Crystal structure, stoichiometry, and dielectric relaxation in Bi3.32Nb7.09O22.7 and structurally related ternary phases

The crystal structure of the phase previously reported to occur at 4:9 Bi2O3:Nb2O5 has been determined using single-crystal X-ray and powder neutron diffraction (P63/mmc; a=7.4363(1) Å, c=19.7587(5) Å; Z=2). The structural study combined with phase equilibrium analyses indicate that the actual composition is Bi3.32Nb7.09O22.7. This binary compound is the end-member of a family of four phases which form along a line between it and the pyrochlore phase field in the Bi2O3:Fe2O3:Nb2O5 system. The structures are derived from the parent pyrochlore end-member by chemical twinning, and can also be described as unit-cell intergrowths of the pyrochlore and hexagonal tungsten bronze (HTB) structures. The dielectric properties of the three chemically twinned pyrochlore phases, Bi3.32Nb7.09O22.7, Bi9.3Fe1.1Nb16.9O57.8 and Bi5.67FeNb10O35, were characterized. All exhibit low-temperature, broad dielectric relaxation similar to that of the Bi–Fe–Nb–O pyrochlore. At 1 MHz and ≈175 K the observed relative permittivites were 345, 240, and 205, respectively, compared to 125 for the Bi–Fe–Nb–O pyrochlore. The higher relative permittivities observed for the chemically twinned pyrochlore derivatives are ascribed to the presence of HTB blocks in their structures: The Bi atoms located in the HTB blocks feature highly asymmetric coordination environments compared to pyrochlore, and the magnitude of the relative permittivity increases with the proportion of Bi located within the HTB portions of the structures.