Hydrothermal Synthesis and Crystal Structure of a Novel Bismuth Oxide: (K _0.2Sr _0.8)(Na _0.01Ca _0.25Bi _0.74)O ₃

Abstract

A novel distorted perovskite-type (K _0.2Sr _0.8)(Na _0.01Ca _0.25Bi _0.74)O ₃ was prepared by a hydrothermal method using the starting materials NaBiO ₃· nH ₂O, Sr(OH) ₂·8H ₂O, Ca(OH) ₂, and KOH. Single-crystal X-ray diffraction of the novel compound revealed a GdFeO ₃-related structure belonging to the monoclinic system of the space group Cc with the following cell parameters: a = 11.8927 (17) Å, b = 11.8962 (15) Å, c = 8.4002 (10) Å, and β = 90.116 (9)°. The final R-factors were obtained as R ₁ = 0.0354 and w R ₂ = 0.0880 (using all the data). K ⁺ and Sr ²⁺ ions were distributed at four types of A-sites. On the other hand, four Bi ⁵⁺-sites (Bi1, Bi2, Bi3, and Bi4) were occupied by four Ca ²⁺ ions (Ca1, Ca2, Ca3, and Ca4), and the first three B-sites were occupied predominantly by Bi ⁵⁺ with Na ⁺ ions. The forth B-site was occupied predominantly by the Ca ²⁺ ion with Bi ⁵⁺ ions. Two types of B-sites, thus forming tilted distorted (Na/Ca/Bi)O ₆ and (Bi/Ca)O ₆ octahedra, have an ordering of 3:1 represented as (K/Sr) ₄(Na/Ca/Bi) ₃(Bi/Ca)O ₁₂. The distorted (Na/Ca/Bi)O ₆ and (Ca/Bi)O ₆ octahedra formed a perovskite-type network by corner sharing with features closely matching those of a GdFeO ₃-type structure. The novel compound is the first example of a perovskite-type bismuth oxide containing only Bi ⁵⁺ in a system without a Ba atom and has a unique ordering (3:1) of the B site. The compound showed photocatalytic activity for phenol degradation under visible light irradiation.