Anna Moliterni

SIR97: a new tool for crystal structure determination and refinement

SIR97 is the integration of two programs, SIR92 and CAOS, the first devoted to the solution of crystal structures by direct methods, the second to refinement via least-squares–Fourier procedures. Several new features have been introduced in SIR97 with respect to the previous version, SIR92: greater automatization, increased efficiency of the direct methods section, and a powerful graphics interface. The program also provides publication tables and CIF files.

EXPO: a program for full powder pattern decomposition and crystal structure solution

EXPO is the integration of two programs, EXTRA and SIRPOW.92, the first devoted to full powder pattern decomposition and the second to the solution and refinement of crystal structures. The program is able to exploit the prior information obtained during the crystal structure solution process for improving the classical decomposition. EXPO also allows preliminary cycles of Rietveld refinement.

EXPO2009: structure solution by powder data in direct and reciprocal space

The program EXPO2009 is the evolution of EXPO2004 [Altomare, Caliandro, Camalli, Cuocci, Giacovazzo, Moliterni & Rizzi (2004). J. Appl. Cryst. 37, 1025–1028]. EXPO2009 performs all the steps of ab initio structure solution by powder data: indexing, space-group determination, estimation of the reflection integrated intensities, structure solution by direct/Patterson methods and/or by a direct-space/hybrid approach, and model refinement by the Rietveld technique. New procedures have been introduced in EXPO2009 for enhancing the structure solution process, particularly in the case of low-resolution data and/or organic compounds, when traditional approaches like direct methods may fail. The EXPO2009 graphical interface has been optimized and made very user friendly.

EXPO2013: a kit of tools for phasing crystal structures from powder data

EXPO2013, the heir of EXPO2009, has been enriched by a variety of new algorithms and graphical tools aiming at strengthening the individual steps of the powder structure solution pathway. Particular attention has been addressed to the procedures devoted to improving structural models provided by direct methods in ab initio approaches. In addition, a new procedure has been implemented, working in direct space, which may be chosen by the user as an alternative to the traditional simulated annealing algorithm.

EXTRA : a program for extracting structure-factor amplitudes from powder diffraction data

Extraction of structure-factor amplitudes from a powder diffraction pattern is not a straightforward procedure. Peak overlapping and background estimation are the main obstacles to the process : they may introduce strong correlations among reflection intensities and heavy errors in their estimates. The program EXTRA is described, which, on the basis of the Le Bail algorithm, is able reliably to estimate the structure-factor amplitudes in a fully automatic way.

Automatic structure determination from powder data with EXPO2004

EXPO2004 is the updated version of the EXPO program [Altomare et al. (1999). J. Appl. Cryst. 32, 339–340]. The traditional steps of the ab initio powder solution process are performed automatically: indexing, space-group determination, decomposition of the pattern for extracting the observed structure-factor moduli, structure solution by direct methods, model refinement by Rietveld technique. Special strategies may be applied to improve both the estimates of the extracted structure-factor moduli and the quality of the structure model. In addition, the use of special procedures exploiting available supplementary information on molecular geometry can be successfully adopted. The graphical interface has also been improved.

New techniques for indexing: N-TREOR in EXPO

Indexing of a powder diffraction pattern is still a critical point in procedures aiming at solving crystal structures from powder data. New code has been associated to the program TREOR90 in order to define an efficient peak search procedure, to modify the crystallographic decisions coded into TREOR90 to make it more exhaustive, to refine the selected unit cell automatically, and to make the entire procedure user friendly, via a graphical interface. The new program, called N-TREOR, has been integrated into the package EXPO to create a suite of programs able to provide a structural model from the analysis of the experimental pattern. N-TREOR is also available as a stand-alone program.

Solving Crystal Structures from Powder Data. IV. The Use of the Patterson Information for Estimating the |F|'s

The positivity of the Patterson function is used as prior information for the decomposition of a powder diffraction pattern. An automatic procedure is described which, integrated with the Le Bail method, is able to provide estimates of |F| values that are often remarkably better than those obtained by application of standard techniques. The procedure, implemented in a computer program, has been applied to a set of test structures (experimental data); the results show that, particularly for structures with some heavy atoms, the new estimates of the structure-factor moduli are far better (on average) than those obtained via the default application of EXTRA [Altomare et al. (1995). J. Appl. Cryst. 28, 842–846]. 1. Symbols and abbreviations The papers by Altomare, Carrozzini et al. (1996), Altomare, Foadi et al. (1996) and Carrozzini et al. (1997) are referred to as papers I, II and III, respectively.

Quanto: a Rietveld program for quantitative phase analysis of polycrystalline mixtures

Quantitative determination of phase abundance in a multicomponent polycrystalline mixture is a basic goal in materials characterization. Because of several advantages compared with traditional techniques, the Rietveld method has been increasingly applied to this task. Further progress in this direction would be to carry out the analysis automatically. The new Rietveld package Quanto is devoted to the automatic estimation of the weight fraction of each crystalline phase in a mixture. The amorphous content can be estimated by means of the internal-standard method. Corrections for preferred orientation and microabsorption effects are available. A user-friendly graphical interface facilitates interaction. A data bank including several organic and inorganic phases is supplied with the program.

On the number of statistically independent observations in a powder diffraction pattern

When in a powder diffraction pattern two or more peaks heavily overlap, the integrated intensities of the overlapping reflections are correlated. This paper describes a method, based on the study of the peak overlap, which estimates the number of statistically independent intensities associated with a powder diagram. Such a number can be useful for forecasting the rate of success of direct methods procedures for ab initio crystal structure solution and for evaluating a priori the efficiency of least-squares programs devoted to crystal structure refinement.