The crystal structures of some hydrogen-bonded phosphates in relation to their structural phase transitions

• Main Author: Choudhary, Ram Naresh Prasad
• Published: University of Edinburgh 1979
• Subjects: 548
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.643142

This thesis is mainly concerned with structural studies of some hydrogen-bonded phosphate compounds (all monoclinic) - namely T1H2P04, CsH2PO4, NaH2PO4 and PbHP04. All these compounds, except NaH2PO4, have a ferroelectric transition. Structural studies of the above compounds have been undertaken for the following primary purposes (i) to contribute accurate structural information to the understanding of the transition mechanism, (ii) to contribute experimental information about hydrogen bond dimensions. Full three-dimensional X-ray and neutron diffraction data from single crystals of T1HZP04 and CsH2PO4 were collected at room temperature on four-circle diffractometers. Full three-dimensional neutron-diffraction data were also collected from a single crystal of NaH2P04 at room temperature. The previously unknown structure of T1H2P04 has been solved from a Patterson map with the X-ray data followed by Fourier difference-syntheses with the neutron data. Accurate positional parameters of the hydrogen atoms in all the above compounds have been determined from the neutron data. The symmetry-restricted hydrogen bonds of the T1H2P04 and CsH2PO4 structures have been found to be disordered. All the other hydrogen bonds, including those of NaH2PO4, have been found to be asymmetric. The crystal structures of T1H2P04 and CsH2PO4 are closely similar: the P04 groups are linked by hydrogen bonds into puckered sheets perpendicular to the long axis of the cell. By contrast, the P04 groups are H-bonded into a continuous network in NaH2P04, rather like monoclinic 021,04. The most extensive study of crystal structure in relation to a structural phase transition has been performed in PbHPO4. Three-dimensional neutron-diffraction data were collected at six different temperatures from well below to just above the transition. The crystal structure (previously uncertain) in the high-temperature, paraelectric phase, has been determined. It has been shown that the hydrogen atoms are disordered in this phase. Then the onset of order of the proton system in the ferroelectric phase, and accompanying heavy ion displacements, have been shown to vary with temperature very much like the onset of the spontaneous polarization.