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 atT c=373.5 K was observed over the rangeT c−50 K<T<T c+7 K. The regular lattice expansion follows a generalized Grüneisen's law with a Debye-temperature θ D =660 K and a uniaxial pressure-dependence of∂θ D /∂p b . The critical behaviour close toT c 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−T c|<12 K. Closer toT c 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 toT c are interpreted as a consequence of a weak 1st order transition.
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Johansen, T.H., Feder, J. & Jøssang, T. High-resolution measurements of thermal expansion along the uniqueb-axis of squaric acid. Z. Physik B - Condensed Matter 56, 41–49 (1984). https://doi.org/10.1007/BF01470211
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DOI: https://doi.org/10.1007/BF01470211