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

Abstract

Ab initio crystal structure solution via powder diffraction data is often incomplete: frequently, the heavy atoms are correctly located but the lightatom 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.

Automatic structure determination from powder data withEXPO2004

“…To recover the full structure, the approach described by Altomare et al (2000a) and by Giacovazzo et al (2002) has been implemented in EXPO2004 (see also Grosse-Kunstleve et al, 1997): it is able to combine the information on the cation coordination, usually available a priori, with Monte Carlo methods. To recover the full structure, the approach described by Altomare et al (2000a) and by Giacovazzo et al (2002) has been implemented in EXPO2004 (see also Grosse-Kunstleve et al, 1997): it is able to combine the information on the cation coordination, usually available a priori, with Monte Carlo methods.…”

4. Single crystal and powder XRD techniques: An overview

“…Developed and implemented in some computing programs more recently than the two previously mentioned solution approaches, the hybrid methods [36][37][38] combine the global optimization techniques with the phasing procedures, trying to exploit the best features of the two methods.Of particular interest is the combination of non-ab initio techniques with direct methods results, giving rise to a couple of hybrid procedures implemented in the EXPO program: POLPO1 [39] and POLPO2 [40]. The main features of POLPO1 will be described below.…”

Obtaining models suitable for Rietveld refinement: the EXPO2011 techniques

“…This reduced efficiency is due to the loss of information usually met in powder crystallography (see § 1). The limits can be overcome in case of inorganic structures with known heavy-atom polyhedral coordination (tetrahedral and/or octahedral), via the application of POLPO [64,65], a hybrid approach combining DM results (i.e., the positions of the cations), with a Monte Carlo procedure exploiting the prior information on the cation coordination and on the expected cationanion bond distances and angles. The full knowledge of the molecular geometry, required by typical non-ab initio techniques (see § 5), is therefore not necessary.…”

“…If DM are unable to locate all the heavy atoms POLPO2 [65] can be carried out for positioning the missing cations. Once these last have been located POLPO1 may be applied for completing the structure.…”

“…Of particular interest is the combination of non-ab initio techniques with Direct Methods, dictated by the necessity of exploiting the prior information on the molecular geometry in the cases in which DM are unable to provide a fully interpretable electron density map. This idea has been implemented in the package EXPO2011, where two types of hybrid approaches have been developed: 1) the POLPO procedure [64,65], which combines DM with Monte Carlo techniques. It aims at completing inorganic structural models, and is described in § 6; 2) the combination of DM with SA, addressed to organic structures [66,67].…”