Diffraction, Non-Crystallinity, and the PDF Database pdf

Diffraction, Non-Crystallinity, and the PDF Database Cyrus E. Crowder, ICDD Tim Fawcett, ICDD Classical Diffraction • Classic ‘Bragg’ scattering is characterized by discrete peaks arising from long-range crystallographically ordered planes: λ = 2d hkl sinθ hkl • The 2009 PDF databases have > 660,000 entries based on the concept of discrete Bragg peaks arising from crystalline structures. • The existing concept of ‘Phase Identification’ is based on matching accurately-determined positions and intensities of peaks in an experimental patterns to those for PDF database entries. Classical XRPD NaCl – An Example 1 1 1 2 0 0 2 2 0 2 2 2 3 1 1 The XRPD Pattern of a Common Pharmaceutical Excipient The process of identifying this material by conventional XRPD search/match techniques is compromised by the small number of clearly defined diffraction maxima and the difficulty in specifying the precise positions of these maxima. Further, these ‘maxima’ are likely shifted from the true underlying positions of the major Bragg peaks due to the significant overlap. Use of full pattern matching for phase identification • The ICDD has begun compiling a database (PD3) of XRPD profiles for such materials. • Such a database can be used for manual comparison with an experimental pattern, however manual comparisons with larger databases for identifying true unknown materials would be slow and subjective. • An automated full-pattern comparison would be desirable to screen the database for most-likely matches. The Integral Index • The Integral Index is a numerical value based on a point-by- point comparison of two digitized X-ray diffraction pattern This numerical value ranges from 0-100 and is based on the Hofmann and Kuleshova similarity index [1]: • The lower the index value, the better the match, giving 0 a perfect match and 100 a complete non-match. • We can use this relationship to rank the match between full patterns from a given database set with the pattern from the unknown material ∑ ∑ ∑ ∑ ∑ = = = = = ⎥ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎢ ⎣ ⎡ −= n i n j count i i count n j calc i i calc jI iI jI iI n S 1 1 1 1 1 int 0 00 1 Example Using Integral Index to Identify Nanocrystalline Material • To facilitate comparison, the background is removed before computing the integral indices. • To facilitate speed, the database is limited to a likely subset of entries, in this case anything with cellulose in the name. • If unsuccessful, other subfiles, i.e., polymers, fillers, or forensics, could be specified. Perform Integral Index Calculations • An estimated crystallite size parameter is used to compute comparible FWHMs for peaks, in this case, 3.5 nm. • The software simulates a pattern for each database entry, using the d-space and intensity values. • Each simulated database pattern is compared with the unknown pattern to generate integral index values. Patterns Generated from Crystal Structure Where Available References for Form I alpha, Form I beta and Form II Simulation of microcrystalline states of cellulose Via PDF-4+ Scardi, Leoni Faber PDF 00-056-1717 Cell II PDF 00-056-1718 Cell I beta PDF 00-056-1719 Cell I alpha Patterns Generated from Experimental d-I list if Crystal Structure is Not Available [...]... amorphous cellulose and that for the cellulose Iβ crystalline form with a simulated crystallite size of 1.5 nm Nanocrystalline Cellulose II? Comparison of X-ray diffraction patterns for amorphous cellulose and that for the cellulose II crystalline form with a simulated crystallite size of 1.5 nm Amorphous Standards • Clearly the database should be expanded to include full  patterns for standard amorphous materials since these ... Integral Index Comparing with all 16 PDF experimental entries that contain ‘cellulose’ in the name, the integral index values vary from 2.26 up to 19.28 for these entries The best integral index fit is with 50-2241 – cellulose Iβ Integral Index = 2.26 Cellulose 1β Comparison of Simulated Cellulose Patterns to Experimental Pattern Red – experimental pattern Blue – ‘Standard’ database pattern Cellulose Iβ Integral... Pattern PDF- 4+ 2009 database searched for entries containing ‘poly’ in the name and having elements only within the set of C, H, N, O, F, and Cl Found: 651 Using a crystallite size of ~15 nm, Integral Index values were computed for this experimental pattern that ranged from ~2.5 to 45 (Roughly 20 seconds to compute on a 2-year-old Dell Inspiron.) The top three are examined closer on the next slide Integral... 5.93 Clearly, the cellulose 1β polymorph is a better match than the cellulose II polymorph Non-crystalline Material This is an X-ray ‘diffraction’ pattern for amorphous cellulose (Sigma Cellulose ground 13 hours) The same pattern is obtained after long periods of grinding, regardless of whether the starting cellulose was form Iα, Iβ, or II (Courtesy Ewa Bucher, International Paper) The lack of long... International Paper) The lack of long range order means we have no conventional “Bragg” diffraction, but instead, rather broad features based on the distribution of interatomic distances within the disordered structure Nanocrystalline Cellulose Iα? Comparison of X-ray diffraction patterns for amorphous cellulose and that for the cellulose Iα crystalline form with a simulated crystallite size of 1.5 nm Nanocrystalline... patterns for standard amorphous materials since these  cannot be generated from Bragg peak positions or  crystallographic information.  • This would allow integral index comparisons to be  performed for both amorphous standards as well as  nanocrystalline variations of crystalline materials  already in the database Raw Polymer Pattern for Identification using Integral Index Background Removal Integral Index Calculations for Polymer Pattern PDF- 4+... Structures Faulted clay materials The Editorial Challenge of a “Full Pattern” Database • How many ‘different’ entries are needed for a given material (i.e chain branching, molecular weight, melting point, degree of crystallinity, etc)? Nanomaterials and blends St Johns Wort Echinacea Benadryl Powdered Bone Lipoic Acid Effexor Amorphous Materials and Blends Saw Palmetto SunTheanine *Courtesy of Ewa Bucher... Semi-crystalline Polypropylene Crystalline Polypropylene Polymers such as these are really 2phase systems, one being crystalline, the other being amorphous Unlike polypropylene, many have only one or two significant Bragg peaks making conventional search/match identification difficult Semi-crystalline Polypropylene Automated full pattern comparisons have the potential to be a better identification tool for XRPD patterns... Sources Cellulose* Tools that assist in identifying materials with poor crystallinity • Data and databases • Simulations – crystallite size, pair distribution functions, cluster analyses, total pattern fitting, “random-walk”models, Rietveld refinement, integral index • Any knowledge of specimen chemistry, processing and composition • Complimentary data – melting point, infrared,nmr, functional groups . 0-100 and is based on the Hofmann and Kuleshova similarity index [1]: • The lower the index value, the better the match, giving 0 a perfect match and. compromised by the small number of clearly defined diffraction maxima and the difficulty in specifying the precise positions of these maxima. Further, these ‘maxima’