Molten Salt Synthesis, Polymorphism, and Microwave Dielectric Properties of Ba8NiTa6O24 Perovskite

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.