Solvate sponge crystals of (DMF)3NaClO4: reversible pressure/temperature controlled juicing in a melt/press-castable sodium-ion conductor

A new type of crystalline solid, termed “solvate sponge crystal”, is presented, and the chemical basis of its properties are explained for a melt- and press-castable solid sodium ion conductor. X-ray crystallography and atomistic simulations reveal details of atomic interactions and clustering in (DMF)<inf>3</inf>NaClO<inf>4</inf>and (DMF)<inf>2</inf>NaClO<inf>4</inf>(DMF =N-N′-dimethylformamide). External pressure or heating results in reversible expulsion of liquid DMF from (DMF)<inf>3</inf>NaClO<inf>4</inf>to generate (DMF)<inf>2</inf>NaClO<inf>4</inf>. The process reverses upon the release of pressure or cooling. Simulations reveal the mechanism of crystal “juicing,” as well as melting. In particular, cation-solvent clusters form a chain of octahedrally coordinated Na⁺-DMF networks, which have perchlorate ions present in a separate sublattice space in 3 : 1 stoichiometry. Upon heating and/or pressing, the Na⁺⋯DMF chains break and the replacement of a DMF molecule with a ClO<inf>4</inf>⁻anion per Na⁺ion leads to the conversion of the 3 : 1 stoichiometry to a 2 : 1 stoichiometry. The simulations reveal the anisotropic nature of pressure induced stoichiometric conversion. The results provide molecular level understanding of a solvate sponge crystal with novel and desirable physical castability properties for device fabrication.