Antiferroelectric Phase Transition in a Proton-Transfer Salt of Squaric Acid and 2,3-Dimethylpyrazine

A proton-transfer
reaction between squaric acid (H2sq)
and 2,3-dimethylpyrazine (2,3-Me2pyz) results in crystallization
of a new organic antiferroelectric (AFE), (2,3-Me2pyzH+)­(Hsq–)·H2O (1), which possesses a layered structure. The structure of each layer
can be described as partitioned into strips lined with methyl groups
of the Me2pyzH+ cations and strips featuring
extensive hydrogen bonding between the Hsq– anions
and water molecules. Variable-temperature dielectric measurements
and crystal structures determined through a combination of single-crystal
X-ray and neutron diffraction reveal an AFE ordering at 104 K. The
phase transition is driven by ordering of protons within the hydrogen-bonded
strips. Considering the extent of proton transfer, the paraelectric
(PE) state can be formulated as (2,3-Me2pyzH+)2(Hsq23–)­(H5O2+), whereas the AFE phase can be described as (2,3-Me2pyzH+)­(Hsq–)­(H2O).
The structural transition caused by the localization of protons results
in the change in color from yellow in the PE state to colorless in
the AFE state. The occurrence and mechanism of the AFE phase transition
have been also confirmed by heat capacity measurements and variable-temperature
infrared and Raman spectroscopy. This work demonstrates a potentially
promising approach to the design of new electrically ordered materials
by engineering molecule-based crystal structures in which hydrogen-bonding interactions
are intentionally partitioned into quasi-one-dimensional regions.