Temperature and Frequency Dependence of the Dielectric Constant and Proton NMR near the Structural Phase Transition of Squaric Acid

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Abstract
The frequency‐and temperature‐dependent dielectric constant is measured in directions parallel to the molecular layers of squarie acid crystals. There is a monodispersive relaxational dispersion step in the low‐frequency regime Which is ascribed to a thermally activated diffusional motion. The dielectric constant, which is the high‐frequency limit of this dispersion step, is influenced by the 100°C structural phase transition in a characteristic manner. In order to account for the observed behaviour different models are discussed. The measured orientational dependence of the proton NMR spectrum is related to the structural data.