A tangent formula derived from Patterson-function arguments. VII. Solution of inorganic structures from powder data with accidental overlap

2000 ◽  
Vol 33 (5) ◽  
pp. 1208-1211 ◽  
Author(s):  
J. Rius ◽  
X. Torrelles ◽  
C. Miravitlles ◽  
L. E. Ochando ◽  
M. M. Reventós ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Powder Diffraction ◽  
Inorganic Compounds ◽  
Solution Process ◽  
Direct Methods ◽  
Powder Diffraction Data ◽  
Patterson Function ◽  
Structure Factors ◽  
Structure Solution ◽  
Overlapped Peaks

Accidental overlap constitutes one of the principal limitations for the solution of crystal structures from powder diffraction data, since it reduces the number of available intensities for direct-methods application. In this work, the field of application of the direct-methods sum function is extended to cope with powder patterns with relatively large amounts of accidental overlap. This is achieved by refining not only the phases of the structure factors but also the estimated intensities of the severely overlapped peaks during the structure solution process. This procedure has been specifically devised for inorganic compounds with uncertain cell contents and with probable severe atomic disorder, a situation often found when studying complex minerals with limited crystallinity. It has been successfully applied to the solution of the previously unknown crystal structure of the mineral tinticite. Finally, an estimation of the smallest ratio (number of observations to number of variables) for the procedure to be successful is given.

The crystal structure of sarmientite, Fe23+ (AsO4)(SO4)(OH)·5H2O, solved ab initio from laboratory powder diffraction data

2014 ◽  
Vol 78 (2) ◽  
pp. 347-360 ◽  
Author(s):  
F. Colombo ◽  
J. Rius ◽  
O. Vallcorba ◽  
E. V. Pannunzio Miner
Keyword(s):  
Crystal Structure ◽  
Powder Diffraction ◽  
Diffraction Data ◽  
Rietveld Method ◽  
Direct Methods ◽  
Hydroxyl Group ◽  
Monoclinic Space Group ◽  
Powder Diffraction Data ◽  
Patterson Function ◽  
Unit Cell Dimensions

AbstractThe crystal structure of sarmientite, Fe23+ (AsO4)(SO4)(OH)·5H2O, from the type locality (Santa Elena mine, San Juan Province, Argentina), was solved and refined from in-house powder diffraction data (CuKα1,2 radiation). It is monoclinic, space group P21/n, with unit-cell dimensions a = 6.5298(1), b = 18.5228(4), c = 9.6344(3) Å, β = 97.444(2)º, V = 1155.5(5) Å3, and Z = 4. The structure model was derived from cluster-based Patterson-function direct methods and refined by means of the Rietveld method to Rwp = 0.0733 (X2 = 2.20). The structure consists of pairs of octahedral-tetrahedral (Fe−As) chains at (y,z) = (0,0) and (½,½), running along a. There are two symmetry-independent octahedral Fe sites. The Fe1 octahedra share two corners with the neighbouring arsenate groups. Both individual chains are related by a symmetry centre and joined by two symmetry-related Fe2 octahedra. Each Fe2 octahedron shares three corners with double-chain polyhedra (O3, O4 with arsenate groups; the O8 hydroxyl group with the Fe1 octahedron) and one corner (O11) with the monodentate sulfate group. The coordination of the Fe2 octahedron is completed by two H2O molecules (O9 and O10). There is also a complex network of H bonds that connects polyhedra within and among chains. Raman and infrared spectra show that (SO4)2− tetrahedra are strongly distorted.

Molecular, crystallographic and algorithmic factors in structure determination from powder diffraction data by simulated annealing

2002 ◽  
Vol 35 (4) ◽  
pp. 443-454 ◽  
Author(s):  
Kenneth Shankland ◽  
Lorraine McBride ◽  
William I. F. David ◽  
Norman Shankland ◽  
Gerald Steele
Keyword(s):  
Crystal Structure ◽  
Simulated Annealing ◽  
Powder Diffraction ◽  
Diffraction Data ◽  
Solution Process ◽  
Search Space ◽  
Systematic Investigation ◽  
Monoclinic Space Group ◽  
Powder Diffraction Data ◽  
Structure Solution

The crystal structure of famotidine form B has been solved directly from powder diffraction data by the application of simulated annealing. The molecule crystallizes in the monoclinic space groupP21/cwith refined unit-cell dimensionsa = 17.6547 (4),b= 5.2932 (1),c= 18.2590 (3) Å and β = 123.558 (1)° atT= 130 K. The core of this work is a systematic investigation of the influence of algorithmic, crystallographic and molecular factors on the structure solution process. With an appropriate choice of annealing schedule, molecular description and diffraction data range, the overall number of successes in solving the crystal structure is close to 100%. Other factors, including crystallographic search space restrictions and parameter sampling method, have little effect on the structure solution process. The basic principles elucidated here have been factored into the design of theDASHstructure solution program.

