Unconventional and Powerful Ion Sources for Solid-State Ion Exchange, Cu₂SO₄ and Cu₃PO₄: Exemplified by the Synthesis of Metastable β-CuGaO₂ from Stable β-LiGaO₂

This study introduces a new method for synthesizing Cu⁺-containing metastable phases through ion exchange. Traditionally, CuCl has been used as a Cu⁺ ion source for solid-state ion exchanges; however, its thermodynamic driving force is often insufficient for complete ion exchange with Li⁺-containing precursors. First-principles calculations have identified Cu₂SO₄ and Cu₃PO₄ as more powerful alternatives, providing a higher driving force than CuCl. It has been experimentally demonstrated that these ion sources can open up new reaction pathways through experimental ion exchanges, such as from β-LiGaO₂ to β-CuGaO₂, which were previously unattainable. An important perspective provided by this study is that the potential of such simple compounds to act as powerful ion sources has been overlooked and that they were identified through straightforward first-principles calculations. This work presents the initial strategic design of an ion-exchange reaction by exploring suitable ion sources, thereby expanding the potential for synthesizing metastable materials.