Clarification of crystal structure of calcium having highest superconducting transition temperature among all elements

July 31st, 2013

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.

Researchers from the National Institute of Advanced Industrial Science and Technology have determined the crystal structure of calcium under ultrahigh pressure, which has the highest superconducting transition temperature (Tc) among all elements.

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 experiment at SPring-8 and quantum chemical calculation and, as a result, a unique and complex host-guest structure 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.

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.

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.

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

Provided by Advanced Industrial Science and Technology