General comments: Take care with scattering angle: sometimes you need to use q and othertimes 2q.
Symmetry answers generally very good

1. Thickness of the lead foil = 6,20165 mm.

   Formula used:
   I(l)/ I₀(l)= exp{-(m/r)_l rx}
   or:
   10/10⁸ = exp{-(230.11,3).x}

   Comment: You seem to have made a mistake with the units?
   2 Marks

   Usually the beam stop is thicker because the X-ray beam is unfiltered and non-monochromated. As a result, X-rays with another energy (higher as well as lower) have to be captured too.

   Comment: Correct, but you need to point out that the higher energy X-rays are more penetrating.
   1 Mark

2. Organic materials consist mainly of light elements (C, H, O, N,...). The X-rays penetrate deeper into the sample and therefore they are also scattered at atoms below the surface. The resulting detection angle for these scattered X-rays is always a little lower then for the X-rays scattered at the surface. So, transparency leads to broader peaks shifted towards lower angles.

   This can be overcome by using a silicon single crystal sample holder covered by a very thin "coating" of the organic powder (dusting the powder on the holder.)
   Result: the peaks will be much sharper, but the intensities will be much less reliable and smaller.

   2 Marks

   The characteristic feature of synchrotron radiation that is particularly beneficial for powder diffraction studies of organic powder samples,is the possibility to select the incident X-ray wavelength from the highly intense continuous synchrotron radiation source. Another wavelength then the usual Cu K_a1 radiation can be used as monochromatic indicent radiation. So the wavelength for organic materials can be chosen slightly above or well below any of the absorption edges of the elements in the sample.

   The two design features of synchrotron angle-despersive powder diffractometers used: Debye Scherrer and Flat plate.

   Comment: See model answer for this part
   0 Mark

3. Formula's used:

   E = 12.398 / l
   and
   n.l = 2.d.sinq
   results in:
   C = E.d.sinq (E in keV, d in Å, the C = 6,1993keVÅ)

   So, d = C / (E.sinq)
   for E = 20keV and 2q = 3.5° or q = 1.75° is d = 10.15Å
   for E = 100keV and 2q = 3.5° or q = 1.75° is d = 2.03Å

   Comment: You need to state explicitly the range; otherwise answer is fine
   4 Marks

4. Given:

   Si(111) - d = 3.3136Å
   l = 2.d.sin(q_M/2)
   wavelength l₀ = 0.75Å
   wavelength l₁ = 0.65Å

   Problem: change of the angle 2q_M

   Answer:
   sin(q_M0/2) = 0.75Å/(2x3.3136Å)
   q_M0 = 0.2268 rad
   sin(q_M1/2) = 0.65Å/(2x3.3136Å)
   q_M0 = 0.1965 rad
   So the change in 2q = - 0.0303 rad
   or the change in 2q = - 1° 44'

   Comment: You need to take care with the angle here (see model answer).
   Also, you want to work in degrees and avoid minutes (and seconds) of a degree as well.
   3 Marks

5. 1. -x,y,-z : a twofold rotation axis parallel to the y-direction and passing through the origin, (0,0,0), symmetry element: 2.

      1 Mark

   2. 1/2+x,1/2+y,1/2+z : translation in the [1,1,1] direction by 1/2a, 1/2b and 1/2c. (??)

      1 Mark

   3. -x,-y,-z : centre of inversion at the origin, symmetry element: -1 (minus sign above the digit)

      1 Mark

   4. -x,-y,1/2+z : a two-one screw axis with a twofold rotation about the c-axis, i.e. about the line 0,0,z , followed by a translation of 1/2c along it, symmetry element: 2₁

      1 Mark

   5. x,y,-z : mirror plane perpendicular to the c-axis and passing through the origin (plane x,y,0), symmetry element: m

      1 Mark

   6. 1/2+x,-y,z : mirror plane perpendicular to the b-axis and passing through the origin (plane x,0,z) followed by a translation of 1/2a along a, symmetry element: a (because translation in "a" direction)

      Minor comment: State explicitly that a is a glide plane
      1 Mark

   7. 1/2+x,-y,-z : a twofold rotation about the x-axis, followed by a 1/2a translation along it, element: 2₁

      1 Mark

   8. 1/2+x,1/2+y,z : a translation 1/2a + 1/2b (??)

      1 Mark

   9. 1/2-x,1/2-y,z : a twofold rotation about z-axis followed by a translation along 1/2a + 1/2b, symmetry element: 2₁

      Comment: Wrong symmetry element
      0 Marks

   10. x,1/2-y,z : mirror plane perpendicular to b-axis passing through the origin (plane x,0,z) followed by a translation of 1/2b along b, symmetry element: m

       Comment: Symmetry element correct, but location in space wrong
       0.5 Marks

   Comment: For ii and viii, you need to specify more detail, i.e. translation is of type I and C, respectively.
   -0.5 Marks