Direct space methods of structure determination from powder diffraction: principles, guidelines and perspectives

Černý, Radovan; Favre‐Nicolin, V.

doi:10.1524/zkri.2007.222.3-4.105

Cited by 100 publications

(29 citation statements)

References 67 publications

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“…Algorithms are known from the global optimization field of mathematics, and the simplest algorithms, like reversed Monte Carlo (RMC) in simulated annealing and parallel tempering mode or evolution algorithms, are used. For more information, see Chapter 15 in [3] or [22].…”

Section: Methods Using Pattern Modellingmentioning

confidence: 99%

Crystal Structures from Powder Diffraction: Principles, Difficulties and Progress

Černý

2017

Crystals

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Abstract:The structure solution from powder diffraction has undergone an intense evolution during the last 20 years, but is far from being routine. Current challenges of powder crystallography include ab initio crystal structure determination on real samples of new materials with specific microstructures, characterization of intermediate reaction products from in situ, in operando studies and novel phases from in situ studies of phase diagrams. The intense evolution of electron diffraction in recent years, providing an experimental (precession) and theoretical (still under intense development) solution to strong dynamic scattering of electrons, smears the traditional frontier between poly-and single-crystal diffraction. Novel techniques like serial snapshot X-ray crystallography point in the same direction. Finally, for the computational chemistry, theoreticians hand-in-hand with crystallographers develop tools where the theory meets experiment for crystal structure refinement, which becomes an unavoidable step in the validation of crystal structures obtained from powder diffraction.Keywords: powder diffraction; structure solution; X-ray diffraction; DFT calculations; electron diffraction The structure solution means in this article determination of the (average) 3D-periodic arrangement of the atoms in the crystal using mainly the information content of Bragg scattering in the diffraction pattern. The case of aperiodic crystals (modulated and quasicrystals), of short-range order (diffuse scattering), as well as of nano-crystals (crystals without measurable Bragg signals) will not be discussed here. The structure prediction, on the other hand, will be used in the sense of the determination of the 3D arrangement of the atoms in the crystal using mainly first-principle calculations, and any experimental observation, like the diffraction pattern, is used only for the validation of the predicted structural model. Indexing: Still a BottleneckDespite an intense development of indexing algorithms in the last 20 years, the detection of the correct lattice may still be a bottleneck of the structure solution in the case of low resolution powder patterns or in the case of monoclinic and triclinic symmetry. A review of known indexing algorithms may be found in Chapter 7 of [3], Chapter 5 of [6] and Chapter 7 of [7]. Each crystallographer and each crystal may work better with a particular algorithm, but as is said in another excellent indexing The advantage of the dichotomy algorithm is its speed (especially in Fox) and its robustness towards low data quality. Indexing is followed by space group determination, which is based on systematic extinction analysis supported by the knowledge of crystal physical properties, like piezoelectricity, the presence of which excludes centrosymmetric space groups. Fairly robust algorithms for space group determination are based on full pattern fitting (Le Bail or Pawley), available in most software packages. The indexing can be quite difficult for the crystals lying on opposite ends of the s...

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Section: Methods Using Pattern Modellingmentioning

confidence: 99%

Crystal Structures from Powder Diffraction: Principles, Difficulties and Progress

Černý

2017

Crystals

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“…In order to eliminate an uncertainty introduced by a non-ideal centering of a capillary, it should be rotated within the same angular range during exposures, providing a smooth variation of the cell parameters. Useful guidelines for successful indexing of powder data can be found in Shankland, 2008, andČ erný andFavre-Nicolin, 2007. Space group determination can be another critical point: "While observing lattice-centering extinctions is usually relatively easy, the determination of the correct space group symmetry elements is generally more challenging" (David and Shankland, 2008). This is even more valid for poorly crystallized hydrides with broad peaks and strong pseudosymmetry, which is difficult to detect in an X-ray pattern due to the weak X-ray scattering power of hydrogen.…”

Section: Indexing and Space Group Determinationmentioning

confidence: 99%

Complex inorganic structures from powder diffraction:case of tetrahydroborates of light metals

Černý¹,

Filinchuk

2011

Zeitschrift Für Kristallographie

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Abstract. Powder diffraction plays a central role in the characterization of light metal tetrahydroborates (borohydrides), novel boron based hydrides recognized as a potential solution for hydrogen storage. Numerous novel BH À 4 based materials have been investigated during the past few years and this class of materials has a fascinating structural chemistry. We review the powder diffraction methods, problems and solutions which are specific for these weakly diffracting and badly crystallized materials. Examples of highlights and pitfalls of the powder diffraction are given. Complex structures with as many as 55 independent atoms were fully characterized, and P-T phase diagrams accessible from in-situ powder diffraction enable understanding of these ionic crystals with important directional bonding.

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“…In this system ab all-milled sample contains two new phases, KSc(BH 4 ) 4 ,K 3 ScCl 6 and non-reacted KBH 4 .O nly T-ramping has allowed the assignment of observed peaks to individual phases, and led to as uccessful indexing of powder patterns. Usefulg uidelines for successful indexing can be found in Shankland, 2008, Č erný andFavre-Nicolin, 2007). Space group determination can be another critical point: "While observing lattice-centering extinctions is usually relatively easy,t he determination of the correct space group symmetry elements is generally more challenging" (David and Shankland, 2008).…”

Section: Ab-initio Structured Eterminationmentioning

confidence: 99%

Powder diffraction methods for studies of borohydride-based energy storage materials

Ravnsbæk¹,

Filinchuk

Černý

et al. 2010

Zeitschrift für Kristallographie

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Abstract. The world today is facing increasing energy demands and as imultaneous demand for cleaner and more environmentallyf riendlye nergy technologies. Hydrogen is recognized as ap ossible renewable energy carrier,b ut its large-scale utilization is mainly hampered by insufficient hydrogens torage capabilities. In this scenario, powder diffractionh as ac entral position as the mosti nformative and versatile technique availablei nm aterials science. This is illustrated in the present reviewb ys ynthesis, physical, chemical and structural characterisationo fn ovel boron based hydrides for hydrogen storage.N umerous novel BH À 4 based materials have been investigated during the past fewy ears and this class of materials has af ascinating structural chemistry.T he experimental methods presented can be applied to av ariety of other materials.

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Direct space methods of structure determination from powder diffraction: principles, guidelines and perspectives

Cited by 100 publications

References 67 publications

Crystal Structures from Powder Diffraction: Principles, Difficulties and Progress

Crystal Structures from Powder Diffraction: Principles, Difficulties and Progress

Complex inorganic structures from powder diffraction:case of tetrahydroborates of light metals

Powder diffraction methods for studies of borohydride-based energy storage materials

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