Crystal Structures of Calcium IV and V under High Pressure

Abstract

High-pressure phases IV and V of calcium discovered in 2005 have the highest superconducting transition temperature of 25 K among all the elements; however, their crystal structures have not been determined. From the x-ray powder diffraction data, both Ca IV and V have been found to form unique and complex structures with a coordination number of 7. They were confirmed to be identical to the theoretical models that were recently predicted [Ishikawa, Phys. Rev. B 77 020101(R) (2008)].

Temperature-driven phase transitions from first principles including all relevant excitations: The fcc-to-bcc transition in Ca

“…(6). Focussing first on the temperature region relevant for the phase transition (716 K) we find vacancy concentrations which are well below 10…”

“…Much interest in calcium is due to its remarkable superconducting properties. 6 As a consequence pressure-induced transitions have been intensively studied with ab initio methods. Moreover, calcium shows also a temperatureinduced transition from fcc to bcc.…”

Distortions and stabilization of simple-cubic calcium at high pressure and low temperature

“…The series of surprising high-pressure phase transitions (6)(7)(8)(9) that change the elemental Ca from an ordinary metal to the highest known T c (>25 K) elemental superconductor (8) has emerged as a central topic as demonstrated in many concurrent investigations (10-17) but also presented a seemingly irreconcilable gap between theory and experiment. X-ray diffraction (XRD) at 300 K indicates that Ca transforms from its ambient condition fcc structure Ca-I to bodycentered cubic (bcc) Ca-II at 20 GPa and then to simple-cubic (sc) Ca-III at 32 GPa (7,9,18). At higher pressures, Ca-III undergoes two additional transformations, to Ca-IV and V at 119 and 143 GPa, respectively (7,9,18).…”

“…X-ray diffraction (XRD) at 300 K indicates that Ca transforms from its ambient condition fcc structure Ca-I to bodycentered cubic (bcc) Ca-II at 20 GPa and then to simple-cubic (sc) Ca-III at 32 GPa (7,9,18). At higher pressures, Ca-III undergoes two additional transformations, to Ca-IV and V at 119 and 143 GPa, respectively (7,9,18). Ca-III, IV, and V are all superconducting (8).…”

Competing phases, strong electron-phonon interaction, and superconductivity in elemental calcium under high pressure

“…So far the higher pressure phases Ca-IV and Ca-V have attracted the most attention, and considerable progress has been made in identifying these phases through a combination of experimental 9,22,23 and theoretical 24,25,26 work. However, satisfactory agreement between experimental and theoretical work is still lacking.…”