Pyroelectric Bi_5-x(Bi₂S₃)₃₉I₁₂S: Fibonacci Superstructure, Synthesis Options and Solar Cell Potential

Abstract

Previously, synthetic hexagonal bismuth sulfide iodide (polar space group P6 3 , a = 15.629(3) ? , c = 4.018(1) ? , Z = 2) has been described by the rather unsatisfactory fractional formula Bi 19/3 IS 9 [1] 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003400310037003600350038003400370039000000 - [3] 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003400310037003600350038003400370036000000 . A redetermination of the structure using old but reliable photographic intensity data indicated the presence of additional split positions and reduced atomic occupancies. From the observed pattern of this “averaged” structure a consistent model of a superstructure with lattice parameters of a' = √13·a = 56.35(1) ? , c' = c, and a formula Bi 5-x (Bi 2 S 3 ) 39 I 12 S emerged, with 2 formula units in a cell of likewise P6 3 space group. Structural modulation may be provoked by the space the lone electron pair of Bi requires. When Bi on the 0, 0, z position of the “averaged” cell is transferred to two general six-fold sites and one unoccupied twofold one of the super-cell, more structural stability is guaranteed due to compensation of its basal plane dipole momentum. Owing to the limited intensity data available, more details of the superstructure are not accessible yet. Some physical properties and solar cell application are discussed together with suggestions of ambient temperature synthesis routes of c-axis oriented nano-rod sheets.