BIOVIA MATERIALS STUDIO X-CELLDATASHEETBIOVIA Materials Studio X-Cell1 is a patented, novel, robust,
efficient, integrated, and easy to use indexing algorithm whichuses an extinction-specific dichotomy procedure to perform an
exhaustive search of parameter space to establish a complete
list of all possible unit cell solutions. BIOVIA Materials
Studio X-Cell provides the essential algorithmic support for
researchers investigating both organic and inorganic crystal
structures from medium-quality conventional laboratory
powder diffraction data to high-quality synchrotron powder
diffraction data obtained from X-ray, neutron, and electron
radiation sources.

WHAT ARE THE CHALLENGES?
Crystal structure determination frequently is a prerequisite
for the rational understanding of the solid-state properties of
new materials. The ideal method for solving crystal structures
is through single-crystal X-ray diffraction. Growing single
crystals of appropriate size, however, is often difficult or even
impossible, whereas powder samples are readily available foranalysis by powder diffractometry. Indexing the experimental
powder pattern has emerged as a significant bottleneck forcrystal structure determination from powder diffraction data.
The failure rate of the commonly used indexing algorithms
is fairly high. Most indexing programs do not determine thezero-point shift of the powder diffraction pattern together withthe unit cell parameters, that may prevent the success of thesearch for unit cell parameters. The robustness with respect
to impurity peaks is, in general, fairly low, and none of theexisting programs offers a choice of specific impurity tolerance
levels. Indexing a flat unit cell is a particular challenge, since allor most of the selected reflections may belong to the dominantzone in reciprocal space, and the unit cell may not be uniquely
defined.A high ratio of reflections may not be observed due topeak overlap, preferred orientation, or poor statistics. In suchcases, the correct unit cell is difficult to distinguish from thelarge number of incorrect solutions with similar figures of merit.

WHAT DOES BIOVIA MATERIALSSTUDIO X-CELL DO?
Developed at Accelrys, BIOVIA Materials Studio X-Cell is aninnovative indexing algorithm that addresses many of theshortcomings of conventional indexing algorithms. By taking
into account systematic absences during the search, the ability
to identify the correct indexing solution is greatly enhanced.

The volume of parameter space and the number of incorrect
solutions increase rapidly with the number of calculated
peaks. BIOVIA Materials Studio X-Cell performs a search from
low to high peak numbers while explicitly taking into accountsystematic absences.
The correct unit cell is therefore found earlier and thus reduces
the required CPU time. In addition, the correct unit cell is lesslikely to be hidden among a large number of incorrect solutions.
BIOVIA Materials Studio X-Cell is the first indexing program touse explicit impurity tolerance levels to specify the maximum
number of unindexed reflections among the experimentallyobserved reflections. Correct unit cells can be found even in thepresence of impurity contamination in a main crystalline phase,a task that is very difficult if not impossible for current indexing
programs. In some cases, the dominant phase of mixtures canbe indexed in the presence of even up to 50% impurity peaks if
high-quality synchrotron data are available.
For a given range of allowed calculated peak numbers, a given
pattern of systematic absences, and a given impurity tolerance
level, an exhaustive search is performed using a successive
dichotomy procedure. Zeropoint shift is determined along withunit cell parameters. All indexing solutions are fully optimized,compared, and ranked according to a relative figure of merit
Fr. Fr favors solutions with low numbers of calculated peaks,
adjustable parameters, and impurity peaks. Indexing of longand flat unit cells is facilitated by the ability to search for rows
(1D) or zones (2D) in reciprocal space first, and then the searchis refined using the lattice parameters of the dominant row orzone in the final unit cell (3D) search in combination with amodified Pawley refinement2.
The main advantages of BIOVIA Materials Studio X-Cell arecompleteness and robustness. Regardless of whether or nota promising unit cell has already been found, the algorithm
continues to search larger and larger portions of parameter
space until the search is complete or interrupted by the user.
Initial validation1 indicated that BIOVIA Materials Studio X-Cell
has a higher success rate (92%) than DICVOL3 (46%), TREOR4(46%), and ITO5 (33%) for 24 representative cases. BIOVIA
Materials Studio X-Cell copes very well with various difficulties
that are typically encountered in powder indexing, including
contamination with impurity phases, strong peak overlap, peak
position errors, zero-point shift and extreme cell geometries.
Implementation of the successive dichotomy approach issignificantly more efficient in BIOVIA Materials Studio X-Cellthan in DICVOL3.

