Studies on the compounds in BaFeS system. III. Phase relation of Ba1+xFe2S4 with infinitely adaptive structure

Samples of the infinitely adaptive phase Ba_1+xFe₂S₄ (or Ba_p(Fe₂S₄)_q; p, q: integer) were carefully prepared by changing the nominal composition and annealing temperature T_a. The single-phase materials, defined in this paper as a member of the infinitely adaptive series Ba_p(Fe₂S₄)_q, were obtained by the addition of excess sulfur in the nominal composition range 0.05 ≤ x ≤ 0.20 at T_a ranging from 650 to 880°C. X-Ray powder diffraction showed the existence of many members of the Ba_p(Fe₂S₄)_q series. The supercell periodicity was markedly dependent on T_a. The composition of reaction products estimated from X-ray diffraction, the method proposed by Grey based on crystallographic considerations, deviated in practice from the nominal composition. This fact suggests random distribution of Ba and Fe vacancies.