crystallographer Interview Questions and Answers

1. What is crystallography?

   • Answer: Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids. It involves using various techniques, primarily X-ray diffraction, to elucidate the structure and properties of materials at the atomic level.

2. Explain Bragg's Law.

   • Answer: Bragg's Law describes the conditions for constructive interference of X-rays scattered from a crystal lattice. It states: nλ = 2d sinθ, where n is an integer (order of reflection), λ is the wavelength of the X-rays, d is the interplanar spacing, and θ is the angle of incidence (and reflection).

3. What are the different types of crystal systems?

   • Answer: There are seven crystal systems: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and rhombohedral (trigonal). They are defined by the lengths and angles of their unit cell axes.

4. What is a unit cell?

   • Answer: A unit cell is the smallest repeating unit of a crystal lattice. The entire crystal structure can be built by repeating the unit cell in three dimensions.

5. Explain the difference between a crystal lattice and a crystal structure.

   • Answer: A crystal lattice is a three-dimensional array of points representing the periodic arrangement of atoms in a crystal. The crystal structure includes the lattice and the basis (the arrangement of atoms within the unit cell).

6. What are Miller indices?

   • Answer: Miller indices are a system of three integers (hkl) that represent the orientation of a crystal plane. They are the reciprocals of the fractional intercepts of the plane on the crystallographic axes.

7. Describe the process of X-ray diffraction data collection.

   • Answer: X-ray diffraction data is collected by directing a monochromatic X-ray beam onto a crystal. The diffracted beams are recorded by a detector (e.g., CCD or film), resulting in a diffraction pattern of spots. The intensity and position of these spots are used to determine the crystal structure.

8. What is a reciprocal lattice?

   • Answer: The reciprocal lattice is a mathematical construct used in crystallography. It's a lattice in reciprocal space, where each point corresponds to a set of crystallographic planes in real space. It simplifies calculations related to diffraction.

9. Explain the concept of structure factor.

   • Answer: The structure factor (F_hkl) is a complex number representing the amplitude and phase of the X-rays diffracted from a set of crystallographic planes (hkl). It depends on the positions and scattering factors of the atoms in the unit cell.

10. What is the phase problem in crystallography?

    • Answer: The phase problem arises because diffraction experiments only measure the intensity of diffracted beams (|F_hkl|²), not their phases. Determining the phases is crucial for reconstructing the electron density and thus the structure.

11. What are some methods used to solve the phase problem?

    • Answer: Methods include isomorphous replacement, molecular replacement, anomalous scattering, and direct methods.

12. What is refinement in crystallography?

    • Answer: Refinement is the iterative process of adjusting the atomic positions and other parameters of a structural model to improve the agreement between the observed and calculated diffraction intensities.

13. What is R-factor?

    • Answer: The R-factor (or R-value) is a measure of the agreement between the observed and calculated structure factors. A lower R-factor indicates a better model.

14. What is the difference between powder and single-crystal X-ray diffraction?

    • Answer: Single-crystal X-ray diffraction uses a single, well-ordered crystal to obtain a 3D diffraction pattern, providing high-resolution structural information. Powder diffraction uses a powdered sample, resulting in a 1D pattern, which is less precise but suitable for identifying phases.

15. What are some other techniques used in crystallography besides X-ray diffraction?

    • Answer: Neutron diffraction, electron diffraction, and other scattering techniques.

16. What software packages are commonly used in crystallography?

    • Answer: Examples include SHELX, CCP4, PHENIX, and Olex2.

17. What are some applications of crystallography?

    • Answer: Drug discovery, materials science, mineralogy, biochemistry, and many other fields.

18. How do you determine the space group of a crystal?

    • Answer: By analyzing the symmetry elements present in the diffraction pattern and using systematic absences.

19. What is a point group?

    • Answer: A point group describes the symmetry elements of a molecule or crystal that leave at least one point unchanged.

20. Explain the concept of symmetry operations.

    • Answer: Symmetry operations are transformations (rotation, reflection, inversion) that leave the object looking identical.

21. What is a screw axis?

    • Answer: A screw axis combines rotation and translation.

22. What is a glide plane?

    • Answer: A glide plane combines reflection and translation.

23. What are some challenges in crystallography?

    • Answer: Obtaining high-quality crystals, solving complex structures, and interpreting results.

24. How do you assess the quality of a crystal?

    • Answer: By examining its shape, size, and diffraction pattern quality.

25. What is anomalous scattering?

    • Answer: Anomalous scattering occurs when the scattering factor of an atom changes significantly near an absorption edge.

26. What is the role of a crystallographer in drug discovery?

    • Answer: To determine the 3D structures of drug targets and drug-target complexes to aid in drug design.

27. What is the importance of crystallography in materials science?

    • Answer: To understand the relationship between the structure and properties of materials and to design new materials with desired properties.

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