Molten salt synthesis (MSS) of the eight-layer hexagonal perovskite Ba8NiTa6O24 was performed using mixed KCl-NaCl salts in comparison with solid-state synthesis (SSS). In the SSS, the hexagonal Ba8NiTa6O24 formed at 1300 degrees C via a reaction between cubic Ba3NiTa2O9 and hexagonal Ba5Ta4O15. While the MSS did not lower the synthesis temperature of the hexagonal Ba8NiTa6O24 but stabilized an unusual A- and B-site-deficient cubic perovskite polymorph of Ba8NiTa6O24 below 1350 degrees C as an intermediate phase prior to transforming into the hexagonal phase. This cubic polymorph contains similar to 3% A-site and similar to 9.5% B-site vacancies plus similar to 3% Ba cations in the B sites and demonstrated remarkable stability below 1350 degrees C when without presence of the molten salt. The cubic polymorph displayed larger epsilon similar to 36 and (f)similar to 110ppm/degrees C than the hexagonal polymorph from the MSS (epsilon similar to 29 and (f)similar to 67ppm/degrees C). The hexagonal SSS-processed ceramics showed advantageous dielectric properties (Q(f)similar to 52000GHz, (f)similar to 30ppm/degrees C) over both cubic and hexagonal MSS-processed ones (Q(f)similar to 18600-20000GHz), while displaying anisotropic grain growth. The anisotropic grain growth was suppressed significantly by the MSS processing.