Temperature-Tuned Variable Confined Space for Modulating Dipolar Polarization of a Disc-Shaped Ammonium Ion

Free accessible confined space and loose interaction
are crucial
for most solid-state ionic motions. Here, by using a near-spherical
anion and a disc-shaped ammonium as two distinct but rigid building
blocks, we report a new ionic crystal, (HMIm)3[La(NO3)6] (HMIm = 1-methyl-1H-imidazol-3-ium),
in which the different confined spaces of three (HMIm)+ ions are fine-tuned over a broad temperature range. This effect
can be utilized to modulate the dipolar polarization across a wide
temperature/frequency range. Additionally, small-scale substitution
of (HMIm)+ by its isomer of almost identical shape/size
affords molecular solid solutions, which can further tune the dipolar
polarization by varying the doping ratio. It is revealed that the
differences in dipole moment and hydrogen bond rather than that of
shape/size lead to a distorted crystalline environment for these solid
solutions. Overall, we provide an exceptional model for understanding
and regulating the dipole motion of polar aromatic molecules/ions
in a crystalline environment.