The structure and dielectric properties of ionic compounds with flexible ammonium moiety

The 3-morpholin-4-yl-propyl-ammine tetrafluoroborate (MPA+) (BF4−) (1) and the 3-morpholin-4-yl-propyl-ammine perchlorate (MPA+) (ClO4−) (2) were synthesized and separated as colorless block crystal. The (MPA+) (BF4−) undergoes a reversible phase transition at ca. 347.3 K with a hysteresis of 16.1 K. Dielectric measurement also can confirm the transition. The crystal structure determined at 300 K (monoclinic, P21/c, a = 9.978(2) Å, b = 7.5206(15) Ǻ, c = 15.058(3) Å, β = 96.99(3)°, v = 1121.6(4) Å3, z = 4) and 373 K (triclinic, P-1, a = 10.021(7) Å, b = 7.662(3) Å, c = 7.920(4) Å, α = 95.012(14)°, β = 101.67(4)°, γ = 91.48(3)°, v = 592.7(6) Å3, z = 2) reveal that this compound has two phase. The most distinct is the BF4− anion is disorder at 373 K, which is probably the driving force of the phase transition. DSC measurement, dielectric measurement and the crystal structure of the compound 2 did not change at 300 K and 100 K.

The most distinct difference between room temperature and high-temperature structures is the order–disorder transition of the BF4− anion, which is the driving force of the phase transition.Figure optionsDownload as PowerPoint slide