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Simple cubic (SC) phase has been long experimentally determined as thehigh-pressure phase III of elemental calcium (Ca) since 1984. However, recentdensity functional calculations within semi-local approximation showed thatthis SC phase is structurally unstable by exhibiting severely imaginaryphonons, and is energetically unstable with respect to a theoreticalbody-centered tetragonal I41/amd structure over the pressure range of phaseIII. These calculations generated extensive debates on the validity of SCphase. Here we have re-examined the SC structure by performing more precisedensity functional calculations within hybrid functionals ofHeyd-Scuseria-Erhzerhof (HSE) and PBE0. Our calculations were able torationalize fundamentally the phase stability of SC structure over all otherknown phases by evidence of its actual energetic stability above 33 GPa and itsintrinsically dynamical stability without showing any imaginary phonons in theentire pressure range studied. We further established that the long-thoughttheoretical I41/amd structure remains stable in a narrow pressure range beforeentering SC phase and is actually the structure of experimental Ca-III'synthesized recently at low temperature 14 K as supported by the excellentagreement between our simulated X-ray diffraction patterns and the experimentaldata. Our results shed strong light on the crucial role played by the preciseelectron exchange energy in a proper description of the potential energy of Ca.

Hybrid Functional Study Rationalizes the Simple Cubic Phase of Calcium at High Pressures