Space Group Assignment

Most indexing programs produce a cell, but not a recommended space group. There are two main methods of determining the space group from powder data, which are discussed below. 

---

These values were used to correct the intensities for nine representative data sets, organic crystals, organometallics and minerals, and the data compared with respect to systematic absences and space group assignment (i) with no corrections to the data, (ii) with an absorption correction (SADABS [9]), (iii) with only 1/2 correction, and (iv) with both absorption and X/2 correction. Analogously four different refinements per sample were carried out based on F2. 

The magnitudes of the corrections varied quite widely over the nine data sets, the average correction being less than one esd, however the maximum correction was 47.4s. In all cases there was an improvement in the number of observed systematic absences and, hence, space group assignment. 

The structure of /3-cristobalile has been determined several times over the past 60 years, but crystal disorder has led to Uncertainty in the space group assignment (Hyde, B. Q. Andersson, S. Inorganic Crystal Structural Wiley New York. 1989 pp 393-395. 

Recent single crystal studies on LiF UF4 14) and 7NaF 6ZrF4 15) established the space group assignments and stoichiometry so that powder diffraction data on the new actinide compounds could be more... 

The single-crystal structure of AgFBp4 was successfully refined in space group P4ln. The earlier supposition (I) of a linear (Ag-F)" chain, based on the tetragonal unit cell and tentative space group assignment. 

Full profile fitting can be useful for various tasks including unit cell refinement involving peak overlap, space group assignment, extracting intensities prior to structure solution, and pre-structure refinement fitting of the powder pattern. The two main methods are listed below. 

The needle-like crystals of pSs are of yellow color and, typically, they are twinned. Lattice parameters and space group assignment were first reported by Burwell in 1937 [89]. Atomic parameters were reported by Sands [90], and with higher accuracy by Templeton et al. [81]. The latter authors also gave a crystallographic explanation of the twinning of pSs crystals. 

The space group assigned to DL-threonine by these authors is certainly incorrect (27). 

Following the determination of the unit cell and space group assignment, integrated intensities (or structure factor amplitudes) are extracted with a pattern-decomposition technique. For clusters of overlapping lines an equipartition of the overall... 

---