Dielectric properties of the 250 K phase transition in BaMnF4 and of CsH2PO4 and CsD2PO4

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Abstract
The dielectric constant of BaMnF4 has been measured close to the 250 K phase transition along a, b and c crystal-lographic axes. The dielectric constant along the a axis shows a small peak at -23.3°C (∼250 K), whereas along the other two axes the dielectric constant changes monotonically with the temperature.Orthorhombic CsH2PO4 undergoes a ferroelectric transition at Tc = -119.5°C, whereas the ferroelectric transition temperature in isomorphous CsU2PO4 is Tc = -5.55°C. The transitions are of fust order in both cases. The rather large isotope effect demonstrates the importance of the O-H-O bonds in the transition mechanism.