6 highly insulating samples can be prepared. This may make BiSrZnNbO$_{6}$ a useful alternative Pb-free ferroelec- tric. TABLE II: Comparison of properties of BiPbZnNbO 6 and BiSrZnNbO 6 cells as obtained from our LDA calculations. a denotes the pseudocubic lattice parameter, the z ∗ are the average Born effective charges, δ are the average cation off- centerings with respect to the O cages, and P is the polariza- tion. IV. SUMMARY AND DISCUSSION BiPbZnNbO 6 BiSrZnNbO 6 a (LDA) 3.99 ˚ A 3.97 ˚ A δ (Bi) 0.64 ˚ A 0.62 ˚ A z ∗ (Bi) 4.4 4.4 δ (Pb/Sr) 0.39 ˚ A 0.33 ˚ A z ∗ (Pb/Sr) 3.6 2.6 δ (Zn) 0.27 ˚ A 0.25 ˚ A z ∗ (Zn) 2.9 2.8 δ (Nb) 0.27 ˚ A 0.25 ˚ A z ∗ (Nb) 5.8 5.8 P (111) 85 µ C/cm 2 79 µ C/cm 2 Density functional calculations for small supercells of double perovskite BiPbZnNbO$_{6}$ and BiSrZnNbO$_{6}$ indi- cate that these may be useful ferroelectric materials. They have high polarization due to the large displace- ment of Bi and the displacements of Nb combined with its high charge. They have substantial band gaps and stable chemistry consistent with synthesis of highly insu- lating samples. It will be of interest to measure prop- erties experimentally, and to investigate the solid so- lutions with PbTiO$_{3}$ to determine if a morphotropic phase boundary is present. Finally, we note that be- cause these materials have mixtures of ions with differ- ent charges on both the A - and B -sites, they should have considerable chemical flexibility. While we con- sidered BiPbZnNbO$_{6}$ in the double perovskite structure, Bi$_{x}$Pb$_{1}$$_{−}$$_{x}$Zn$_{1}$$_{/}$$_{3+}$$_{x/}$$_{3}$Nb$_{2}$$_{/}$$_{3}$$_{−}$$_{x/}$$_{3}$O$_{3}$ may exist over a large range of x . BiPbZnNbO$_{6}$ is the x =0.5, while x =0 is the known relaxor ferroelectric PZN. To our knowledge, the x =1 perovskite BiZn$_{4}$$_{/}$$_{3}$Nb$_{1}$$_{/}$$_{3}$O$_{3}$ has not been reported, and instead the Bi$_{2}$O$_{3}$ – ZnO – Nb$_{2}$O$_{5}$ system shows Nb rich pyrochlore phases that presumably compete with the perovskite. 41 As such, it may be possible to synthesize the perovskite under pressure, similar to other low tolerance factor Bi based perovskites. 42,43 In any case, the chemi- cal flexibility implies a tunability of properties that may be very helpful in obtaining useful ferroelectric properties including MPBs. because Pb is considered hazardous. This fact motivates searches for Pb-free replacements for the commonly used Pb-based ferroelectrics such as PZT. We performed cal- culations for 40 atom BiSrZnNbO$_{6}$ supercells, similar to those discussed above for the Pb system. Some properties are compared with the Pb system in Table II. We found a slightly smaller equilibrium volume for the Sr system, corresponding to a pseudocubic a =3.97 ˚ A. As shown in Fig. 5, the material is ferroelectric, with large collinear cation off-centerings along [111]. The calculated polar- ization is 79 µ C/cm $^{2}$. These off-centerings are similar to those for the Pb system, except on the Sr site. This is also the case for calculations at the same volume. For a pseudocubic a =3.98 ˚ A, the calculated off-centerings are 0.62 ˚ A, 0.33 ˚ A, 0.25 ˚ A, and 0.25 ˚ A, for Bi, Sr, Zn and Nb, respectively, as compared with 0.62 ˚ A, 0.38 ˚ A, 0.26 ˚ A, and 0.27 ˚ A, for Bi, Pb, Zn and Nb in the Pb compound at the same volume. This implies only slightly inferior ferroelectric properties for the Sr compound as compared to the Pb compound. The calculated average Born effec- tive charges for BiSrZnNbO$_{6}$ are 4.4, 2.6, 2.8, and 5.8 for Bi, Sr, Zn, and Nb, respectively. These are very similar to BiPbZnNbO$_{6}$, except that the Sr Born charge is lower than that for Pb. Importantly, the LDA band gap of the Sr compound, E$_{g}$ =3.0 eV, is slightly higher than that of the Pb compound. This large band gap combined with the stable valences of the various cations implies that We are grateful for helpful discussions with M. Fornari and M.H. Du. This work was supported by the Di- vision of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of En- ergy (AS,VRC,DJS) and the Office of Naval Research (ST,DJS). Acknowledgments 7 S. V. Halilov, M. Fornari, and D. J. Singh, Appl. Phys. Lett. 81 , 3443 (2002). 8 D. I. Bilc and D. J. Singh, Phys. Rev. Lett. 96 , 147602 (2006). 9 D. J. Singh, M. Ghita, M. Fornari, and S. V. Halilov, Fer- roelectrics 338 , 73 (2006). 10 D. J. Singh and C. H. Park, Phys. Rev. Lett. 100 , 087601 (2008). 11 D. Fu, M. Itoh, S. Y. Koshihara, T. Kosugi, and S. Tsuneyuki, Phys. Rev. Lett. 100 , 227601 (2008). 6 S. M. Choi, C. J. Stringer, T. R. Shrout, and C. A. Randall, J. Appl. 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