THE BIOVIA MATERIAL STUDIO ADVANTAGEBIOVIA Materials Studio X-Cell is implemented in BIOVIA
Materials Studio Reflex, operated within the BIOVIA Materials
Studio® software environment. BIOVIA Materials Studio’s
integrated model building and editing tools enable you toconstruct, visualize, and manipulate structures of crystalline
solids (e.g., drugs, pigments, metals, metal oxides, zeolites,etc.) using BIOVIA Materials Studio Visualizer. Results obtainedby indexing are readily passed on to other modules in BIOVIA
Materials Studio Reflex and BIOVIA Materials Studio Reflex Plusfor a full structure solution determination. It is easy to produce
high quality images. Structural information and diffraction
data can be readily exported to and imported from other PC
applications - allowing you to share them with colleagues
and perform further analysis using a spreadsheet and otherpackages.

HOW DOES BIOVIA MATERIALS STUDIO
X-CELL WORK?
The task of indexing involves finding a unit cell for which thecalculated 20 positions of the diffraction peaks match theexperimentally observed peak positions. Diagram 11 illustrates
the workflow of BIOVIA Materials Studio X-Cell. An example ofinputs for BIOVIA Materials Studio X-Cell calculations is shownin Figure 1. The correctness of a unit cell solution can be verified
easily through a Modified Pawley Refinement as indicated inFigure 2.

Start

Activate all impurity tolerance levels (ITL) up to userChoose active ITL with lowest performed dichotomy steps

Choose range of calculated peack numbers

Loop A
Loop B

Examine all crystal

Examine all powder extinction classes (space groups withsame pattern of systematic absences)

Successive dichotomy search on parameter spaceEnd loopNOYESEnd loop

Search finished forFEATURESNONOYES
Deactivate Current ITLAll ITAFigure 2: Correct unit cell obtained from BIOVIA Materials Studio
X-Cell confirmed with a Modified Pawley Refinement

NOYESEndDiagram 1: Workflow of BIOVIA Materials Studio X-CellFigure 1: The BIOVIA Materials Studio interface displaying
an experimental X-ray powder diffraction pattern and theBIOVIA Materials Studio X-Cell control panel• Reads in a variety of diffractometer file formats including
Bruker, Stoe, Scintag, Jade, Philips, JCAMP, Galactic SPC,GSAS raw, ILL, PAnalytical XRDML• Allows for different X-ray sources with arbritary wavelengths.
• Suitable for X-ray, neutron, and electron diffraction• Ability to pre-process experimental data, for example via
background subtractions, data interpolation, data smoothing,data scaling, and Ka2 strippings• Manual or automatic peak identification• Choice of peak search methods - simple or Savitsky Golay• Choice of dimensionality search on rows (1D), zones (2D),and unit cell (3D)
• Eight different impurity tolerance levels control allowable
impurity peaks among the experimentally observed peaks• Two impurity modes to enable searches at an allowed
impurity tolerance level only, or in parallel at all impurity
levels up to the allowed maximum• Can handle any possible space group, both standard andnonstandard settings, accounting for systematic absences• Controls the bounds of the search dimension by setting
target values and allowed errors for one or three lattice
parameters• User-defined highest allowed number of calculated peaks• Ability to set expected zero-point shift and the range for thezero-point search• Full interactivity with powder pattern and data in tableformat• Returns unit cell parameters, figures of merit, a stability
parameter,number of impurity peaks,number of observed
peaks, number of calculated peaks not overlapped withobserved peaks, zero-point shift, average 20 deviations trial• Enables construction of a unit cell from any successful
solution, for further analysis with BIOVIA Materials Studio
Reflex tools• Benefits from straightforward interaction with other BIOVIA
Materials Studio products as well as other PC applications

REFERENCES:
1. M.A.Neumann, J. Appl. Cryst., 2003, 36, 356-365.
2. G.E. Engel, S.Wilke, O. Konig, K.D.M. Harris, and F.J.J. Leusen, J. Appl. Cryst., 1999, 32, 1169-1179.
3. A. Boultif and D. Louer, J. Appl. Cryst., 1991, 24, 987-993.
4. P.E.Werner, L. Ericksson and M.Westdahl, J. Appl. Cryst., 1968, 1, 108-113.
©2014 Dassault Systèmes. All rights reserved. 3DEXPERIENCE, the Compass icon and the 3DS logo, CATIA, SOLIDWORKS, ENOVIA, DELMIA, SIMULIA, GEOVIA, EXALEAD, 3D VIA, BIOVIA and NETVIBES are commercial trademarks
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5. J.W.Visser, J. Appl. Cryst., 1969, 2, 89-95.

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