Crystal and domain structure of acentric Bi 2 Pb 3 (GeO 4 ) 3 apatite

Bi 2 Pb 3 (GeO 4 ) 3 represents an apatite of the most common P6 3 /m family with almost empty channels along [001], indicating an actual monoclinic symmetry reduction to the P2 1 space group. The driving force for such structural distortions is the existence of lone pairs of electrons of both Bi 3+ and Pb 2+ cations and the resulting dipole momentum that needs to be compensated. Three domains compose a structure with merohedral P6 3 symmetry and lattice parameters of a = 10.0182(7) Å and c = 7.2612(7) Å. Additional racemic twinning was confirmed with nearly equal domain contributions leading to general twinning with six order states. The structure was solved from twinned single crystal diffraction data ( CAD 4, Mo‐Kα radiation) in the acentric space group P6 3 , using threefold off‐centre split positions only for the Bi atoms to adapt monoclinic symmetry. The structure was refined to reliability factors of R F = 0.028 and wR 2 = 0.044. Possible deviation from hexagonal metric could not be found by Rietveld refinement. Instead, minor phase content of 0.2% PbO was detected, consistent with a small reduction of the Pb site occupation. Although ferroic phase transformation may follow from the small deviation of atomic positions from P6 3 /m symmetry, it could not be experimentally verified.