The potential for developing superconductive materials that can conduct electricity without creating resistance or friction remains an elusive long-term research objective. Researchers have been trying to identify superconductive materials that exhibit this unique property at room temperature.

In a previous TLT article,¹ the synthesis of lanthanide superhydride, a rare Earth metal derivative, was discussed. Electrical resistance measurements showed that this material exhibits superconductivity at 250-260 K under a pressure above 175 gigapascals (GPa), which is approximately 30-40 K below room temperature. The researchers did not determine the mechanism for how lanthanum superhydride exhibits superconductivity at a temperature very close to room temperature.

Preparation of superconductive materials has to occur under extreme-pressure conditions. Ranga Dias, assistant professor of mechanical engineering and of physics and astronomy at the University of Rochester in Rochester, N.Y., explains, “Superconductivity can only be achieved when a material is placed in optimum conditions where there is strong electron phonon coupling within the stability of the correct structure. Extreme pressure acts in a similar manner as a search engine to scan the potential phases of a specific material to find the optimum conditions. In conducting experiments to find the right material, the pressure is increased in an effort to change the chemical identity of the material.”

Research efforts to develop a superconductive material at room temperature have focused on synthesis of metallic hydrogen. Dias says, “Hydrogen is not an easy element to work with as enormous pressures are needed to force it into a metallic state. Doping with another element can facilitate the formation of metallic hydrogen at lower pressures.”

Dias determined that one approach to take to improve the prospects of room temperature superconductivity was to produce covalent liquid metals, particularly those that are based on carbon and sulfur. He says, “Carbon disulfide was found to demonstrate superconductivity at extremely low temperatures (6-7 K) but under 50 GPa of pressure. Covalent liquid metals such as magnesium boride also have exhibited superconductivity.”

The appealing aspect of working with carbon and sulfur is that both elements exhibit high degrees of coordination and have extra space enabling them to be converted into various structures under extreme-pressure conditions. New research examining the interactions of carbon and sulfur has now led to the detection of superconductivity in a material at room temperature.

Photochemical process
Dias and his colleagues have identified a material that exhibits superconductivity properties at a maximum temperature of 287 K under a pressure of 267 GPa. This is the first report of superconductivity at room temperature.

The researchers initiated preparation of this material by mixing equimolar amounts of carbon and sulfur, which is ball milled to a particle size less than 5 microns and placed in a diamond anvil cell (see Figure 1). Molecular hydrogen gas is then loaded at 3 kbar to act as both a reactant and as a pressure transmitting medium.


Figure 1. A diamond anvil cell was used by researchers to produce a material that exhibits superconductivity properties at room temperature. Figure courtesy of the University of Rochester.

The initial reaction conditions were to apply 3.9 GPa of pressure and irradiate the starting materials using 532 nanometer laser light. Dias says, “Our reason for irradiating the sample was to trigger a photochemical reaction. The alpha phase of sulfur exhibits its highest degree of photosensitivity under these conditions. Sulfur-sulfur bonds break under these conditions leading to the formation of free radicals that can either self-react to form different structures or react with hydrogen to form hydrogen sulfide.”

The intent of the researchers was to create conditions that can lead to the formation of new species. Raman spectroscopy showed that a mixed alloy of hydrogen sulfide and methane most likely formed under these conditions.

Dias says, “We subjected the reaction mixture to higher pressures in an effort to make a metal. The initial material formed at 3.0 GPa was an organic species that did not show any metallic properties.”

Eventually a superconductive material was produced when the pressure was increased to 267 GPa. The researchers do not know the structure of the material, as it is very difficult to use X-ray diffraction to better understand its composition. Dias believes that the material is a ternary carbon-sulfur-hydrogen system that behaves differently from pure hydrogen sulfide.

He says, “We determined that the room temperature superconductive material exhibited bulk superconductivity, which permeates through the entire material. This is in contrast to filamentary superconductivity that characterizes a material that shows superconductivity just at its surface.”

Dias recognizes that a new analytical technique needs to be developed to better understand the structure of the room temperature superconductive material. He says, “Our ultimate objective is to eventually produce a material that exhibits superconductivity under ambient temperature and pressure. The next step is to first produce a material that is a superconductor at ambient temperature under a lower pressure of approximately 50 GPa.”

The researchers will be using chemical tuning to adjust the ratios of the raw materials and possibly adding a dopant to reach their objective. Dias says, “Our next challenge will be to produce these materials that are stable (or metastable) at ambient pressure via ‘compositional tuning’ so they will be even more economical to mass produce. We have formed a new company called Unearthly Materials Inc., which will be focused on making this material at ambient pressure.”

Additional information can be found in a recent article² or by contacting Dias at rdias@rochester.edu.

REFERENCES
1. Canter, N. (2019), “Lanthanide superhydride: Potential for superconductivity near room temperature,” TLT, 75 (5), pp. 18-19.
2. Snider, E., Gammon, N., McBride, R., Debessai, M., Vencatasamy, K., Vindana, H., Lawler, K., Salamat, A. and Dias, R. (2020), “Room temperature superconductivity in a photochemically transformed carbonaceous sulfur hydride,” Nature, 586 (7829), pp. 373-377.