Hiroshi Fujihisa (Senior Researcher) and his colleagues in Accurate Crystal Structure Analysis Group, Research Institute of Instrumentation Frontier (Director, Masataka Ohkubo), the National Institute of Advanced Industrial Science and Technology (President, Ryoji Chubachi) determined the crystal structure of calcium under ultrahigh pressure, which has the highest superconducting transition temperature (Tc^*1) among all elements. This was achieved in collaboration with the Center for Quantum Science and Technology under Extreme Conditions (Director, Yoshito Tobe), Osaka University (President, Toshio Hirano), and Japan Synchrotron Radiation Research Institute (Director, Tetsuhisa Shirakawa).

In 2011, researchers at Osaka University discovered the presence of phase VII calcium under an ultrahigh pressure of 210 GPa or more and reported that this phase of calcium has the highest Tc of 29 K among all elements. However, the crystal structure of phase VII calcium has not been determined. In this study, the crystal structure of phase VII calcium was analyzed by X-ray powder diffraction^*2 experiment at SPring-8 and quantum chemical calculation and, as a result, a unique and complex host-guest structure^*3 was discovered.

It is expected that the highest Tc of this phase of calcium among all elements will be logically explained on the basis of the crystal structure determined in this study. Such work will also lead to the design of materials with higher Tc, which are desired in the scientific and industrial worlds.

This achievement was published in the American scientific journal Physical Review Letters on June 7, 2013.

*1 Superconducting transition temperature (Tc)

The temperature at which the electrical resistance of a material becomes zero (superconducting state) upon cooling  [ back ]

*2 X-ray powder diffraction

When a powder sample irradiated with X-rays, the X-rays are diffracted in various directions. The inner structure of a sample can be determined without destroying the sample by measuring the intensity of diffracted X-rays and the angle of the diffraction direction.  [ back ]

*3 Host-guest structure

A structure with a host framework having interior cavities that encapsulate guest atoms or guest molecules smaller than the cavities  [ back ]

Figure: Crystal structure of phase VII calcium analyzed in this study The host structure indicated in green and the guest structures indicated in blue and brown are combined.

Fig. 1: Successive phase transitions of calcium Calcium becomes a superconductor in phase III. Tc slightly decreases in phase V; however, it increases again and reaches the highest Tc of 29 K among all elements in phase VII. The question mark in the crystal structure of phase VII indicates that the structure is theoretically predicted but not established.

Fig. 2: (a) Already-known host-guest structure such as that observed in potassium and (b) crystal structure of phase VII calcium at 241 GPa analyzed in this study The top and bottom figures are different views of the same structure. All spheres are calcium atoms; the atoms forming the host structure are indicated in green and those forming the guest structures are indicated in blue and brown.