Effect of supramolecular complexation of alkali hydrogenselenates with crown ethers and solid-solutions with their hydrogensulfate counterparts on the solid-to-solid phase transition behaviors†

This study investigates the structural and phase transition characteristics of supramolecular complexes composed of 18-crown-6 ether and hydrogen selenate (HSeO₄⁻) anions with various cations (K⁺, Rb⁺, Cs⁺). Single crystals of [18-crown-6·K]HSeO₄·2H₂O, [18-crown-6·Rb]HSeO₄·H₂O, [18-crown-6·Cs]HSeO₄·H₂O, [18-crown-6·K]HSeO₄, and [18-crown-6·K](HSeO₄)_0.5(HSO₄)_0.5 were grown and their structures determined via single-crystal X-ray diffraction. Differential scanning calorimetry and variable-temperature powder X-ray diffraction were employed to analyse dehydration and phase transition behaviors. The inclusion of 18-crown-6 ether significantly lowered the superprotonic phase transition temperatures by approximately 40 °C compared to pure solid acids. Additionally, substituting HSO₄⁻ with HSeO₄⁻ decreased phase transition temperatures for K and Cs-complexes and modified the phase transition behavior of the Rb-complex from two-step to single-step isostructural phase transition. Attempts to form solid solutions between the HSeO₄⁻ and HSO₄⁻ complexes yielded mixed results, with notable success in modulating phase transition temperatures in K-complexes.