Investigation on pseudosymmetry, twinning and disorder in crystal structure determinations: Ba(H{sub 2}O)M{sub 2}{sup III}[PO{sub 3}(OH)]{sub 4} (M=Fe, V) as examples

Twinning commonly occurs in monoclinic crystals with dimensionally similar a and c axes and results in pseudo-orthorhombic symmetries with overlapping diffractions. For example, twinning in the new synthetic compound Ba(H{sub 2}O)Fe{sub 2}[PO{sub 3}(OH)]{sub 4}, which varies in space group from P2{sub 1} to P2{sub 1}/c with approximately equal a and c axial lengths, gives rise to a pseudosymmetry of C222{sub 1}. Similarly, the related compound Ba(H{sub 2}O)V{sub 2}[PO{sub 3}(OH)]{sub 4} is commonly twinned and varies in space groups as well, arising from ordered to disordered distributions of the barium cations and water molecules in the cavities. Moreover, analyses of these and other twinned structures show that the small average standard uncertainty of bond distances is a sensitive criterion for structure determinations, especially for those involving crystal twinning as well as order-disorder. A proper structure determination leads to small standard uncertainties of the atomic displacement parameters, which further result in the small standard uncertainties of bond distances. - Graphical abstract: Ba(H{sub 2}O)M{sub 2}{sup III}[PO{sub 3}(OH)]{sub 4} (M=Fe, V) varies in space group from P2{sub 1} to P2{sub 1}/c, arising from ordered to disordered distributions of Ba{sup 2+} and H{sub 2}O in the cavities. Highlights: Black-Right-Pointing-Pointer Twinning commonly occurs in monoclinic crystals with a Almost-Equal-To c. Black-Right-Pointing-Pointer Overlapping diffractions from twin domains hamper with the determination of real space groups. Black-Right-Pointing-Pointer Conventional criteria for evaluating the real space groups are not effective in this case. Black-Right-Pointing-Pointer Small standard uncertainty of bond distances is proposed as a sensitive criterion. Black-Right-Pointing-Pointer Using this criterion we determined the order-disorder structures of Ba(H{sub 2}O)M{sub 2}{sup III}[PO{sub 3}(OH)]{sub 4} (M=Fe, V) from twinned crystals.