Low-Temperature X-Ray Diffraction : Apparatus and Techniques

Low-temperature X-ray diffraction (LTXRD) investigations offer many challenges to the diffractionist, not all of which are technical or scientific in nature. LTXRD studies can be frustrating: There are at least two reports of investigations ruined by the loss of crystals (grown with extreme difficulty) because of the widespread power failure and blackout in the northeastern United States in late 1965. LTXRD studies can cause discomfort: In several instances, "low temperatures" have been attained by opening all the windows in the X-ray laboratory. LTXRD studies can be dangerous: It was once reported that a crys­ tal was lost because a laboratory assistant fell down a flight of stairs and lay unconscious for about an hour on his way to refilling a liquid-nitrogen (LN2 ) dewar. This last report indicated the disposition of the crystal but not that of the laboratory assistant. However, in general, the results of low-temperature X-ray diffraction investigations cannot be obtained in any other manner, and one is well compensated for the effort expended in constructing and maintaining a low-temperature system. Crystal-structure analyses of solidified liquids and gases, phase transformation investigations, accurate crystal-structure analy­ ses and electron-density maps, thermal expansion measurements, and defect structure studies are a few of the many important applications of LTXRD.

I: Introduction and Applications.- 1. Introduction.- 1.1. Historical Background.- 1.2. Cooling Apparatus.- 1.2.1. Provision for Cooling the Sample.- 1.2.1.1. Gas Stream.- 1.2.1.2. Conduction.- 1.2.1.3. Immersion.- 1.2.2. Control and Measurement of the Sample Temperature.- 1.2.3. Frost Prevention.- 1.3. Problem Areas in Designing LTXRD Apparatus.- 1.3.1. Absorption of X-Radiation.- 1.3.2. Size of Instrument.- 1.3.3. Frost Formation.- 1.3.4. Temperature Stability.- 1.3.5. Temperature Calibration.- 1.3.6. Misalignment.- 1.3.7. Sample.- 2. Applications.- 2.1. Solidified Liquids and Gases.- 2.2. Crystal-Structure Analysis.- 2.3. Electron-Density Distribution.- 2.4. Radiation Damage.- 2.5. Protein Crystallography.- 2.6. Defect Structures.- 2.7. Solid-Solid Phase Transition.- 2.7.1. Plastic Crystals.- 2.7.2. Metastable States.- 2.7.3. Phase Transitions.- 2.7.4. High-Pressure, Low-Temperature Studies.- 2.8. Thermal Expansion Coefficients and Precision Lattice Constants.- 2.9. Low-Temperature Powder Patterns.- 2.10. Other X-Ray Diffraction Applications.- 2.11. Application of Low-Temperature Methods to Other Types of Physical Measurements.- II: Low-Temperature X-Ray Diffraction Apparatus.- 3. Gas-Stream Cooling Apparatus.- 3.1. General Principles.- 3.1.1. Gas Generator.- 3.1.2. Transfer Lines and Nozzle.- 3.1.3. Temperature Control.- 3.1.4. Temperature Measurement.- 3.1.5. Frost Prevention.- 3.1.6. Rate of Cryogen Consumption.- 3.1.7. Gas-Stream Systems.- 3.1.8. Protection of X-Ray Apparatus.- 3.2. The Cold-Stream Nozzle.- 3.2.1. Size of the Opening.- 3.2.2. Outer Stream.- 3.2.3. Construction.- 3.2.4. Calibration of Temperature Gradient.- 3.3. Construction of a Simple Gas-Stream Apparatus.- 3.3.1. The Nozzle.- 3.3.2. The Heat Exchanger.- 3.3.3. Dry Gas.- 3.3.4. Temperature Control System.- 3.4 More Sophisticated Low-Temperature Gas-Stream Systems.- 3.4.1. Gas Stream from External Source.- 3.4.1.1. Mechanical Refrigeration.- 3.4.1.2. Heat Exchanger Immersed in Cold Bath.- 3.4.2. Gas Stream from Boiling Liquid Nitrogen.- 3.4.2.1. The Reservoir as Gas Generator.- 3.4.2.2. Reservoir Fills Small Gas Generator.- 3.4.2.3. Reservoir Fills Large Gas Generator.- 3.4.3. Gas Stream from Boiling Liquid Hydrogen or Liquid Helium.- 3.4.3.1. Weissenberg Goniometer.- 3.4.3.2. Single-Crystal Diffractometer.- 3.4.3.3. Rotation Camera.- 3.4.3.4. Powder Camera.- 3.4.4. Chambers Cooled by Gas Streams.- 3.5. Special Considerations for Specific X-Ray Diffraction Instruments.- 3.5.1. Weissenberg Goniometer.- 3.5.1.1. Back-Stream Method.- 3.5.1.2. Layer-Line Screens.- 3.5.1.3. Nozzle Support.- 3.5.1.4. Film Cassette.- 3.5.1.5. Rat-Plate Adapter.- 3.5.1.6. Upper-Level Photographs.- 3.5.2. Precession and Oscillation-Rotation Cameras.- 3.5.3. Single-Crystal Diffractometer.- 3.5.4. Powder Cameras and Diffractometer.- 3.6. Summary of Gas-Stream Methods.