Expanding the Potential of High-Temperature Superconductivity
Electrochemical potential and applied pressure have each been used extensively to prepare new materials, such as pure aluminum (electrochemistry) and synthetic diamond (applied pressure). However, these two fundamentally important approaches to chemical synthesis have not been combined, in part due to the difficulties of designing an apparatus that will perform electrochemistry inside a diamond anvil cell. If the associated technical challenges could be overcome, the combination of electrochemistry and pressure (Fig. 1) could provide a powerful new method for the exploration of composition space in the effort to design new materials with tailored properties.
Recent evidence for room-temperature and even “hot” superconductivity in high-pressure hydrides beyond binary compositions suggests that increasing chemical complexity is a key requirement for increasing the superconducting critical temperature (Tc) for these materials. Static pressures near 200 GPa are required to stabilize superconducting superhydrides, and this is currently only achievable using the diamond anvil cell. More importantly, it is often the case that a complex superhydride decomposes into constituent simpler superhydrides even at such high pressures. Many complex superhydrides therefore may never achieve thermodynamic stability by pressure alone, thus preventing thorough investigation of their interesting and important properties.
Following previous work that demonstrated the concept of combing pressure and electrochemistry to synthesize binary superhydrides, a new theoretical study carried out in a collaboration between researchers from Carnegie-Mellon University, Jilin University and the University of Illinois Chicago has recently extended this concept to a ternary hydride system, Li-Mg-H, where Li2MgH16 was previously calculated to have a superconducting critical temperature of ~ 470 K at 250 GPa. However, this complex phase is thermodynamically unstable against binary hydrides.
Combing first-principles calculations, crystal structure prediction and computational thermodynamics, phase diagrams of the Li-Mg-H system can be mapped over the space of composition, pressure and electrochemical conditions (electrode potential and pH), with the one at a fixed Mg/Li ratio between 0 and 0.25. Two ternary Li-Mg superhydrides, Li2MgH16 and Li4MgH24 can be thermodynamically stabilized at suitable negative electrode potentials, if the hydrogen evolution reaction (HER) can be kinetically suppressed, which may be achieved by superconcentrated electrolytes or other mechanisms. The ground state of Li2MgH16 undergoes two polymorphic phase transitions at 33 and 160 GPa. The highest pressure phase is superconducting, while the two lower pressure phases are not.
This work shows the great potential of combing pressure and electrochemistry to synthesize novel multi-component superhydrides at low pressures, which may not be achieved even by applying multimegabar pressure alone. In practice, the highest achievable hydrogenation will depend on suppressing HER and on engineering issues like maintaining structural integrity of the highly hydrogenated electrode. Such a vast space of novel phases will provide many exciting opportunities for experimental and further theoretical research.
Guan, P.-W., Y. Sun, R. J. Hemley, H. Liu, Y. Ma, and V. Viswanathan, Low-pressure electrochemical synthesis of complex high-pressure superconducting superhydrides. Physical Review Letters 128, 186001 (2022).
Figure 1. Applied pressure and electrochemistry are powerful methods for the synthesis of new materials, and the combination of the two is proposed to provide additional avenues for stabilization of complex superhydrides.