Table 1850. Pca2_{1} (Pbc2_{1}, P2_{1}ab, 29) space group.
Name in the International Tables for Crystallography 
Pca2_{1} 

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

Crystal symmetry 
Is noncentrosymmetric because it has not x,y,z in its symmetry. 
Crystal examples 

Name in the International Tables for Crystallography 
Pbc2_{1 }(is equivalent to Pca2_{1}, i.e. nonconventional setting of space group Pca2_{1}） 


Crystal examples 
Rubidium hydrogen maleate (RbH(C_{4}H_{2}O_{4})): 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)
Na_{2}CaAl_{4}Si_{4}O_{16} (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]
Li_{2}Ge_{7}O_{15}(below T_{c}): Ferroelectric phase.

Ferroelectricity in HfO_{2} is highly sensitive to the dopant species and dopant concentration[4]:
Ferroelectricity in GdDoped HfO_{2} thin films: noncentrosymmetric transition phase [3].
Low Al doping in HfO_{2}.
Sidoped HfO_{2}: a = 5.11714 Å, b = 5.17937 Å, c = 5.28883 Å..

Nhydroxy4phenylbut3en2imine [C_{10}H_{11}NO]: a = 5.591 Å, b =22.019 Å, c = 14.742 Å, β = 90.0 °, Z = 4.
Er(HPO_{4})(NO_{3})·3 H_{2}O: a = 10.1653 Å, b = 11.9178 Å, c = 6.8533 Å, V = 830.26, Z = 4.
C_{13}H_{9}NO: a = 4.6530 Å, b = 15.1910 Å, c = 26.902
Å, α = 90
°, β = 90
°, γ = 90
°, Z = 8.
C_{6}H_{6}N_{2}O_{2}: 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), 517524.
[2] Rossi, G., Oberti, R. & Smith, D.C. (1986): Crystal structure of lisetite, CaNa2Al4Si4O16. Am. Mineral. 71, 13781383.
[3] S. Mueller, C. Adelmann, A. Singh, S. Van Elshocht, U. Schroeder, and T. Mikolajick, Ferroelectricity in GdDoped HfO2 Thin Films, ECS Journal of Solid State Science and Technology, 1 (6) N123N126 (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 Sidoped HfO2, Journal of Applied Physics 115, 034104 (2014).
