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.
PLATON also contains the routine
LePageTwin.
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 4-FEB-2021.
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