Synthesis and characterisation of transition metal-doped lithium niobate and lithium tantalate
The study of ferroelectric LiNbO3 and LiTaO3 doped with transition metals involves the characterisation of LiNbO3/LiTaO3 solid solutions in the systems Li2O-Nb2O5-MxOy and Li2O-Ta2O5-MxOy (M = Cr, Mn, Fe, Co, Ni, Cu). Compounds were made by solid state reaction at temperatures between 1000 and 1500°C, depending on the system studied. The emphasis of this work is on the characterisation of the defect structure of LiNbO3/LiTaO3 solid solutions using phase diagram determination, X-ray and neutron powder diffraction, EXAFS, ESR and optical spectroscopy. The valence of the incorporated cations was studied by magnetic measurements. The electrical properties of these materials have been investigated using AC impedance spectroscopy. It can be shown that the physical properties of LiNbO3 and LiTaO3 depend strongly on the defects in the structure which can be controlled by purposeful doping with other cations. Structural refinements of the X-ray and neutron powder diffraction data have shown that the defects arising from nonstoichiometry are accommodated by vacancies created on the Li site. This affects the structure when doped with third cations, giving rise to complex substitution mechanisms Spectroscopic studies have shown that the dopants (Cr3+, Mn2+/Mn3+, Co2+, Ni2+, Cu+, Cu2+) are shifted from the central octahedral position towards the adjacent empty octahedron. The electrical properties of LiNbO3 and LiTaO3, measured by AC impedance spectroscopy, depend strongly on the dopant content: the conductivity generally rises, whereas the activation energy for the electrical conductivity drops with increasing dopant concentration. The microstructure of electroceramics can also be probed by AC impedance spectroscopy and it was shown that the texture of all samples was bad due to poor sintering of the pellets. Longer sintering times did not improve the quality of the ceramics which exhibit a large pore size distribution. An important aspect for future work would be the quality improvement of LiNbO3/LiTaO3 based ceramics.