Powder Diffraction Project Detailed Marking Scheme

Course Material Index  Course Index  Administration  TLA

Detailed Marking Scheme

Marks are given according to the following categories:

1. Overall presentation and appearance

   The project report should look good and be easy to read when viewed on both medium resolution (800x600 or 1024x768 pixels) and higher resolution (1280x1024 or better) computer screens. The project should be easy to navigate.

   Marks can be given for attractive pictures of the refined crystal structure. However, marks to be deducted for excessive pictures, tables, or figures, that detract from the readability of the report.

   [10 Marks]

2. Quality of HTML source

   Marks are deducted for the following errors or bad practice:
   

   • Broken links to other HTML pages
   • Broken links to figures
   • Excessively long (> 255 characters) lines of text
   • Vendor specific HTML tags
   • HTML files not submitted as ASCII text
   (e.g. use of 8 bit character fonts for, say, for ° or Å)
   • Non-functioning tables
   • Spurious tags
   Missing end tags are not penalised unless the output is affected by the lack of them.

   [10 Marks]

3. Results and Conclusions from the Fit to 5 K Data Set A

   Uses the orthorhombic room-temperature structure as the starting model.

   Actual numbers similar to those obtained in the Project Solution should be obtained.

   Students should refine only an overall B value using this data set given the limited angular range of data provided.

   Separate tables of final parameters must be given so that the conclusions are clear to the reader.

   [10 Marks]

4. Results and conclusions from the fit to 5 K data set B
   1. Using the lower-symmetry space groups P12₁/n1.

      Actual numbers similar to those obtained in the Project Solution should be obtained.

      Students should refine only an individual isotropic B value using this data set given that the sample is at 5 K.

   2. Using the lower-symmetry space groups P2₁/b11 (or P12₁/a1 with x and y interchanged).

      Actual numbers similar to those obtained in the Project Solution should be obtained.

      Students should refine only an individual isotropic B value using this data set given that the sample is at 5 K.

      Separate tables of final parameters must be given so that the conclusions are clear to the reader.

   [20 Marks]

5. Appropriate use of tables.

   A minimum of three tables should be provided with the following information:
   

   • Clear table caption with, for example, sample, instrument, and space group details;
   • Atomic coordinates for each refinement;
   • Atomic thermal parameters for each refinement;
   • Appropriate R-factors;
   • Bond lengths and angles;
   • Appropriate instrumental values (e.g. UVW and any peak shape parameters (but not scale or instrumental zero values);
   • Appropriate number of decimal places;
   • Units where applicable;
   • Estimated standard deviations on all refined parameters.

   [15 Marks]

6. Appropriate use of figures.

   A minimum of four figures should be provided:

   1. Best fit to data set A;
   2. Fit to data set B using parameters from refinement (i);
   3. Best fit to data set B using P12₁/n1
   4. Best fit to data set B using P2₁/b11 (or equivalent space group setting)

   All plots should possess the following:

   • Suitable figure caption;
   • Suitable axes with a reasonable number of values on axes;
   • Observed data;
   • Calculated data;
   • Difference function;
   • Calculated reflection positions;
   • GIF (line diagram) and not JPEG (photographic) format.

   [15 Marks]

7. Correct CIF information for archival purposes.

   Sufficient CIF items must be given to demonstrate a knowledge of CIF. Marks are deducted for errors in the CIF information (especially where the output of any program has been used without question).

   CIF file should be a plain text file and must contain the following:

   • Refined unit cell parameters
   • Atomic parameters with esd's
   • Some structural information such as bond lengths and/or angles

   [10 Marks]

8. Overall conclusions

   Some conclusions may be given in the body of the document, but the document should contain a summary at the end. Marks are to be given for correctly mentioning the following items:

   • Correct space group deduction and distortion for lead sulphate at 5 K;
   • Comments on the uses of a limited range of data;
   • Comments on the lattice parameters compared to RT;
   • Comments on the structural parameters compared to RT;
   • Comments on how the esd's vary from one fit to another;

   [10 Marks]

Course Material Index  Course Index  Administration  TLA