What is the true space group of weberite?

• Published in: Journal of solid state chemistry 1982-01, Vol.43 (2), p.213-221
• Main Authors: Knop, Osvald, Cameron, T.Stanley, Jochem, Klaus
• Format: Article
• Language: English
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/0022-4596(82)90231-6

Analysis of X-ray diffraction evidence obtained from a single crystal of natural weberite, Na 2MgAlF 7, at room temperature and at −140°C rules out all space groups but two, the noncentrosymmetric Imm2 (originally proposed by Byström) and I2 12 12 1. However, comparison of structure refinements in these two groups and in Imma shows that the departure from centrosymmetry is so slight, and the positional and thermal parameters of some of the F atoms in the Imm2 and I2 12 12 1 refinements are so highly correlated, that the descriptions of the weberite structure in the three space groups must be regarded as practically indistinguishable. In the absence of a proof of achirality Byström's space group assignment is provisionally accepted as valid, and Na 2MgAlF 7 is considered as isostructural with the recently refined Na 2NiFeF 7.