Synthesis and stability of hydrogen selenide compounds at high pressure - Dr. Ross Howie

NOVEMBER 20, 2017

The observation of high-temperature superconductivity in hydride sulfide (H2S) at high pressures has generated considerable interest in compressed hydrogen-rich compounds. The heavier hydrogen chalcogenides (i.e., H2Se and H2Te) are predicted to also exhibit high Tc superconductivity, however up until now remained experimentally unexplored. A new study led by Dr. Ross Howie of HPSTAR investigated the synthesis and stability of H2Se at high pressure and finds remarkable similarities with H2S. This study is published in J. Chem. Phys.

Hydrogen selenide is synthesized in situ from elemental Se and molecular H2 at pressures of 0.4 GPa and temperatures of 473 K. On compression at room temperature, the authors observe the high-pressure solid phase sequence (I-I’-IV) of H2Se through Raman spectroscopy and x-ray diffraction measurements, before dissociation into its constituent elements.

Through the compression of H2Se in H2 media, the authors also observe the formation of a host-guest structure, (H2Se)2H2, which is stable at the same conditions as H2Se, with respect to decomposition.

Recent x-ray synchrotron measurements have suggested that high-Tc superconducting phase of H2S could be due to the formation of H3S, which reforms upon partial decomposition of H2S at high pressure. This new study demonstrates that H2Se and H2Se–H2 mixtures behave very similar to H2S and H2S–H2 at high pressure and 300 K. Assuming the decomposition of H2Se can be stabilized by low temperatures, it is very plausible that H3Se could form at high pressures and exhibit superconductivity.

“Given that the behaviour of H2S at high pressure is not completely understood, these experiments show that we can learn a lot from the other hydrogen chalcogenides. Future experiments on H2Se at the conditions in which superconductivity is predicted could prove very insightful”, said Dr. Ross Howie.

This study involved Drs. Jack Binns, Philip Dalladay-Simpson, and Eugene Gregoryanz of HPSTAR and was in collaboration with Mr. Edward Pace and Dr. Miriam Peña Alvarez of the University of Edinburgh, both of whom were visitors at HPSTAR.

Caption: Photomicrographs of the synthesis of H2Se in a diamond-anvil cell sample chamber. The sample chamber is formed by a rhenium gasket. (a) H2 and solid Se at 0.4 GPa and room temperature, (b) H2Se–H2 mixture at 0.4 GPa and 473 K, (c) liquid phase separation of H2Se (central bubble) and H2 (surrounding area) at 0.4 GPa and room temperature after heating, (d) H2Se beginning to solidify, (e) slow coalescence of H2Se regions, (f) completely solidified H2Se at 1.5 GPa.

寻找高温超导材料一直是凝聚态物理研究领域的热点课题, H₂S在高压下~200K高温超导的发现刷新了高温超导的记录。与H2S类似的氢基硫族化合物，H₂Se的高压稳定性及超导性质等也引起了物理学家们的广泛关注。北京高压科学研究中心的Howie研究员领导的团队一直致力于氢，及其化合物的高压研究。该团队使用高压原位X射线衍射，拉曼光谱研究了H₂Se的高压稳定性。他们发现H₂Se在高压下表现出与H2S非常类似的结构稳定性，因此进一步研究或证实H2Se的超导性很有必要。