High-resolution measurements of thermal expansion along the unique b -axis of squaric acid

TLDR: The thermal expansion of squaric acid along the uniqueb-axis was measured between 323 K and 418 K with a resolution of 2·10−7 in strain and 5·10 −4 K in temperature using a capacitance dilatometer as mentioned in this paper.

Abstract: The thermal expansion of squaric acid along the uniqueb-axis was measured between 323 K and 418 K with a resolution of 2·10−7 in strain and 5·10−4 K in temperature using a capacitance dilatometer. The anomaly associated with the antiferro-electric phase transition occuring atTc=373.5 K was observed over the rangeTc−50 K<T<Tc+7 K. The regular lattice expansion follows a generalized Gruneisen's law with a Debye-temperature θD=660 K and a uniaxial pressure-dependence of∂θD/∂pb. The critical behaviour close toTc was studied with a sweep rate of 36 mK/h. A hyteresis loop of width 23 mK was observed in the strain curve, indicating a 1st order transition. However, no discontinuity in dilatation was observed, and therefore the transition is close to a multicritical point. The anomalous increase in the expansion coefficient can be described with critical exponents α=α′=0.56 in the interval 0.4 K<|T−Tc|<12 K. Closer toTc the divergence is stronger. A generalized Pippard relation between the expansion coefficient and specific heat holds to within 1 K from the transition point. The inconsistencies apparent closer toTc are interpreted as a consequence of a weak 1st order transition.