Order parameter of the KDP ferroelectric phase transition measured by neutron diffractometry

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Abstract
Using a novel neutron high resolution double crystal arrangement, the spontaneous shear angle which is a direct measure of the order parameter has been determined in the vicinity of the first-order ferroelectric phase transition in both undeuterated (KDP) and 90%-deuterated (DKDP) potassium dihydrogen phosphate.The temperature homogeneity of the samples was better than 10-3 K, as was the temperature control.The measurements confirm the previously reported results concerning the coexistence of both PE and FE-phases during the isothermal part of the transition. Our data differ markedly from results previously published as to the thermal hysteresis and the lowest values of the shear angle. Within the accuracy of our temperature measurement, no hysteresis could be detected. At the onset of the transition we observed shear angles of 8.2 min. of arc for KDP and 24 min. of arc for DKDP. Whilst the shear angle remains constant for BKDP throughout the isothermal regime, it varies from 8.2 to 10.4 min. of arc in the case of KDP.