Synthesis and structure of ca₆bi₆o₁₅: Its relationship to ca₄bi₆o₁₃

Single crystals and powdered samples of the bismuth(III) calcium oxide Ca₆Bi₆O₁₅ have been synthesized and studied using both X-ray and neutron diffractometry. The compound crystallizes in the space group P1̄ with Z = 2. Cell parameters, obtained with the single-crystal X-ray diffractometer, were a = 10.101(3), b = 10.130(2), c = 10.466(2) Å, and α = 116.86(1), β = 107.18(2), γ = 92.90(2)°. As with other materials in the series M-Bi-O (M = Ca, Sr) studied thus far, at least some Bi is found in threefold coordination with oxygen; the remaining Bi are fourfold coordinated. The structure is related to that of Ca₄Bi₆O₁₃, which has been studied using neutron powder diffraction. Both compounds are composed of layers containing alternating Bi- and Ca-rich segments. These layers stack to form Bi-rich slabs containing tunnels accommodating the Bi 6s² lone pairs.