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Molten Salt Synthesis, Polymorphism, and Microwave Dielectric Properties of Ba 8 NiTa 6 O 24 Perovskite
Author(s) -
Gong Meilu,
Lu Fengqi,
Kuang Xiaojun,
Yu Xiaodi,
Wang Quanchao,
Li Xiaohui,
Fang Liang
Publication year - 2015
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/jace.13610
Subject(s) - hexagonal crystal system , molten salt , crystallography , materials science , hexagonal phase , perovskite (structure) , phase (matter) , polymorphism (computer science) , analytical chemistry (journal) , chemistry , metallurgy , organic chemistry , chromatography , biochemistry , genotype , gene
Molten salt synthesis ( MSS ) of the eight‐layer hexagonal perovskite Ba 8 NiTa 6 O 24 was performed using mixed KCl–NaCl salts in comparison with solid‐state synthesis ( SSS ). In the SSS , the hexagonal Ba 8 NiTa 6 O 24 formed at 1300°C via a reaction between cubic Ba 3 NiTa 2 O 9 and hexagonal Ba 5 Ta 4 O 15 . While the MSS did not lower the synthesis temperature of the hexagonal Ba 8 NiTa 6 O 24 but stabilized an unusual A‐ and B‐site‐deficient cubic perovskite polymorph of Ba 8 NiTa 6 O 24 below 1350°C as an intermediate phase prior to transforming into the hexagonal phase. This cubic polymorph contains ~3% A‐site and ~9.5% B‐site vacancies plus ~3% Ba cations in the B sites and demonstrated remarkable stability below 1350°C when without presence of the molten salt. The cubic polymorph displayed larger ε ~ 36 and τ f ~ 110 ppm/°C than the hexagonal polymorph from the MSS (ε ~ 29 and τ f ~ 67 ppm/°C). The hexagonal SSS ‐processed ceramics showed advantageous dielectric properties ( Q f ~ 52 000 GHz, τ f ~ 30 ppm/°C) over both cubic and hexagonal MSS ‐processed ones ( Q f ~ 18 600–20 000 GHz), while displaying anisotropic grain growth. The anisotropic grain growth was suppressed significantly by the MSS processing.