Phase transitions of sulfur under high-pressure and high-temperature condition

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High-pressure and high-temperature behavior of sulfur was directly observed, using a combination of multi-anvil type high-pressure apparatus"MAX80"and synchrotron radiation. Observable pressure and temperature range was expanded up to 17 GPa and 900℃ by using sintered diamond anvils. Three high-pressure phases were found in the range. All high-pressure phases were unquenchable at ambient condition. Crystal structures of the high-pressure phases were determined from their X-ray powder diffraction patterns. The high-pressure phase 1 has a hexagonal cell, in which sulfur atoms form helical chains. The high-pressure phase 2 has another hexagonal cell and the structure is built up of cyclic molecule of S_6. The high-pressure phase 3 contains another helical chains in a tetragonal cell. Compression behaviors of all high-pressure phases and the low-pressure phase were observed at room temperature condition. An anisotropic compression behavior was found in the case of the high-pressure phase 2. The pressure and temperature phase diagram of sulfur was also determined, based on the in-situ observations.