Completion of crystal structure by powder data: a new method for locating atoms with polyhedral coordination

Ab initio crystal structure solution via powder diffraction data is often incomplete: frequently, the heavy atoms are correctly located but the light-atom positions are usually unreliable. The recently developed procedure POLPO [Altomare et al. (2000). J. Appl. Cryst. 33, 1305-1310], implemented in the EXPO program [Altomare et al. (1999). J. Appl. Cryst. 32, 339-340], aims at completing a partial structure model provided by direct methods by exploiting the prior information on the polyhedral coordination of the located atoms (tetrahedral or octahedral) and their connectivity. The POLPO procedure requires that all the cations are correctly labelled and rightly located. This condition does not always occur, particularly when the data quality is poor. A new method is described which is able to locate missing cations and surrounding anions when the cation coordination is tetrahedral or octahedral. The procedure has been successfully checked on different test structures.