A new type of crystalline solid changes phase and reversibly releases one equivalent of solvent when heated or printed. It’s “a unique behavior that we haven’t seen in any previous material,” says Michael Zdilla of Temple University in the US, part of a team that accidentally discovered the property. “In my experience, accidents are the most exciting thing that can happen in science.”
Zdilla and colleagues were studying solid electrolyte materials with conductivities close to those of ceramic when they came across the unique system they refer to as the sponge solvate crystal. Typically, when crystals are heated, either the material melts, does not lose any solvent, and forms again when cooled; or it melts and then decomposes and the materials cannot reform when cooled. This crystal is different – instead of melting or decomposing, it transforms into a different solvate.
The co-crystal initially forms the 3: 1 solvate (DMF)3NaClO4thHowever, pressure releases one equivalent of DMF from the system, shifting the structure to the 2: 1 solvate (DMF).2NaClO4th. After releasing the pressure, the 2: 1 solvate reabsorbs the DMF and reforms (DMF)3NaClO4th. Since the 3: 1 solvate has a hexagonal crystal shape, high pressure in the x and y directions easily deforms the crystal; with ClO4th– – Ions that were forced to places originally occupied by Na+ Ions, which cause DMF to be ejected from the side of the crystal – resulting in a thin surface layer of DMF. However, when pressure is applied in the z-direction, two or three Na-DMF contacts are broken and replaced by Na-ClO4th Contacts and a shift from the 3: 1 solvate form to the monoclinic 2: 1 solvate form with a 27% lower molar volume can be seen. This reversible shift also occurs when heating and cooling the system.
Jagadese Vittal, an expert in crystal engineering at the National University of Singapore, says, “This is a simple solvated inorganic salt crystal material that, interestingly, behaves like a sponge due to changes in pressure and temperature. In addition, this work opens a new avenue for melt-castable crystalline materials for various applications. ‘
Further examples of this discovery could “lead to a new class of potential energy storage materials,” says Zdilla’s colleague Prabhat Prakash from the Indian Institute for Science Education and Research in Pune. ‘We are investigating battery applications for such materials because these crystals have sodium ion channels with sufficiently small energy barriers to conduct. In the present work, perchlorate and DMF are reduced slightly on a typical battery anode – our plan is to discover more such crystals with more redox-stable components. ‘