Space group $P3m1$ #156 allows an arbitary placement of the origin of the $z$-axis. We follow (Shevelkov, 1995) and set $z_{1} = 0$ for the bismuth atom.
A. V. Shevelkov, E. V. Dikarev, R. V. Shpanchenko, and B. A. Popovkin, Crystal Structures of Bismuth Tellurohalides BiTeX (X = Cl, Br, I) from X-Ray Powder Diffraction Data, J. Solid State Chem. 114, 379–384 (1995), doi:10.1006/jssc.1995.1058.
Found in
S. Güler-K{\i}l{\i}ç and Ç. K{\i}l{\i}ç, Crystal and electronic structure of BiTeI, AuTeI, and PdTeI compounds: A dispersion-corrected density-functional study, Phys. Rev. B 91, 24524 (2015), doi:10.1103/PhysRevB.91.245204.
First cited in
N. Anderson, M. J. Mehl, H. Eckert, S. Divilov, X. Campilongo, S. Curtarolo, The AFLOW Library of Crystallographic Prototypes: Part 5. Submitted to Computational Materials Science (2026).