Comprehensively understanding the distinct properties of crystal structure, phase transition and dielectrics in [M(crown)n]ReO4 (M+ = Na+, K+, NH4+; crown = 15-crown-5, 18-crown-6; n = 1 or 2)

Abstract

In this study, three supramolecular compounds, namely [Na(18-crown-6)]ReO₄ (1), [K(18-crown-6)]ReO₄ (2) and [NH₄(15-crown-5)₂]ReO₄ (3) were synthesized and characterized using microanalysis, thermogravimetric, differential scanning calorimetric and powder X-ray diffraction techniques, among others. Herein, 15-crown-5 and 18-crown-6 represent 1,4,7,10,13-pentaoxacyclopentadecane and 1,4,7,10,13,16-hexaoxacyclooctadecane, respectively. Both 1 and 2 experienced one-step reversible phase transition at ∼436 K and ∼360 K, respectively, while 3 underwent three steps of reversible phase transitions at ∼289 K, ∼352 K and ∼380 K. A dielectric anomaly occurred at ∼433 K (∼363 K) for 1 (2), consistent with the temperature obtained from DSC measurement. In the dielectric spectra of 3, anomalies were observed at ∼386 K and ∼433 K, corresponding to two thermal anomalies observed at higher temperatures in the DSC plot. However, there was an absence of dielectric response around 289 K where a thermal anomaly appeared in the DSC plot. The distinct crystal structures, phase transitions and dielectric properties in 1–3 stem from the degree of compatibility between the radii of M⁺ (Na⁺, K⁺ and NH₄⁺) and the ring sizes of crown ethers (18-crown-6 and 15-crown-5).