P2D2
Predicted Powder Diffraction Database


A. Le Bail - March 2008

Introduction
Data
Content
Identification by Automatic Search-Match
Availability
Future
References

Introduction

The P2D2 (Predicted Powder Diffraction Database) is built up by calculating the powder patterns corresponding to the predicted crystal structures from the latest PCOD update [1]. The job is done by the CIF2POW software [2] able to read a large multiple CIF and to provide the corresponding powder patterns in a format described below.


Data

Example of a powder pattern produced by the CIF2POW software :

     15
PCOD1000014                         Click here to see the CIF
Al F3
  10.2102  10.2102   7.2408   90.000   90.000   90.000
P 4/N M M
 6
99
 5.9063  409.   1   0   1
 5.1125  283.   1   1   1
 5.1051  197.   2   0   0
 4.1723  999.   2   0   1
 3.8623  835.   1   2   1
 3.6204   35.   0   0   2
 3.6099  238.   2   2   0
 3.4122  453.   0   1   2
 3.2363  336.   1   1   2
 3.2306  570.   2   2   1
 3.2287  544.   3   1   0
 3.0801  530.   0   3   1
 2.9532  102.   2   0   2
 etc, etc
 1.2596   54.   0   4   5
 1.2586   70.   5   3   4
 1.2501   12.   1   4   5
 1.2475   82.   8   1   1
 1.2408   58.   3   3   5
     1.742
Ref: A. Le Bail, J. Appl. Cryst. 38 (2005) 389-395.


Format of the data in the P2D2 text file :

line 1 : No of the phase in the P2D2 (from 1 to 107449)
line 2 : No of the phase in PCOD
            this is a 7 digits number after the four letters PCOD
            ex : PCOD1000000
            That number allows for retrieving the atomic coordinates in PCOD
            at http://www.crystallography.net/pcod/
line 3 : chemical formula
             ex : Si O2  or   B4 Si3 O12  ,   Si3 Ti O9, etc
             There is a space between each group of (element + number)
line 4 : cell parameters a, b, c, alpha, beta, gamma
line 5 : probable space group (it is a prediction...)
            ex : P -4 M 2   or    P 4/N M M,  I 41/M, etc
            - there is a space between the symmetry elements
            - sometimes noted "unknown" (data coming from the
               first GRINSP version)
line 6 ; a symmetry code number (1-14)
   1 = Cubic
   2 = Body-centered cubic
   3 = Face-centered cubic
   4 = Rhombohedral
   5 = Hexagonal
   6 = Tetragonal
   7 = Body-centered tetragonal
   8 = Orthorhombic
   9 = Body-centered orthorhombic
  10 = Face-centered orthorhombic
  11 = Base-centered orthorhombic
  12 = Monoclinic
  13 = Base-centered monoclinic
  14 = Anorthic
line 7 : number N of pairs [d(A) and  Intensity] + hkl
lines 8 and following : N lines of pairs d(A) and  I + the hkl
                - Imax = 999.
                - intensities at delta(d) < 0.0002 A were added
                - calculated by   L x F**2 x multiplicity
                         (L = Lorentz factor)
                - for X-ray, wavelength = 1.54056 A
                - peaks with I < 10 are discarded
                         (all models are optimized in the P1 space group,
                          so that there could be weak-intensity peaks
                          violating the existence conditions of the "probable"
                          space group...)
                - the d, I, hkl list stops either at 99 lines or at dmax = 1A
next line : I/Icor
last line : reference


Content

The P2D2 contains > 100 000 open framework structures of inorganic oxydes and fluorides. Silicates, phosphates and sulfates of Ti, V, Ga, Nb, Zr; zeolites, etc.

Massive predictions were done by the GRINSP software [3,4] producing N- or N-N'-connected 3D frameworks. The values explored for N-N' are 3, 4, 5, 6, corresponding to polyhedra such as triangles, tetrahedra, pentahedra and octahedra connected exclusively by corners, forming either binary or ternary compounds [5-7].

