[Truncated abstract] New, double-doped, Bi2O3-based materials in the Bi2O3 Ln2O3 PbO (Ln = La, Nd, Er and Yb) and Bi2O3 WO3 PbO systems were prepared using solid-state reactions. For the Bi2O3 Er2O3 PbO and Bi2O3 Yb2O3 PbO systems, the air-quenchable compositional domain of the fcc fluorite-type phase was partially established. Temperature dependent conductivity measurements were performed on these quenched-in fluorite-type materials using AC impedance spectroscopy. Conductivity at 750[degrees Celsius] generally increased with increasing Pb2+/Ln3+ and decreasing (Ln3++Pb2+)/Bi3+ ratios. The material (BiO1.5)0.70(ErO1.5)0.15(PbO)0.15 had a conductivity of 0.66 [plus-minus] 0.05 S cm-1 at 750[degrees Celsius], placing it among the most highly conductive Bi2O3-based materials, and was the best new fluorite-type material from a combined conductivity and structural stability viewpoint. Some of the new materials in the Bi2O3 La2O3 PbO and Bi2O3 Nd2O3 PbO systems appeared to have the quenched-in fluorite type structure based on powder X-ray diffraction data. These materials had very high conductivities at 750[degrees Celsius] of `~ 1 S cm-1, but underwent rapid symmetry lowering transformations during heating, thus making them unsuitable for use as solid electrolytes. The fluorite-type structure was not air-quenchable in the Bi2O3 WO3 PbO system, for the compositions synthesised. Room temperature neutron powder diffraction data were collected for quenched-in fluorite-type materials in the (BiO1.5)0.80(LnO1.5)0.20-x(PbO)x, Ln = Er and Yb, x = 0, 0.03, 0.06 and 0.09, and (BiO1.5)0.97-y(ErO1.5)y(PbO)0.03, y = 0.27, 0.17 and 0.12, series. ... This suggests that Pb2+ dopant cations occupy face-centre positions in the fcc unit cell, and the Pb2+ lone pair electrons are likely to be orientated towards an oxide ion vacancy in an adjacent tetrahedral site. Pb2+/oxide ion vacancy interactions affect the migration of oxide ions/oxide ion vacancies through the structure, and are responsible for the significantly larger activation energy for oxide ion migration in the Pb2+-doped materials relative to the Pb2+-free materials. For example, the activation energies of (BiO1.5)0.80(ErO1.5)0.20-x(PbO)x, x = 0.03 and 0.06, were 1.50 [plus-minus] 0.02 and 1.54 [plus-minus] 0.02 eV, respectively, while the activation energy for (BiO1.5)0.80(ErO1.5)0.20 was 1.25 [plus-minus] 0.04 eV. Long-term annealing of the quenched in fluorite-type materials in the Bi2O3 Er2O3 PbO and Bi2O3 Yb2O3 PbO systems at 500 and 600[degrees Celsius] resulted in conductivity lowering structural transformations, making these materials unsuitable for practical use as solid electrolytes at these temperatures. For example, the materials (BiO1.5)0.80(ErO1.5)0.20-x(PbO)x, x = 0.03, 0.06 and 0.09, underwent a fluorite-type to tetragonal transformation during annealing at 500[degrees Celsius] due to <100> oxide ion vacancy ordering, and the rate of conductivity decay at 500[degrees Celsius] increased with increasing Pb2+/Er3+ ratio. Long-term annealing experiments at 500[degrees Celsius] performed on air quenched (Bi2O3)0.705(Er2O3)0.245(WO3)0.050 showed that the disordered fluorite-type structure of this material was not fully stabilised, as evidenced by the presence of superlattice reflections in selected area electron diffraction patterns for the material annealed for 2000 hours, and a gradual conductivity decay after ~ 150 hours annealing.