Distortions and stabilization of simple-cubic calcium at high pressure and low temperature.
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.
Mao
Wendy L
WL
Geological and Environmental Sciences, SLAC, [corrected] Stanford University, Stanford, CA 94305, USA. wmao@stanford.edu
Wang
Lin
L
Ding
Yang
Y
Yang
Wenge
W
Liu
Wenjun
W
Kim
Duck Young
DY
Luo
Wei
W
Ahuja
Rajeev
R
Meng
Yue
Y
Sinogeikin
Stas
S
Shu
Jinfu
J
Mao
Ho-kwang
HK