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Metastable solid electrolytes exhibit superior conductivity compared to stable ones, making them a subject of considerable interest. However, solid-state synthesis of the metastable phase is affected by multiple thermodynamic and kinetic parameters, leading to ambiguity in the categorization of stability and metastability. This study categorizes remnant and intermediate metastability based on thermodynamic principles. 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, such as the high-temperature stable phase obtained by fast cooling, becomes the most stable phase, and annealing of the remnant metastable phase causes the phase transition to the low-temperature stable phase. Investigating Li+ conducting chlorides, Li3MCl6 (M = Y and Ho), this study shows that heating starting materials to approximately 600 K produced low-temperature Li3MCl6 phase with one formula unit; high-temperature Li3MCl6 with three formula units were observed by further heating. Annealing of quenched Li3MCl6 at 573 K resulted in a phase transition from high-temperature to low-temperature, indicating that the high-temperature phase was remnant metastable at low temperatures. XRD patterns of low-temperature phases suggest the presence of stacking faults that stabilize low-temperature structures with high symmetry.

The structural and electronic properties of BiCoO(3) under high pressure have been investigated. Synchrotron X-ray and neutron powder diffraction studies show that the structure changes from a polar PbTiO(3) type to a centrosymmetric GdFeO(3) type above 3 GPa with a large volume decrease of 13% at room temperature revealing a spin-state change. The first-order transition is accompanied by a drop of electrical resistivity. Structural results show that Co(3+) is present in the low spin state at high pressures, but X-ray emission spectra suggest that the intermediate spin state is present. The pressure-temperature phase diagram of BiCoO(3) has been constructed enabling the transition temperature at ambient pressure to be estimated as 800-900 K.