I E. Grey,
, W G. Mumme, , Jorge T. Guzman, Juan C. Nino,
Ceramic materials which exhibit high relative dielectric permittivities and are processible at temperatures in the 1000 C to 1200 C range are of interest for embedded elements such as capacitor, resonators, and filters. Complex titanates, niobates, and tantalates often display the desired properties and are of interest for this application as well as numerous others in communications systems as well as photocatalysis. As part of ongoing studies of phase equilibria and structure-property relations in this class of electroceramics, a selected portion of the Bi2O3-Nb2O5 system has been determined. The crystal structure of the phase previously reported to occur at 4:9 Bi2O3:Nb2O5 has been refined using single-crystal X-ray and powder neutron diffraction (P63/mmc, a = 7.4363(1) , c = 19.7587(5) , Z = 2). The structural study combined with phase equilibrium analysis suggests 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 and the ternary members 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.
Journal of Solid State Chemistry
4:9 Bi2O3-Nb2O5, bismuth iron niobates, bismuth niobates, chemically twinned pyrochlore, dielectric relaxation, high permittivity