Single polyhedra compounds :

- Triangles : B2O3 virtual compounds
- Tetrahedra : zeolites and dense SiO2 phases, ordered (Al/Si)O2, (Al/P)O2 and (Al/S)O2 compounds
- Pentahedra : V2O5 compounds with square-based pyramids sharing corners
- Octahedra : MF3 hypothetical crystal structures with M = Al, Cr, Fe, Ga, and mixte compounds with octahedra of 2 different sizes (CaF6/AlF6; NaF6/AlF6).

Mixte polyhedra compounds :

- Triangles/octahedra : BO3/AlO6
- Octahedra/tetrahedra : MO6/M'O4, M = Ti, V, Ga, Nb, Zr; M' = Si, P, S.
- Octahedra/pentahedra : MO6/M'O5; M = Ti, V ; M' = V
- Pentahedra/tetrahedra : MO5/M'O4; M = Ti, V; M' = Si, P, S
- etc

Lists of formulations are available corresponding to the recent PCOD update of 107449 entries, together with classifications according to the framework density (...-FD.txt files), the quality R-factor (...-R.txt files) the complete lists of connectivity sequences (CS) (...-con.txt files) and the multiple CIF corresponding to each series.


Identification by Automatic Search-Match

The main purpose of the P2D2 is to allow for identification of unknown compounds which would even not be recognized by a search-match [8] through the powder diffraction file (ICDD - PDF) (containing essentially the powder data for known compounds).

Up to now, the P2D2 was combined with the EVA search-match software (Bruker) and Highscore (PANalytical).

A demonstration of the performances with EVA is available inside of a  .doc file for MS Word 2000.

See also the powerpoint file corresponding to a recent conference in Brazil. A paper is to be published [9] in the Powder Diffraction Journal, supplementary issue June 2008.


Availability

The 107 449 entries in the PCOD are delivered in full open access under the CIF format.
A search engine allows for retrieval of the PCOD entry number, formula, cell parameters, volume, text :
http://www.crystallography.net/pcod/
or
http://sdpd.univ-lemans.fr/cod/pcod/

If you wish the big multiple CIF containing everything, this is also possible, 114Mo zipped.

The P2D2 may also be delivered as a text file (67 Mo, zipped) for compilation purposes for other search-match software. If your usual search-match program does not give access to the P2D2, ask to your search-match software provider.

The files necessary for EVA are also available here (72Mo, zipped).

Contact : A. Le Bail


Future

If you are interested in the addition of particular families of compounds, isostructural with other families already inside of the PCOD, the structures may be built on demand.

More development of the GRINSP software is intended in order to allow for other connectivities than by corner-sharing (edge, face, mixed). Hole location, filling them with appropriate ions in order to ensure electrical neutrality. Using of the valence bond theory. Etc.

A question is to keep or not several descriptions of the same structure in different space groups and cells, as they may appear during the GRINSP predictions. In PCOD, the choice was made to keep only one model corresponding to a given connectivity sequence, selected for its highest symmetry and best R factor (so that each zeolite model is unique, etc).


References

[1] A. Le Bail. PCOD : The Predicted Crystallography Open Database.
      http://www.crystallography.net/pcod/

[2] A. Le Bail. PPP - Powder Pattern Prediction, IUCr Commission on Powder Diffraction Newsletter 31 (2004) 51-53.

[3] A. Le Bail. Inorganic structure prediction with GRINSP, J. Appl. Cryst. 38 (2005) 389-395.
      http://www.cristal.org/grinsp

[4] A. Le Bail. Geometrically restrained inorganic structure prediction : GRINSP, IUCr Computing Commission Newsletter 4 (2004) 37-45.

[5] A. Le Bail & F. Calvayrac. Hypothetical AlF3 crystal structures, J. Solid State Chem., 179 (2006) 3159-3166.

[6] A. Le Bail. Inorganic structure prediction: too much and not enough, Solid State Phenomenon, 130 (2007) 1-6.

[7] A. Le Bail. Predicted corner-sharing titanium silicates,  Z. Kristallogr., suppl. 26 (2007) 203-208.

[8] J-M. Le Meins, L.M.D. Cranswick and A. Le Bail. Results and conclusions of the internet based Search/Match Round Robin 2002, Powder Diffraction, 18 (2003) 106-113.

[9] A. Le Bail, Frontiers between crystal structure prediction and determination by powder diffractometry, Powder Diffraction (2008) in the press.