Stability and Metastability of Li3YCl6 and Li3HoCl6

Metastable solid electrolytes exhibit superior conductivity compared to stable ones, making them a subject of considerable interest. However, their synthesis of the metastable phase is affected by multiple thermodynamic and kinetic parameters, leading to ambiguity in the organization of stability and metastability. In this study, we organized remnant and intermediate metastability based on temperature. The intermediate metastable phase, which is less stable than the temperature-independent stable phase, typically transforms into the stable phase(s) at high temperatures. In contrast, the remnant metastable phase is formed by first obtaining most stable phase at specific temperatures and then “trapping” it by rapidly changing the temperature. By investigating Li+ conducting chlorides, Li3MCl6 (M = Y and Ho), we demonstrated that heating starting materials to approximately 600 K produced low-temperature Li3MCl6 phase with one formula unit while further heating resulted in high-temperature Li3MCl6 phase with three formula units. Annealing of quenched Li3MCl6 at 573 K resulted in a phase transition from the high-temperature to low-temperature phase, indicating that the high-temperature phase was remnant metastable at low temperatures. This study organizes remnant and intermediate metastability. The intermediate metastable phase, which is less stable than the thermodynamically independent stable phase, typically transforms into the stable phase(s) at high temperatures. In contrast, the remnant metastable phase once becomes the most stable phase under specific thermodynamic conditions, like such as ice in a freezer, and is then trapped by upon changing the conditions rapidly.