Untersuchung der strukturellen Phasenübergänge und Domänenbildung in den ferroischen Modellsubstanzen RbH 2 PO 4 und RbD 2 PO 4
RDP (RbH2PO4) belongs to the well known family of hydrogen bonded ferroelectrics with the composition A(H,D)2XO4 (A=K,Rb,Cs,Tl and X=P,As). On KDP (KH2PO4), the root member of the family, Busch and Scherrer discovered in 1935 the ferroelectric behaviour. A rich number of works concerning the ferroelectric - paraelectric phase transition of KDP investigated by a great variety of methods have been published. But the principal mechanism of the phase transition is still not understood. One of the basic problems here is the so called isotope effect. Just by an exchange of the protons with deuterium atoms Tc increases from 123K (KDP) to 220K (DKDP (KD2PO4)). The present work is divided into two parts. The first part is regarded to the polymorphism of RDP and DRDP. Both are less detailed investigated then in KDP and DKDP. Therefor single crystal diffraction experiments with neutron and X-ray radiation have been undertaken. As a result two new structures have been discovered and disagreements in the polymorphism are understood. The second part concentrates on the paraelectric - ferroelectric phase transition. Here are first high resolution X-ray single diffraction experiments as a function of electrical field and temperature presented. Both parts of this work have revealed a new role of the hydrogen/deuterium atoms in the structure of all phases in RDP and DRDP. The hydrogen/ deuterium atoms were interpreted in former works as individual parts of the structure. But in contrast to this they are forming [PO4H2]-groups. The former separation into ordered and disordered (H,D)-distributions is just a consequence of the in time and space averaging investigation methods. In terms of [PO4H2]-groups the paraelectric - ferroelectric phase transition of RDP(II)/(I) is of disorder - order type. In the tetragonal HT-structure local orthorhombic short range clusters with a resulting dipol moment are formed. In approaching Tc, with decreasing temperature, these clusters are growing and the resulting polarisation increases. The clusters with anti parallel polarisation are arranged by long range dipol - dipol interactions. With a critical size the growing of the clusters is blocked and 180°-twin lamellae of the ferroelectric phase are formed. Although the phase transition has a group - subgroup relation of index two, which is regarding to Landau the sufficient condition for a order disorder type, with the abrupt formation of the twin lamellae the transition is principally discontinuously and of first order.
info:eu-repo/classification/ddc/530, Physik, ferroelektrisch Phasenübergang, Röntgenbeugung, Neutronenbeugung
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