Electron microscopy
 
P21/m (11) Space Group
- Practical Electron Microscopy and Database -
- An Online Book -
Microanalysis | EM Book                                                                                   http://www.globalsino.com/EM/        

=================================================================================

 

Table 1669a. P21/m (11) space group.

Name in the International Tables for Crystallography
P21/m (Primitive base-centered <z-axis unique> monoclinic Bravais-lattice symmetry group) P
10 2/m
Crystal system
Monoclinic 2/m
Asymm
0 ≤ x ≤ 1 and 0 ≤ y ≤ 1/4 and 0 ≤ z ≤ 1
4 Absence of reflections

 

Translation
 
Symmetry (atomic coordinates)
X, Y, Z; -X, 1/2+Y, -Z; -X, -Y, -Z; X, 1/2-Y, Z
Crystal examples
 

Table 1669b. P1121/m (11) space group.

Name in the International Tables for Crystallography
P1121/m P
   
Crystal system
Monoclinic  
Asymm
 
  Absence of reflections

0 0 l: l ≠ 2n.

A 21-screw axis along the c-axis
Translation
 
Symmetry (atomic coordinates)
 
Crystal examples
La4Cu3MoO12: a ≈ 8 Å, b ≈ 7 Å, c ≈ 11 Å, γ ≈ 90° (see Figure 1669)

For La4Cu3MoO12 crystals, both the absent reflections in the SAED [100] pattern in Figure 1669 (a) and the G-M lines in the 00l discs for odd l in the ZOLZ-CBED pattern taken along the [010] zone axis in Figure 1669 (b) indicate the presences of a 21 screw axis parallel to the c-axis and the forbidden reflection (00l: l≠2n). Moreover, the [010] WP (whole-pattern) in Figure 1669 (c) presents a mirror plane perpendicular to the c-axis. Therefore, according to the tables above, the La4Cu3MoO12 crystals belong to the P1121/m (11) space group.

SAED pattern along [100] zone axis
[010] ZOLZ-CBED pattern
Whole pattern presenting a mirror plane perpendicular to the c-axis
(a)
(b)
(c)

Figure 1669. La4Cu3MoO12 (a ≈ 8 Å, b ≈ 7 Å, c ≈ 11 Å, γ ≈ 90°) crystals: (a) SAED pattern along [100] zone axis; (b) ZOLZ-CBED pattern along [010] zone axis. The black arrows indicate G-M lines visible in the 00l discs for odd l, and (c) Whole pattern presenting a mirror plane perpendicular to the c-axis. Adapted from [1]

 

 

 

 

 

[1] Sylvie Malo, Douglas A. Vander Griend, Kenneth R. Poeppelmeier, Yanguo Wang, Vinayak P. Dravid, Crystal symmetry of La3Cu2VO9 and La4Cu3MoO12 derived from the YAlO3 hexagonal structure by transmission electron microscopy, Solid State Sciences 3 (2001) 17–23.

 

 

=================================================================================