2fold Screw Axes

1
2-fold Screw Axes

• Along b have x,1/2y,-z
• For points on the axis (i.e. 0,y,0) this looks
  like a translation only.
• Just like 1-d centered cell
• Result for 0k0 h2n systematic presence
• If the screw does not run through origin the same
  presences exit because cannot detect the origin
  shift!

2
31 Along C axis

• In this case only waves with l3 are allowed
• Thus the condition is 00l, l3n

3
Glide perpendicular to b

• As before, points in the ac plane see only
  translation.
• Translate ½ along c
• Allowed h0l l2n
• For n glide translation is ½ along the ac
  diagonal resulting in h0l hl2n

4
Centering

• C centering adds (1/2,1/2,0) to all points.
• Therefore for all hkl hk2n
• I centering adds (1/2,1/2,1/2) to all points
• Therefore for all hkl hkl2n

5
(No Transcript)
6
C2/c
7
Pnma
8
Data Equivalence

• The symmetry of the data is given by the Laue
  group.
• Friedels law F(hkl)F(-h-k-l)

9
(No Transcript)
10
Space Group Determination

• By Hand
• Automatically by programs
• NOTESpace Group determination canNOT be done
  until after the data is collected.

11
By Hand
12
Test Crystal (ambi)

• Orthorhombic
• Must check
• h,0,0 0,k,0 0,0,l h,k,0 h,0,k h,k,0 and
  h,k,l
• Data file is called xl.hkl
• (Note LINUX is case sensitive)

13
Programs

• Find pattern and weight by frequency of known
  space groups.
• This approach can find patterns that are not
  thereP2/c vs P21/c P212121 vs P21212
• Look for weak reflections I/sigma(I)
• Use some high cutoff i.e. 10sigma
• This at time fails to find absences

14
Programs

• xpreppart of SHELXTL
• Uses method 1
• Does all processing
• absen
• Uses method 2

15
Space Group Determination

• To refine the structure satisfactorily the
  correct space group must be used.
• If there is a centric-accentric pair try the
  centric space group first unless there are
  reasons to rule it out.
• Sometimes you can refine in an incorrect space
  group. This results in a mess and you can get
  Marshed