Electrical conductivity, DSC, XRD, and 7Li NMR studies of rotator crystals n-C21H43COOLi x K(1 − x) (0.33 ≤ x ≤ 0.50), n-C m H(2m + 1)COOLi, and n-C m H(2m + 1)COOK (m = 13, 15, 17, 19, and 21)

Abstract

Differential scanning calorimetry (DSC) thermograms, X-ray diffraction (XRD) analysis, electrical conductivity (σ), and 7Li NMR spectroscopy characterization of n-CmH(2m + 1)COOM solids (M = Li, Na, K; m = 13, 15, 17, 19, 21) and mixed crystals n-C21H43COOLixK(1 − x) (0.25 ≤ x ≤ 0.75) was performed as a function of temperature. DSC thermograms of n-CmH(2m + 1)COOM revealed several solid-solid phase transitions with large entropy changes. Electrical conductivity studies established that n-CmH(2m + 1)COOLi crystals are poor electrical conductors. In contrast, n-CmH(2m + 1)COOK salts were found to have σ values of 10 − 7–10 − 8 S·cm − 1. Since the crystal structures and phase-transition temperatures of both n-CmH(2m + 1)COOLi and n-CmH(2m + 1)COOK crystals were similar, they were able to form mixed crystals with the structure n-CxH(2m + 1)COOLixK(1 − x). DSC thermograms of the mixed crystals showed a small entropy change at the melting point (ΔSmp < 13 J K − 1 mol − 1), in addition, large ΔS values at the solid-solid phase transition temperature. The σ values obtained for mixed crystals were roughly one order of magnitude greater than those determined for n-C21H43COOK crystals. 7Li NMR spectra of the mixed crystals recorded at various temperatures suggested that the self-diffusion of Li + ions was excited in the highest-temperature solid phase. Based on these results, we have classified these mixed crystals as rotator crystals.