Crystal Structure And Thermal Expansion In Lead- Niobate-based Tungsten-bronzes

Tetragonal tungsten-bronze (TTB) structured compounds built up the largest family of ferroelectric materials after the perovskites. These materials are widely studied and used as dielectrics, piezoelectrics, ferroelectrics, and so on. In particular, the lead niobate-based TTBs have attracted lots of attention because of their structural diversity and excellent physical properties such as the high Curie temperature and strong polarization. Knowledge of the thermal expansion in TTBs will be highly helpful to understand the nature of solid state chemistry and to deal with the problems caused by thermal stress in their technological applications. This dissertation focused on the thermal expansion behaviors of the TTBs. A series of TTB-structured lead-niobates-based compounds were designed and synthesized. The crystal structures and structural evolutions were investigated systematically. The thermal expansion behaviors were determined, and the mechanism of the negative and positive thermal expansion were elucidated in term of their relationships with crystal structures and electronic structures, which can be used as a guide to tailor the thermal expansions. Moreover, a material with excellent nonlinear optical (NLO) property were found with non-stoichiometric compositions, which provides a new strategy for exploring NLO materials.First, the unixial negative thermal expansion along the polar b axis of Pb2KNb5O15 was determined by various temperature X-ray diffraction method. The refinements against high-temperature neutron diffraction data reveals the presence of in-plane (a-b plane) polar displacements for both Pb and Nb atoms. The negative thermal expansion along the b direction is directly induced by the synergetic rotation and distortion of the NbO6 octahedra. Firsts-principle calculation suggests that the enhanced ferroelectric polarization was caused by the covalent bond between Pb2+ with 6s2 lone-pair and O2-. The unixial negative thermal expansion in Pb2KNb5O15 is the result of framework structure effect and ferroelectrostriction, while the latter dominates.By the modification of metal cations, the thermal expansion behavior was tailored in a series of TTBs niobates with formulas of Pb2RNb5O15 (R= Na, K0.5Li0.5, K, Rb, Ag) and Pb3TiNb4O15 (PTN). It shows compounds with smaller R cations tend to form superstructures regularly in both the a-b plane and the c direction. The crystal structures for R= K0.5Li0.5 and Ag were determined. The smaller R cations can effectively raise the Curie temperature and strengthen the ferroelectricity, which give rise to strengthened negative thermal expansion along the polar b axis; however, the smaller R cations could also cause tilting to the NbO6 octahedra, which, in reverse, give rise to extra positive thermal expansion along the layered c axis. The apparent volumetric thermal expansion behavior is resulted from the balance of the two factors.By replacing (PbR)3+ with Bi3+, a cation possessing the same 6s2 lone-pair to Pb2+, the new TTB-structured composition, PbBiNb5O15, was designed and synthesized. Unusual five orders of satellite reflections were observed under selected area electron diffraction, implying the structure is highly modulated with large amplitudes. Structural analysis reveals large displacement and occupancy modulation amplitudes for both the Pb2+ and Bi3+ in the A sites, which result in large local dipole moments. The strong modulation in PbBiNb5O15 seems to suppresses the large local polarizations over long ranges, and result in weak apparent polarizations.Finally, by the incorporation of Li, a non-stoichiometric TTB-structured compound, Pb2.15Li0.7Nb5O15, was found to possess excellent NLO property. The accurate crystal structure of Pb2.15Li0.7Nb5O15 was ambiguously determined by a combination of neutron and X-ray diffraction. The small Li+ cation was found to be located in the quadrangular tunnels together with minor amounts of Pb2+, which clarify the controversy of where Li prefers to locate in TTB structures. Atomic level scanning transmission electron microscope experiments reveal the existence of enhanced local polarizations, which is likely to be the origin of strong second-harmonic generation responses in Pb2.15Li0.7Nb5O15.