Crystal Structures, Modeling, and Dielectric Property Relationships of 2:1 Ordered Ba3MM2O9 (M = Mg, Ni, Zn; M = Nb, Ta) Perovskites

The preparation, modeled and refined crystal structures, and structure-dielectric property relationships of five Ba3MMʹ2O9 (M = Mg, Ni, Zn; Mʹ = Nb, Ta) perovskites are reported. Crystal structure modeling was used to generate initial structure models for Rietveld refinements of the neutron powder diffraction data. Bond valence sums calculated from the bond distances indicate Ba-O bonds are compressed and the M-O and Mʹ-O6 bonds are expanded from ideal lengths. A shift of Ta5+ and Nb5+ out of center of the [MʹO6] octahedra forms three short and three long Mʹ-O bonds. The octahedral distortion is driven by the asymmetry in the O bonding network and aided by the second-order Jahn-Teller distortion of the d0 Nb5+ and Ta5+ cations. Differences in the atomic coordination environments in the crystal structures were utilized to propose crystal structure-dielectric property relationships. The coordination of the divalent B-site cation (M2+) was correlated with the temperature coefficient of the resonant frequency (f); a more under-bonded M2+ exhibited a more negative f. Also, if the bond valences sums of the B-site cations were closer to the formal oxidations states, then a higher Q×f was observed.