Crystal Structures of Bismuth Tellurohalides BiTeX (X = Cl, Br, I) from X-Ray Powder Diffraction Data

Abstract

Crystal structures of the bismuth tellurohalides BiTeCI (prepared for the first time), BiTeBr, and BiTeI were refined based on X-ray powder diffraction data using a full-profile Rietveld method. All three phases crystallize in a hexagonal system: BiTeCl; space group P63mc (No. 186), a = 4.2426(1) Å, c = 12.397(1) Å, z = 2, R1 = 0.065, RPr = 0.104; BiTeBr, space group P3̄m1 (No. 164), a = 4.2662(1) Å, c = 6.487(1) Å, z = 1, RI = 0.042, RPr = 0.082; BiTeI, space group P 3m1 (No. 156), a = 4.3392(1) Å, c = 6.854(1) Å, z = 1, RI = 0.054, RPr = 0.129. Bismuth tellurohalides do not belong to the SbSBr structure type. BiTeBr has a 2H-CdI2-type structure with tellurium and bromine atoms statistically distributed within a two-layered packing. BiTeI exhibits a distorted variant of the former structure with an ordering of tellurium and iodine atoms in an anionic subcell. In BiTeCl, layers of tellurium and chlorine atoms alternate along the c direction of the unit cell, forming a four-layered (hc)2 packing. Bismuth atoms occupy all octahedral interstices in each second layer. The common feature that the three structures share is the corrugated (BiTe)+ layers which resemble the geometry of bismuth layers in metallic bismuth. The structures are described in terms of a semi-ionic model, according to which bismuth and tellurium atoms form positively charged (BiTe)+ layers, while bismuth-halogen contacts are considered to be ionic.