Electron microscopy
 
Pca21 (Pbc21, P21ab, 29) space group Space Group
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Table 1850. Pca21 (Pbc21, P21ab, 29) space group.

Name in the International Tables for Crystallography
Pca21 P
47 mm2
Crystal system
Orthorhombic mmm
Asymm
0 ≤ x ≤ 1/4 and 0 ≤ y ≤ 1 and 0 ≤ z ≤ 1
4 Absence of reflections

Symmetry (atomic coordinates)

X, Y, Z; -X, -Y, 1/2+Z; 1/2+X, -Y, Z; 1/2-X, Y, 1/2+Z

Translation
  Crystal symmetry Is non-centrosymmetric because it has not -x,-y,-z in its symmetry.
Crystal examples
 

 

Name in the International Tables for Crystallography
Pbc21 (is equivalent to Pca21, i.e. non-conventional setting of space group Pca21    
Crystal examples
Rubidium hydrogen maleate (RbH(C4H2O4)): Very nearly centrosymmetrical, i. e., only one H atom position in the structure violates the centrosymmetry [1].
RbHMaleate: a = 4.674, b = 7.667, c = 16.455, α = 90, β = 90, γ = 90.
KHMaleate: a = 4.578, b = 7.791, c = 15.953, α = 90, β = 90, γ = 90.
CoAsS (a = 5.589 Å, b = 5.583 Å and c = 5.581 Å): Co (4a) at (0.259, -0.005, 0), S (4a) at (0.631, 0.383, 0.380) and As (4a) at (0.869, 0.619, and 0.617)
Na2CaAl4Si4O16 (a = 8.260 Å, b = 17.086 Å and c = 9.654 Å) : nonstandard origin (offset by [0.25, 0.25, 0]); Si and Al are fully ordered at the tetrahedral positions of the framework, and the intraframework Ca and Na cations are distributed in the common Ca + 2Na layers. [2]
Li2Ge7O15(below Tc): Ferroelectric phase.
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Ferroelectricity in HfO2 is highly sensitive to the dopant species and dopant concentration[4]:
Ferroelectricity in Gd-Doped HfO2 thin films: non-centrosymmetric transition phase [3].
Low Al doping in HfO2.
Si-doped HfO2: a = 5.11714 Å, b = 5.17937 Å, c = 5.28883 Å..
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N-hydroxy-4-phenylbut-3-en-2-imine [C10H11NO]: a = 5.591 Å, b =22.019 Å, c = 14.742 Å, β = 90.0 °, Z = 4.
Er(HPO4)(NO3)·3 H2O: a = 10.1653 Å, b = 11.9178 Å, c = 6.8533 Å, V = 830.26, Z = 4.
C13H9NO: a = 4.6530 Å, b = 15.1910 Å, c = 26.902 Å, α = 90 °, β = 90 °, γ = 90 °, Z = 8.
C6H6N2O2: four molecules per unit cell of dimensions a = 6.501 Å, b = 19.330 Å and c = 5.082 Å.

 

 

[1] Fleck, M; Bohaty, L: Syntheses, crystal structures and an overview of alkali metal maleates. Zeitschrift für Naturforschung B 64b (2009), 517-524.
[2] Rossi, G., Oberti, R. & Smith, D.C. (1986): Crystal structure of lisetite, CaNa2Al4Si4O16. Am. Mineral. 71, 1378-1383.
[3] S. Mueller, C. Adelmann, A. Singh, S. Van Elshocht, U. Schroeder, and T. Mikolajick, Ferroelectricity in Gd-Doped HfO2 Thin Films, ECS Journal of Solid State Science and Technology, 1 (6) N123-N126 (2012).
[4] Lili Zhao, Matthew Nelson, Henry Aldridge, Thanakorn Iamsasri, Chris M. Fancher, Jennifer S. Forrester, Toshikazu Nishida, Saeed Moghaddam, and Jacob L. Jones, Crystal structure of Si-doped HfO2, Journal of Applied Physics 115, 034104 (2014).

 

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