Abstract
Barium nitrate crystallizes in the crystal class T–23. It had been assumed to belong to the space group Th6–P21x3i since no exceptions to the extinction rule for this space group (hkO with h = odd) had been found with x-ray diffraction methods. It was, however, possible to detect the existence of these forbidden reflections by means of neutron diffraction. The distribution of intensities indicated an only very small deviation from centrosymmetry (space group T4–P21x3). From the new parameters obtained by the least-squares refinement method, it was found that the NO3 groups build flat pyramids with the oxygen atoms in the basal plane (h = 0.10 Å, angle O–N–O approximately 119°). Computation on data obtained from the measurement of a different zone lead to the same results. The orientation of the NO3 groups explains the pronounced tetrahedral habit of the crystals.

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