The heterogeneous phase distribution found in Ba 2 Ti 9 O 20 ceramic resonators results from a temperature‐dependent phase boundary and slow reaction kinetics. When sintered at 1350°C or higher in oxygen the Ba 2 Ti 9 O 20 phase becomes slightly reduced and barium‐rich. Thus a stoichiometric composition forms rutile and ″Ba 2 Ti 9 O 20 'phase. On slow cooling the excess barium diffuses to the oxygen‐rich surface where it reacts to form an envelope of rutile‐free material surrounding a core containing a small amount of rutile.
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O’Bryan et al. (1983) studied this question.
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