Abstract

The barylite structure can be refined to an R index of 0.060 in the orthorhombic centrosym-metric space group Pnma, despite the fact that the structure is intrinsically non-centrosymmet-ric as disclosed by a second harmonic generation (SHG) test. Refinement in the non-centrosymmetric space group Pn21a yielded no significant changes in positional parameters, possibly because of the dominant role of the centrosymmetrically arranged Ba atoms in the scattering. The F0 and ΔF maps provided no clues as to the source of the acentric distribution of structure factors. We therefore suggest that a sensitive physical test, such as SHG, be routinely performed before a crystal structure is accepted as centrosymmetric

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