Synthesis and Crystal Chemistry of Two New Fluorite-Related Bismuth Phosphates, Bi<sub>4.25</sub>(PO<sub>4</sub>)<sub>2</sub>O<sub>3.375</sub> and Bi<sub>5</sub>(PO<sub>4</sub>)<sub>2</sub>O<sub>4.5</sub>, in the Series Bi<sub>4+</sub><i><sub>x</sub></i>(PO<sub>4</sub>)<sub>2</sub>O<sub>3+3</sub><i><sub>x</sub></i><sub>/2</sub> (0.175 ≤ <i>x</i> ≤ 1)
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Two new phosphates, Bi4.25(PO4)2O3.375 and Bi5(PO4)2O4.5, have been analyzed by single-crystal X-ray diffraction in the series Bi4+x(PO4)2O3+3x/2 (0.175 ≤ x ≤ 1). The syntheses of the compositions ranging from x = 0.175 to 0.475 were carried out by the ceramic route. The compositions from x = 0.175 to 0.475 form a solid solution with a structure similar to that of Bi4.25(PO4)2O3.375, while Bi5(PO4)2O4.5 was isolated from a mixture of two phases. Both of the phases form fluorite-related structures but, nevertheless, differ from each other with respect to the arrangement of the bismuth atoms. The uniqueness in the structures is the appearance of isolated PO4 tetrahedra separated by interleaving [Bi2O2] units. ac impedance studies indicate conductivity on the order of 10-5 S cm-1 for Bi4.25(PO4)2O3.375. Crystal data: Bi4.25(PO4)2O3.375, triclinic, space group P1̄ (No. 1), with a = 7.047(1) Å, b = 9.863(2) Å, c = 15.365(4) Å, α = 77.604(4)°, β = 84.556(4)°, γ = 70.152(4)°, V = 980.90(4) Å3, and Z = 4; Bi5(PO4)2O4.5, monoclinic, space group C2/c (No. 15), with a = 13.093(1) Å, b = 5.707(1) Å, c = 15.293(1) Å, β = 98.240(2)°, V = 1130.95(4) Å3, and Z = 8.
