Abstract:Application of Direct Methods to reflection intensities obtained by powder diffraction is often obstructed by the fact that not all intensities are reliable. In this paper a weighting scheme is proposed to overcome this problem. The scheme is implemented in the Direct Methods program SIMPEL88 in such a way that phase relations containing overlapping reflections are less probable than they would have been containing very reliable reflections only. Preliminary test results show that this way of tackling the prob… Show more
“…Tremayne et al applied the concepts of maximum entropy to the problem with some success, [48] but considerable manual intervention was required. In the early to mid 90s, the groups of Schenk, [49] Giacovazzo [50] and Rius [51] began adapting their single-crystal direct methods programs to address the problems posed by powder diffraction data, and as a result, structure solution from XPD data using direct methods improved substantially. The programs EXPO [52] and XLENS [53] represent the current state of that art.…”
The development of powder diffraction (PD) techniques for structure analysis is traced from its inception almost 100 years ago to the present day, with a brief glimpse of what SwissFEL can contribute in the near future. Although PD data were used in the early days to deduce some simple high-symmetry structures, it was not until computers, instrumentation and synchrotrons arrived on the scene that the true potential of PD data could be realized. In the last 25 years, PD has blossomed into a viable method, not only for structure refinement, but also for structure solution. This means that scientists with polycrystalline materials that cannot be grown as single crystals can still obtain the structural information they need. Historically, structure solution from PD data began with model building, progressed through the application of single-crystal methods to simpler structures and the adaptation of those methods to the special problems posed by PD data, the development of automated model-building algorithms, and most recently to the application of charge flipping. As X-ray sources and detectors continue to develop, the boundary between a powder and a single crystal is narrowing. Laue microdiffraction techniques and the prospects offered by SwissFEL will allow single-crystal data to be collected on some polycrystalline materials.
“…Tremayne et al applied the concepts of maximum entropy to the problem with some success, [48] but considerable manual intervention was required. In the early to mid 90s, the groups of Schenk, [49] Giacovazzo [50] and Rius [51] began adapting their single-crystal direct methods programs to address the problems posed by powder diffraction data, and as a result, structure solution from XPD data using direct methods improved substantially. The programs EXPO [52] and XLENS [53] represent the current state of that art.…”
The development of powder diffraction (PD) techniques for structure analysis is traced from its inception almost 100 years ago to the present day, with a brief glimpse of what SwissFEL can contribute in the near future. Although PD data were used in the early days to deduce some simple high-symmetry structures, it was not until computers, instrumentation and synchrotrons arrived on the scene that the true potential of PD data could be realized. In the last 25 years, PD has blossomed into a viable method, not only for structure refinement, but also for structure solution. This means that scientists with polycrystalline materials that cannot be grown as single crystals can still obtain the structural information they need. Historically, structure solution from PD data began with model building, progressed through the application of single-crystal methods to simpler structures and the adaptation of those methods to the special problems posed by PD data, the development of automated model-building algorithms, and most recently to the application of charge flipping. As X-ray sources and detectors continue to develop, the boundary between a powder and a single crystal is narrowing. Laue microdiffraction techniques and the prospects offered by SwissFEL will allow single-crystal data to be collected on some polycrystalline materials.
“…A probably nonexhaustive list of published dedicated SDPD software from which structure solution could be expected for one or both samples is as follow (alphabetical order): DASH (David et al , 2006), EAGER (Harris et al , 1998), ENDEAVOUR (Putz et al , Figure 2.X-ray powder pattern for sample 1 (calcium tartrate tetrahydrate). The second reflection at low angle culminates at ∼116 000 counts.Figure 3.Synchrotron powder pattern for sample 2 (lanthanum tungstate). 1999), ESPOIR (Le Bail, 2001), EXPO (Altomare et al , 2004), FOX (Favre-Nicolin and Cerny, 2002), GEST (Feng and Dong, 2007), OCTOPUS (Harris et al , 1994), ORGANA (Brodski et al , 2005), POWDERSOLVE (Engel et al , 1999), PSSP (Stephens and Huq, 2002), SAFE (Brenner et al , 2002), SA (Andreev et al , 1997), SIMPEL (Jansen et al , 1993), SUPERFLIP (Palatinus and Chapuis, 2007), TOPAS (Coelho, 2000), and XLENS (Rius, 2004). A list of computer programs for the prediction of the packing of molecular structure, which could have been used for the structure solution of sample 1, can be found in Day et al , 2005.…”
Section: Round-robin Organization Samples and Timetablementioning
The results from a third structure determination by powder diffractometry ͑SDPD͒ round robin are discussed. From the 175 potential participants having downloaded the powder data, nine sent a total of 12 solutions ͑8 and 4 for samples 1 and 2, respectively, a tetrahydrated calcium tartrate and a lanthanum tungstate͒. Participants used seven different computer programs for structure solution ͑ESPOIR, EXPO, FOX, PSSP, SHELXS, SUPERFLIP, and TOPAS͒, applying Patterson, direct methods, direct space methods, and charge flipping approach. It is concluded that solving a structure from powder data remains a challenge, at least one order of magnitude more difficult than solving a problem with similar complexity from single-crystal data. Nevertheless, a few more steps in the a͒
Aus vielen kristallinen Festkörpern lassen sich keine hinreichend großen und/oder für die Untersuchung mit Einkristall‐Röntgenbeugungsverfahren qualitativ geeigneten Einkristalle erhalten. Darum ist für das Verständnis der strukturellen Eigenschaften solcher Stoffe die Möglichkeit zur Strukturbestimmung aus Pulverbeugungsdaten absolut notwendig. Obwohl man den Schritt der Verfeinerung bei der Strukturbestimmung aus Pulverbeugungsdaten mit der Rietveld‐Methode zur Verfeinerung des Profils routinemäßig durchführen kann, sind mit der Lösung der Kristallstruktur direkt aus den Daten von Pulveraufnahmen einige Schwierigkeiten verbunden. Gleichwohl hat man in den letzten Jahren auf diesem Gebiet hinsichtlich der Anwendungsmöglichkeiten und der Aussagekraft wesentliche Fortschritte erzielt. In diesem Beitrag werden die strukturellen Probleme solcher Strukturbestimmungen hervorgehoben, die man inzwischen direkt ausgehend von Pulverbeugungsdaten angehen kann, ebenso werden neue Anwendungen aus anderen Bereichen der Chemie vorgestellt. Die zugrunde liegenden Methoden werden kurz behandelt, wobei wir schwerpunktmäßig auf kürzlich entwickelte Methoden für den Schritt der Strukturlösung im Strukturbestimmungsverfahren eingehen.
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