This thesis is concerned with structural studies of ferroelectric crystals,
in particular the structure of CsD₂AsO₄ in its tetragonal paraelectrie phase and orthorhombic ferroelectric phase and the structures of
Co₃B₇O₁₃I and Cu₃B₇7O₁₃Br in their cubic paraelectric phases.
Both X-ray and neutron single-crystal diffraction techniques were used.
The methods of constrained least -squares refinement and significance testing
were used to determine the significance of certain structural features
Details of the development of a technique for the collection of single -
crystal data from an unpoled and physically unconstrained crystal in its
ferroelectric phase are given.
The structural studies of tetragonal CsD₂AsO₄ revealed that the D ions
are disordered in double minimum potential wells. In the orthorhombic
phase the D ions order onto one of their two possible sites in the tetragonal phase. The displacements of the Cs, As and O ions are related to
the choice of site of the D ion on the O. . . . . O bond. The As ions move
away from the O ions which the D ions have moved towards.
The unusual feature of the ferroelectric transition in Cs(DₓH₁₋ₓ)₂AsO₄
suggests an inhomogeneous mixing of the hydrogen and deuterium
The studies of Co₃B₇7O₁₃I and Cu₃B₇7O₁₃Br show that it is not necessary to
postulate disorder to explain the crystal structures of these boracites
in their cubic phase.