Zirconia doped with 3.2-4.2 mol% (6-8 wt%) yttria (3-4YSZ) is currently the material of choice for thermal barrier coating topcoats. The present study examines the ZrO2-Y2O3- Ta2O5/Nb2O5 systems for potential alternative chemistries that would overcome the limitations of the 3-4YSZ. A rationale for choosing specific compositions based on the effect of defect chemistry on the thermal conductivity and phase stability in zirconia-based systems is presented. The results show that it is possible to produce stable (for up to 200 h at 1000degrees-1500degreesC), single (tetragonal) or dual (tetragonal + cubic) phase chemistries that have thermal conductivity that is as low (1.8-2.8W/m K) as the 3-4YSZ, a wide range of elastic moduli (150-232 GPa), and a similar mean coefficient of thermal expansion at 1000degreesC. The chemistries can be plasma sprayed without change in composition or deleterious effects to phase stability. Preliminary burner rig testing results on one of the compositions are also presented.