Djurleite Twinning (pseudohexagonal)

Djurleite: Cu1.97S

Situation

Besides the general features of twinning we find:
Large hexagonal twin lattice. Many reflections weak or missing. All strong reflections obey the conditions:
h = 0 mod 4
k = 0 mod 4
l = 0 mod 2
(The converse is not true: some reflections whose indices obey these criteria are not strong.)

Solution

The orthorhombic unit cell has the cell parameters
a = 26.92 Å, b = 15.71 Å, c=13.56 Å
a is approx. sqrt(3)*b

From the systematic extinctions, the extinction symbol is P-n-, which is compatible with the space groups Pmnm, P21nm or Pmn21.

By applying the transformation matrix
(0.00 2.00 0.00)
(0.00 -1.00 2.00)
(1.00 0.00 0.00)

you get the pseudohexagonal unit cell
a = 31.42 Å, b = 31.34 Å, c= 26.92 Å, al = 90.0°, be = 90.0°, ga = 120.08°

Graphics

Note: A valuable tool for finding sub-/supercells is the PC version of the program LEPAGE. The program LEPAGE is also implemented in the program PLATON, but in this version it does not look for supercells.

Twin Index

The determinant of the transformation matrix is 4. Therefore the twin index is 4.

Twin Law

The operations of pseudo-symmetry in the (pseudo-hexagonal) twin lattice are the possible twin operations.
If you rotate the orthorhombic lattice about [100] counterclockwise through 120° and 300°, you end up with the pseudo-hexagonal twin lattice. So, the twin lattice consists of three orthorhombic cells.

Solid State Chemistry

The crystal structure of djurleite Cu1.97S can be derived from the high temperature structure of chalcocite Cu2S. In the high chalcocite structure the sulfide ions form a hexagonal-close packed framework, in which half of the triangle centers are occupied alternately by copper ions. Space group is P63/mmc and the cell parameters are a = 3.96 Å, c = 6.72 Å (Evans, 1979).

Final Remark

By the crystal structure determination on an untwinned single crystal, the space group turned out to be monoclinic P21/n with a beta-angle of 90.13(3)°(Evans, 1979).

References

H. Takeda, J.D.H. Donnay, D.E. Appleman, Z. Krist. (1967) 125, 414-422.
A. Santoro, Acta Cryst. (1974) A30, 224-231.
H.T. Evans, jr., Z. Krist. (1979) 150, 299-320.

Page maintained by Martin Lutz. Last change 7-OCT-98.
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