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