A Supramolecular Ferroelectric Realized by Collective Proton Transfer

Hydrogen-bonded donor–acceptor compounds for organic ferroelectric materials

The pressure dependences of the dielectric properties and phase transitions of squaric acid, H2C4O4, and its deuterated analog, D2C4O4, were investigated. The transition temperatures at 1 bar are 375.5±1.0 K and 527.5±2.0 K, respectively, and decrease with pressure at initial rates of 105.6±2.0 K/GPa and 102.5±2.0K/GPa. It is argued that the large hydrogen isotope effect on Tc can be accounted for on the basis of the elongation of the O–H⋅⋅⋅O bond resulting from deuteration. This elongation causes a modification of the double‐well potential along this bond. It is not necessary to invoke proton tunneling. The lack of an appreciable isotope effect on dTc/dP supports the absence of tunneling. An interesting aspect of the results is the appearance of a dome‐shaped feature in the real part of the dielectric constant just above Tc in both crystals. This feature, which is reminiscent of the behavior of the magnetic susceptibility of antiferromagnets above the Neel temperature, is discussed. The results on D2C4O4...

Effects of pressure on the dielectric properties and phase transitions of the 2‐D antiferroelectric squaric acid: (H2C4O4 and D2C4O4)

Temperature dependence of the order parameter of the two dimensional antiferroelectric squaric acid as determined from birefringence measurements

The crystal structure of squaric acid [3,4-dihydroxy-3-cyclobutene-1,2-dione, H 2 C 4 O 4 , M = 114.1] has been determined from neutron diffraction data at 15 K, T c - 21K, T c - 13 K and T c + 5 K below and above the phase transition at 374 K. The space group of the material is monoclinic P2 1 /m from 15 K and up to T c and tetragonal 14/m above. After corrections for absorption and thermal diffuse scattering, least-squares refinements were carried out with 69 variable parameters in the model below T c and 25 above T c . The analysis of difference vibrational parameters along the C-O internuclear direction strongly suggest that the high-temperature structure has a disordered C 4 O 4 unit. In agreement with previous findings, the protons are distributed on two principal positions 0.53 A apart along the O... O hydrogen bond above T c . In the low-temperature phase, an increasing amount of disorder with increasing temperature is found, particularly in the protonic positions. However, the protons in the two different hydrogen bonds show distinctly different behavior, and hence immediately below T c different degrees of order are found along the x- and z-directions. These structural findings are discussed in the light of the 1-D correlations observed in the material.

Dag Semmingsen

Papers

The phase transition of two-dimensional squaric acid H2C4O4 studied by neutron scattering