- 4. Conduction-Cooling Apparatus.- 4.1. General Principles.- 4.1.1. Introduction.- 4.1.2. Advantages of Conduction-Cooling Techniques.- 4.1.3. Disadvantages of Conduction-Cooling Techniques.- 4.1.3.1. Limitations on Data Collection.- 4.1.3.2. Absorption of X-Ray Beam.- 4.1.3.3. Thermal Gradients.- 4.1.3.4. Sample Preparation in Situ.- 4.2. Cryostat Design Requirements.- 4.2.1. Frost.- 4.2.2. Beryllium Windows.- 4.2.3. Temperature Gradients.- 4.2.4. Continuous-Flow Cryostat.- 4.3. Special Considerations for Individual X-Ray Instruments.- 4.4. Conduction Cooling via Liquefied Gases.- 4.4.1. Single-Crystal Cryostats for Use with Liquid Helium.- 4.4.2. Single-Crystal Cryostats for Use with Liquid Nitrogen.- 4.4.3. Powder-Sample Cryostats for Use with Liquid Helium or Liquid Nitrogen.- 4.4.3.1. Camera Apparatus.- 4.4.3.2. Liquid-Helium Cryostats for Powder Diffractometers.- 4.4.3.3. Liquid-Nitrogen Cryostats for Powder Diffractometers.- 4.4.3.4. Diffractometer Adapters.- 4.5. Conduction Cooling via Mechanical Refrigeration.- 4.6. Conduction Cooling via Thermoelectric Cooling.- 4.7. Conduction Cooling via Joule-Thomson Expansion.- 5. Immersion-Cooling Apparatus.- 5.1. General Principles.- 5.2. Immersion of Sample in Cold Liquid.- 5.2.1. Spraying.- 5.2.2. Dripping.- 5.2.3. Dipping.- 5.3. Immersion of Camera in Cold Liquid.- 5.4. Use of Cold Room.- III: Low-Temperature X-Ray Diffraction Techniques.- 6. Sample Preparation.- 6.1. Sample Preparation: Samples Solid at Room Temperature.- 6.1.1. Well-Behaved Solids.- 6.1.2. Unstable Samples.- 6.1.3. Protein Samples.- 6.1.4. Phase Transitions.- 6.2. Sample Preparation: Samples Liquid at Room Temperature.- 6.2.1. Crystallization Prior to Mounting.- 6.2.2. Crystallization Simultaneously with Mounting.- 6.2.3. Crystallization after Mounting.- 6.2.3.1. Filling and Sealing of Capillary Tubes.- 6.3. Sample Preparation: Samples Gaseous at Room Temperature.- 6.4. Preparation of Reactive or Radioactive Samples.- 6.5. Techniques for Preparing Randomly Aligned Powder Samples.- 7. Crystal-Growing-Techniques.- 7.1. Samples Solid at Room Temperature.- 7.2. Samples Liquid or Gaseous at Room Temperature.- 7.2.1. Crystal Growth at Low Temperatures Prior to Mounting the Crystal.- 7.2.2. Crystal Growth from the Melt; Sample in Capillary Tube.- 7.2.2.1. Growth Not on the X-Ray Instrument.- 7.2.2.2. Growth by Controlling the Temperature of a Gas Stream.- 7.2.2.3. External Heating.- 7.2.2.4. Special Problems.- 7.2.3. Crystal Growth from the Melt; Sample Not in Capillary Tube.- 7.3. Crystal Growth below the Temperature of Phase Transition.- 7.3.1. Slow Cooling.- 7.3.2. Annealing.- 7.3.3. Growth from Solution.- 7.4. Test for Centrosymmetric Crystal.- 8. Data Collection and Reduction.- 8.1. Temperature of Data Collection.- 8.1.1. Choice of Temperature.- 8.1.2. Temperature Calibration.- 8.2. Apparatus to Be Used for Data Collection.- 8.2.1. Protection of the X-Ray Instrument.- 8.2.2. Film at Low Temperature.- 8.2.3. Alignment and Calibration of Apparatus.- 8.3. Data Collection.- 8.3.1. Strategy of Data Collection.- 8.3.2. Check for Single Crystal.- 8.3.3. Alignment of Crystal.- 8.3.4. Radiation Damage.- 8.4. Data Reduction.- 8.4.1. Absorption Corrections.- 8.4.1.1. Powder Sample.- 8.4.1.2. Absorption by Windows and Sample Holder.- 8.4.1.3. Sample in Capillary Tube and Completely Bathed in the Beam.- 8.4.1.4. Sample in Capillary Tube and Larger than the Beam.- 8.4.1.5. Effect of Capillary Tube on Background Measurement.- 8.4.2. Refinement of Disordered Models.- 9. Future Trends in Low-Temperature X-Ray Diffraction.- IV: Appendices, Bibliography, and Index.- Appendix 1. Temperature Measurement.- Appendix 2. Cryogenic Liquids and Solids.- Appendix 3. Mechanical Refrigeration.- Appendix 4. Joule-Thomson Expansion Cooling.- Appendix 5. Thermoelectric Devices.- Appendix 6. Dewars.- Appendix 7. Transfer Lines.- Appendix 8. Insulation.- Appendix 9. Frost Prevention.- Appendix 10. Miscellaneous Gas-Flow Accessories.- Appendix 11. Low-Temperature Adhesives and Greases.- Appendix 12. List of Manufacturers of Low-Temperature Equipment and Accessories.- Appendix 13. Recommended Procedure for Describing Low-Temperature Apparatus in Publications.- B.1. Introduction.- B.2. Alphabetical Listing.- B.3. Apparatus Code-Number Listing.- B.4. Techniques and Applications Code-Number Listing.