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Stability Phase‐Fields and Pyrochlore Formation in Sections of the Bi 2 O 3 –Al 2 O 3 –Fe 2 O 3 –Nb 2 O 5 System
Author(s) -
Vanderah Terrell A.,
Guzman Julian,
Nino Juan C.,
Roth Robert S.
Publication year - 2008
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2008.02648.x
Subject(s) - pyrochlore , aurivillius , materials science , bismuth , ternary operation , ternary numeral system , phase (matter) , solid solution , solidus , mineralogy , natural bond orbital , analytical chemistry (journal) , crystallography , dielectric , ferroelectricity , chemistry , metallurgy , density functional theory , computational chemistry , optoelectronics , organic chemistry , alloy , chromatography , computer science , programming language
Bismuth‐niobate‐based ceramic materials are of interest for embedded elements such as capacitors and filters because they exhibit high relative dielectric permittivities and tend to be processible at temperatures in the 1000°–1200°C range. As part of ongoing studies of phase equilibria and structure–property relations in this class of electroceramics, a limited study of sections in the Bi 2 O 3 –Al 2 O 3 –FeO 3 –Nb 2 O 5 system was conducted. In contrast to published reports, a pyrochlore phase was not found in the ternary subsystem Bi 2 O 3 –Al 2 O 3 –Nb 2 O 5 , despite detailed syntheses, which included controlled heatings up to and above the solidus: except for solid solutions of Al 2 O 3 in binary Bi 2 O 3 –Nb 2 O 5 phases (i.e., fluorite‐related Bi 3 NbO 7 and Bi 5 Nb 3 O 15 ), no distinct ternary compounds were observed to form. In the quaternary Bi 2 O 3 –Al 2 O 3 –FeO 3 –Nb 2 O 5 system, a pyrochlore solid solution was confirmed along a line from Bi 1.657 Fe 1.092 Nb 1.150 O 7 (=42.50:28.00:29.50 Bi 2 O 3 :Fe 2 O 3 :Nb 2 O 5 ) up to the limiting composition Bi 1.657 Al 0.328 Fe 0.764 Nb 1.150 O 7 (=42.50:8.400:19.60:29.50 Bi 2 O 3 :Al 2 O 3 :Fe 2 O 3 :Nb 2 O 5 ), corresponding to substitution of 30% of the Fe 3+ with Al 3+ . This result suggests that the large pyrochlore phase‐field in the Bi–Fe–Nb–O system extends into the quaternary Bi–Al–Fe–Nb–O system to form a single‐phase volume of pyrochore solid solutions by dissolving appreciable amounts of Al 2 O 3 . The Bi–Al–Fe–Nb–O pyrochlore phases exhibited low‐temperature dielectric relaxation characteristic of bismuth‐based pyrochlores. The substitution of Al for Fe resulted in a small increase in relative dielectric permittivity (from 125 to ≈135 at 1 MHz, ≈200 K), and a decrease in the temperature (from ≈130 to ≈120 K) corresponding to the peak in the dielectric loss.