Hydrothermally Stabilized (Na0.25Bi0.25)Bi3O2(PO4)2: Crystal Structure and Crystal Chemical Relationships with Minerals of the Preisingerite Group

Single crystals of bismuth sodium oxophosphate (Na0.25Bi0.25)Bi3O2(PO4)2 were prepared by hydrothermal synthesis. The compound crystallizes in the triclinic space group P1 with the following unit-cell parameters: a =6.9299(4), b = 7.5480(4), c = 9.2013(4) Å, α = 106.978(5), β = 93.665(4), γ = 112.374(5) o, and Z = 2. The crystal structure was determined by low-temperature single-crystal X-ray diffraction and refined to an R-value of 0.023. This compound is a sodium-bearing derivative of the Bi3.33O2(PO4)2 phase. The title crystal structure is built by oxygen polyhedra surrounding bismuth atoms. Linked via edges and faces, these polyhedra form ribbons parallel to the c axis. The ribbons are interconnected into a tri-periodic construction by phosphate tetrahedra. Refinement of the structure revealed that the 1b Wyckoff position at the symmetry center is surrounded on both sides by atoms in a general 2i position at a distance of 0.545(2) Å. This general-type position is statistically occupied by Na and Bi atoms in equivalent amounts. Na/Bi polyhedra near the symmetry center are located between the ribbons. Thus, Na and Bi are disordered around the center of symmetry. The crystal chemical relationships between the (Na0.25Bi0.25)Bi3O2(PO4)2 and minerals of the preisingerite group are discussed. The work provides a crystal-chemical basis for future studies of the Bi3.33O2(PO4)2 derivative phosphates, which are of interest in the context of ion transport.