loading cell is used to enable the increase of pressure at low temperature.

8. Temperature (K) δ ？ α Pressure (GPa) Experimental process 1 isothermal compression or decompression・・・・ at 19, 180, 240, 277, 300 K

9. X-ray diffraction pattern • From diffraction pattern at 19 K ・・・・ • 0.96-7.2 GPa: diffraction patterns for the α phase • 7.6 GPa: two phases (α phase and ε phase） coexist • 8.1 GPa: diffraction pattern for ε phase Y. Akahama, Phy. Rev. B, 64, 054105 (2001)

10. ・・・α phase ・・・β phase ・・・ε phase ・・・δ phase Result 1 • No other phases（α´ and δ´ phases） are detected. • The α phase directly transforms to the ε phase. α-δ boundary E. Uemura et al, J. Phys. Condens. Matter 14, 10423 (2002)

11. Experimental process 2 to produce a fine-grained polycrystalline powder・・・・ very rapid pressure increase up to about 40–50 GPa Temperature (K) decrease pressure until the ε-δ phase transition δ ？ from δ phase・・・・ decreasing slowly temperature and pressure α Pressure (GPa)

12. c 90° b a β´ β´ c monoclinic (α phase) orthorhombic (δ phase) shift of the ab plane (angle β´turns exactly to 90°) b a Structure of α- and δ-oxygen δ- o2 orthorhombic (Fmmm) α- o2 monoclinic （C2/m）

13. β´≠90°・・・monoclinic (α) β´＝90°・・・orthorhombic (δ) The α-δ phase transition and the anisotropy of compressibility are observed. line ・・・ R.D Etters et al., Phys. Rev. B 32, 4097 (1985) Y. Akahama et al., Phys. Rev. B 64, 054105 (2001) The α-δ phase transition δ

14. ○ ・・・・α phase ● ・・・・δ phase ・・・・α-δ transition Result 2 • No other phases（α´ and δ´ phase） are detected. • The δ phase is stable at wide pressure.

15. δ Result 1 65 K Result 2 α Hysteresis Liquid 300 β Temperature(K) 200 ε 100 0 12 4 10 8 6 Pressure(GPa) • The α phase transforms again to the δ phase increasing the temperature up to 270K.

16. Summary • No other phases（α´ and δ´ phase） are detected. • By tow X-ray diffraction studies, position of phase-transition boundaries are determined,respectively. • The hysteresis is observed at low temperature and high pressure.