CENTROSYMMETRIC OR NONCENTROSYMMETRIC - CASE-STUDY, GENERALIZATION AND STRUCTURAL REDETERMINATION OF SR5NB5O17

The possibility that the structure of the novel semiconducting perovsk ite-related material strontium niobium oxide, Sr5Nb5O17, refined by Sc hmalle et al. [Acta Cryst. (1995), C51, 1243-1246] in space group Pnn2 , might instead belong to space group Pnnm has been investigated follo wing an analysis of the atomic coordinates that indicated the latter s pace group to be more likely. All lobs were carefully remeasured, firs t those within a hemisphere containing c, then all that lay within th e full sphere of reflection. Refinement was undertaken, with each of t wo different sets of weights, in each space group. Each data set was u sed under three limiting intensity conditions: I-obs > 4 sigma(I-obs): I-obs > 2 sigma(I-obs) and finally with all reflections, but setting magnitudes with I-obs less than or equal to 0 equal to 0. Fourteen sep arate tests based only upon the X-ray diffraction data may be used to distinguish between Pnn2 and Pnnm. Nine tests favored the latter choic e, four were indeterminate and one was not used. Seven further tests m ay be made on the basis of physical measurement; of these, three stron gly indicated Pnnm, one was indeterminate and three could not be used. The evidence clearly suggests the space group is Pnnm. The use of all reflections, including those with negative magnitude set equal to zer o, is essential to avoid ambiguity in the X-ray diffraction tests and achieve the highest reliability. Refinement with weights based on vari ances of Type A and Type B [Schwarzenbach et ai. (1995). Acta Cryst. A 51, 565-569] resulted in improved reliability compared with that obtai ned from a popular empirical weighting scheme. The revised structure d iffers in several respects from that published previously.