期刊
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
卷 107, 期 22, 页码 9965-9968出版社
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1005279107
关键词
high pressure; phase transition; superconductivity
资金
- Department of Energy, Office of Basic Energy Sciences (DOE-BES)
- Division of Materials Sciences and Engineering [DE-AC02-76SF00515]
- DOE-BES [DE-SC0001057, DE-AC02-06CH11357]
- DOE-National Nuclear Security Administration (Carnegie/Department of Energy Alliance Center)
- National Science Foundation
- Wenner-Gren stiftelserna
Ca-III, the first superconducting calcium phase under pressure, was identified as simple-cubic (sc) by previous X-ray diffraction (XRD) experiments. In contrast, all previous theoretical calculations showed that sc had a higher enthalpy than many proposed structures and had an imaginary (unstable) phonon branch. By using our newly developed submicrometer high-pressure single-crystal XRD, cryogenic high-pressure XRD, and theoretical calculations, we demonstrate that Ca-III is neither exactly sc nor any of the lower-enthalpy phases, but sustains the sc-like, primitive unit by a rhombohedral distortion at 300 K and a monoclinic distortion below 30 K. This surprising discovery reveals a scenario that the high-pressure structure of calcium does not go to the zero-temperature global enthalpy minimum but is dictated by high-temperature anharmonicity and low-temperature metastability fine-tuned with phonon stability at the local minimum.
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