Extreme pressure could force hydrogen into a high-temperature superconductor

A simple compound with lots of hydrogen may be a high-temperature …

Matthew Francis – Apr 4, 2012 7:00 am | 27

Crystal structure of clathrate calcium hydride. The smaller spheres are hydrogen, which surround the central calcium atom like a cage. Credit: Photograph by courtesy of Hui Wang

Superconductivity typically demands very low temperatures, requiring liquid helium or similar means to bring the temperature down to where electrical resistance is zero. Even the high-temperature superconductors have yet to come anywhere close to room temperature, topping out at approximately 110 degrees C above absolute zero (which is still 163 degrees below freezing). However, physicists have postulated that certain hydrogen-rich compounds may exhibit significantly higher transition temperatures.

A new simulation by Hui Wang et al. suggests that a calcium hydride compound (CaH₆) could have a critical transition temperature as high as 235 K (-38° C). The catch: the material must be subjected to pressures of approximately 150 gigapascals (150 GPa, or approximately 1.5 million atmospheres), pressures more typical of geological processes. The key to the pressure-driven transformation is the formation of a clathrate, or cage-like structure in the crystal lattice. The predicted electronic structure may allow the coupling between vibrations of the atoms (phonons) and electrons, leading to superconductivity.

The idea that simple molecular solids like calcium hydride can superconduct isn’t new. The addition of hydrogen molecules (H₂) at high pressures has been predicted to enable the required free flow of electrons. In this case, the researchers began with solid CaH₂ and studied what would happen if extra molecular hydrogen gas is added while the crystal is compressed.

Of the possible structures that resulted, CaH₄ was the most stable, but CaH₆ had the most promising electronic configuration: a stable, cage-like structure known as a clathrate, where the additional hydrogen formed new bonds to surround each calcium atom.

To model the calcium hydride compound, the researchers used ab initio calculations, which are standard in materials science. In this method, the electronic properties are calculated as directly as possible from first principles, while attempting to make as few assumptions as possible about the complex properties of the material.

The structure of CaH₆ proved to be remarkably stable and led to the types of electron-phonon interactions known to produce superconductivity in other materials. The temperature of transition (known as the critical temperature) was between 220 and 235 K, much higher than any known superconductor, and within reach of high-end refrigeration systems.

We know that pressure can create new superconducting phases in iron-based superconductors, so it wouldn’t be surprising if large pressures also play a role in novel superconductor types. Note, however, that pressures in the iron-based superconductors are in the 13 GPa range, so the predicted 150 GPa value for calcium hydride superconductors is much greater.

The model proposed by Wang et al. appears promising, but it should be noted that only one superconductor of a similar type (SiH₄) has been discovered experimentally, and its actual superconducting mechanism is uncertain. On the other hand, high-temperature superconductors have been frustrating to study in general, so if this relatively simple model corresponds to reality (despite requiring crushingly large pressures), it is certainly worth testing in the lab.

PNAS, 2012. DOI: 10.1073/pnas.1118168109 (About DOIs).