Investigation of nonstoichiometric ordered niobate perovskites for microwave applications

This dissertation describes the preparation, structure, and dielectric properties of several new families of nonstoichiometric ordered niobate perovskites. First examples of non-stoichiomeeryryc 1:2 ordered perovskites were discovered in the (1-x)A2+(Li1/4Nb,Ta3/4)O 3-xA2+(Li2/5W3/5)O3 (A2+=Ba, Sr and Ca) and [A2+2(1+x)/3La (1-2x)/3][Li1/3(Nb,Ta1-xWx)2/3 ]O3 systems, with trigonal P3¯ m1 (Ba) and monoclinic P21/c symmetries (Sr- and Ca-based). In contrast to all known A(BI1/3B II2/3)O3 perovskites, these 1:2 ordered solid solutions do not include any composition with a 1:2 cation distribution and the structure exhibits extensive non-stoichiometry. Structure refinements support a cation occupancy model with A2+[(Li3/4+y/2Nb 1/4-y/2)1/3(Nb1-yWy)2/3]O 3 (where y = 0.9x). The formation of the non-stoichiometric 1:2 ordered phases is associated with the large site charge/size differences, and restricted by local charge imbalances at the A-sites for W-rich compositions. Single phase B-site vacancy-containing ordered perovskite solid solutions are formed within a very limited range 0≤x≤0.02 in the (1-x)BZN-(x)Ba 3W2O9 system with a stoichiometry: Ba(Zn 1-x x)1/3(Nb1-xWx)2/3O 3. In contrast to other substituted 1:2 perovskites the small concentrations of BW improve relative stability of order, enhance Torder of BZN by 35°C, and accelerate the kinetics of ordering and domain growth. BW additions also improve the sintering. The BZN-BW ceramics exhibit the lowest dielectric losses recorded for all niobate perovskite dielectrics with Q.f\u27s ∼ 120,000 at 8 GHz. Ten pseudo binaries in BaO-ZnO-Nb2O5 ternary were studied to examine the extent of any nonstoichiometry in Ba(Zn1/3Nb2/3)O3. BZN was found to support a limited range of non-stoichiometry and exceptionally small changes in bulk chemistry produce order of magnitude alterations in the dielectric loss properties. The effect of muffling on structure and properties of bulk BZN was studied. Non-stoichiometric BZN with ZnO vapor protections remain well ordered and they have a very high Q ( Q.f = 105,000) in their as-sintered state. τf of the nonstoichiometric BZN perovskites were tuned through the incorporation of Ni and Co. The ordering gradient in the partially ordered ceramics associated with the increased constraints on the growth of the 1:2 ordered structure within the interior of larger pellets was found to produce a resonant frequency dependence of their Qf value and can be minimized by extended annealing