Discriminating between the displacive vs. order-disorder character of a phase transition by magic angle spinning NMR

Abstract

Using squaric acid (H2C4O4) as an example, it is shown that the temperature dependence of the isotropic chemical shift (δISO), as measured by magic angle spinning (MAS) NMR, can serve as a probe of the displacive vs. order-disorder character of a phase transition. It is also found that the utilization of single crystals in the MAS measurements leads to significantly narrower peaks than using powders. In the case of MAS on squaric acid. single crystals exhibit a four-fold narrowing of the 13C peaks, as compared to those from powders. This additional gain in resolution has made it possible to measure a sharp anomaly in the δISO at the paraelectric-antiferroelectric phase transition of squaric acid (Tc ∼ 373 K). This anomaly is interpreted as a direct evidence that the transition mechanism involves a displacive component for the hydrogens as T- < Tc, suggesting that MAS/NMR can indeed serve as a new complement to the current methodologies for investigating phase transitions.