Squaric acid, phase transition

Fig. 7 CP-MAS spectra of squaric acid, peak assignment and temperature variation in the close vicinity of its paraelectric-antiferroelectric phase transition [24], Note the pronounced coexistence of the spectra from the two phases...

Fig. 8 Temperature dependence of the four NMR peaks of squaric acid [20]. Note how the four peaks coalesce to one above the phase transition, but that the average of the peak positions does not stay constant, as required for a pure order/disorder transition. It increases around the transition temperature, emphasizing an additional displacive component, coexisting with the order/disorder one...

Fig. 9 Temperature dependence of 5iso for the NMR peaks in squaric acid in the close vicinity of the phase transition temperature left panel). Note that the change starts as a smooth curve, followed by a jump caused by the first-order character of the phase transition. This is considered evidence for the coexistence of an order/disorder and displacive character in the phase transition mechanism [20]. The right panel gives a comparison between theoretical and experimental data. For details, see text...

Fig. 10 Temperature dependence of the average 0-H bond length as measured via the isotropic chemical shift for all carbon atoms in squaric acid. The change in the O bond length appears to play the role of an order parameter in the phase transition [20]...

An interesting example is available of inequivalent nuclei becoming equivalent on raising the temperature and going through a phase transition, reversibly. This occurs in squaric acid C4(0)2(0H)2, studied by single-crystal 13C NMR,29 and later (2004) also by 170 NMR. Here, MAS was used to narrow the lines sufficiently to observe the changes in the isotropic parts of the chemical-shift matrices. The phase transition occurs at 373 K. 

A. N. Klymachyov and N. S. Dalai, Magic angle spinning NMR on single crystals as a new aid in characterizing phase transitions application to squaric acid. Z. Phys. B, 1997,104, 651-656. 

Application of high external pressures influences the transition temperature to the antiferroelectric phase (Yasuda et al., 1979 Samara and Semmingsen, 1979). The Tq becomes lower as the applied pressure increases. Under an ultra-high pressure of about 3 GPa, the antiferroelectric transition itself disappears and the high dielectric constant of ca. 200 is maintained even at cryogenic temperatures (Moritomo et al., 1991). Since Raman diffraction measurements under 3-4.5 GPa revealed that squaric acid exists still as an alternating bond form, the tautomerization coupled with intermolecular proton transfer occurs even at low temperatures (Moritomo et al., 1990). 

Squaric acid 4,4 -Bipyridine Structure and characterization of the 1 1 cocrystal, and study of the single-crystal to single-crystal phase transition around 175 °C [50]... 

Fig. 11. (left) Relative volume concentration of the low- and high-temperature phases of squaric acid vs. temperature, (right) C CPMAS spectra of squaric acid at temperatures close to the phase-transition point. (Adapted with permission from ref. 174, Copyright 1996 Elsevier.)... 

Dalai, N., Klymachyov, A. and Bussmann-Holder, A. (1998). Coexistence of order-disorder and displacive features at the phase transitions in hydrogen-bonded solids squaric acid and its analogs. Phys. Rev. Lett. 81, 5924-5927. 

The first illustration is provided by ferroelectrics belonging to the family of pyridinium salts. Complex interplay between the contributions of van der Waals, Coulomb, dipolar and hydrogen-bonding interactions are expected because of the hybrid nature of the compound. The majority of reported NMR experiments are proton second-moment and relaxation studies on polycrystalline samples. The most sophisticated NMR methods with regard to resolution, symmetry and time-scale interpretations applied to the historical problem of assigning a pure order disorder or displacive mechanism to a ferroelectric phase transition will provide the second example with the study of squaric acids and perovskites compounds like BaTi03. 

N.S. Dalai, K.L. Pierce, J. Palomar, R. Fu, Single-crystal magic-angle spinning O NMR and theoretical studies of the antiferroelectric phase transition in squaric acid,... 

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