Crystal structure determination by neutron powder diffraction using structural and packing constraints

1991 ◽  
Vol 24 (6) ◽  
pp. 1005-1008 ◽  
Author(s):  
P. G. Byrom ◽  
B. W. Lucas
Keyword(s):  
Crystal Structure ◽  
Powder Diffraction ◽  
Structure Determination ◽  
Direct Methods ◽  
Neutron Powder Diffraction ◽  
Crystal Structure Determination ◽  
Diffraction Profile ◽  
Peak Separation ◽  
Structure Factors ◽  
Structure Solution

In the past, crystal structure determination of solids consisting of molecules (or atom groups) whose geometry and size are known approximately has often been attempted using neutron powder diffraction profile refinement techniques, but without inclusion of this information. A method of structure solution has therefore been developed to include it. The proposed method does not require a set of structure factors and thus avoids the problems encountered in separating peaks in a powder diffraction scan. A successful test was conducted with a previously determined (yet treated as unknown) crystal structure, where direct methods had failed to solve the structure due to incorrect peak separation. Two computer programs, MODEL and PARAM, that implement the method are described.

Completion of Crystal Structure with Polyhedral Coordination: A New Procedure

2004 ◽  
Vol 443-444 ◽  
pp. 23-26
Author(s):  
Angela Altomare ◽  
Corrado Cuocci ◽  
Carmelo Giacovazzo ◽  
Anna Grazia ◽  
Anna Grazia Giuseppina Moliterni ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Powder Diffraction ◽  
Diffraction Data ◽  
Prior Information ◽  
Direct Methods ◽  
Structure Model ◽  
Powder Diffraction Data ◽  
Partial Structure ◽  
Structure Solution ◽  
Cation Coordination

The ab-initio crystal structure solution via powder diffraction data is often uncomplete. A recent procedure POLPO [1] 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 has been developed. 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.

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

2002 ◽  
Vol 35 (4) ◽  
pp. 422-429 ◽  
Author(s):  
Carmelo Giacovazzo ◽  
Angela Altomare ◽  
Corrado Cuocci ◽  
Anna Grazia Giuseppina Moliterni ◽  
Rosanna Rizzi
Keyword(s):  
Crystal Structure ◽  
Powder Diffraction ◽  
Diffraction Data ◽  
Direct Methods ◽  
New Method ◽  
Structure Model ◽  
Powder Diffraction Data ◽  
Heavy Atoms ◽  
Structure Solution ◽  
Cation Coordination

Ab initiocrystal structure solutionviapowder diffraction data is often incomplete: frequently, the heavy atoms are correctly located but the light-atom positions are usually unreliable. The recently developed procedurePOLPO[Altomareet al.(2000).J. Appl. Cryst.33, 1305–1310], implemented in theEXPOprogram [Altomareet 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. ThePOLPOprocedure 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.

A tangent formula derived from Patterson-function arguments. VI. structure solution from powder patterns with systematic overlap

1999 ◽  
Vol 32 (1) ◽  
pp. 89-97 ◽  
Author(s):  
Jordi Rius ◽  
Carles Miravitlles ◽  
Hermann Gies ◽  
Josep M. Amigó
Keyword(s):  
Hexagonal Lattice ◽  
Direct Methods ◽  
High Alumina ◽  
High Symmetry ◽  
Powder Diffraction Data ◽  
Patterson Function ◽  
Space Groups ◽  
Peak Overlap ◽  
Structure Solution ◽  
Alumino Silicate

Besides accidental peak overlap, systematic overlap constitutes one of the principal limitations for the solution of crystal structures from powder diffraction data. Unlike accidental overlap which affects all types of structures, systematic overlap is restricted to high-symmetry structures (e.g.65% of the space groups compatible with a hexagonal lattice). In this work, the direct-methods sum function is adapted to cope with data extracted from patterns containing systematic overlap. Preliminary results indicate that at least for moderate-size inorganic structures, systematic overlap should not represent a serious drawback for the application of direct methods. In contrast to the usual two-stage procedures employed for solving structures with accidental overlap, here both multiplet decomposition and phase refinement are carried out simultaneously. This procedure is illustrated using two examples: the dominant crystalline phase of a hydrated high-alumina cement and the fibrous alumino-silicate `aerinite'.

Real-space technique applied to crystal structure determination from powder data

2002 ◽  
Vol 35 (2) ◽  
pp. 182-184 ◽  
Author(s):  
Angela Altomare ◽  
Corrado Cuocci ◽  
Carmelo Giacovazzo ◽  
Antonietta Guagliardi ◽  
Anna Grazia Giuseppina Moliterni ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Powder Diffraction ◽  
Structural Model ◽  
Real Space ◽  
Structure Parameters ◽  
Powder Diffraction Data ◽  
Structure Solution ◽  
Phase Extension ◽  
Space Technique ◽  
New Process

Real-space techniques used for phase extension and refinement in the modern direct procedures forab initiocrystal structure solution of proteins have been optimized for application to powder diffraction data. The new process has been implemented in a modified version ofEXPO[Altomareet al.(1999).J.Appl.Cryst.32, 339–340]. The method is able to supply a structural model that is more complete than that provided by the standardEXPOprogram. The model is then refinedviaa diagonal least-squares procedure, but only when the ratio of the number of observations to the number of structure parameters to be refined is larger than a given threshold.

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