PTCDA is of interest because the brick-like shaped PTCDA molecule easily forms well-ordered films on various substrates due to its unique crystal structure which is characterized by flat lying molecules in the (102) plane of the bulk. Up to now, two different morphological forms are known, referred to as a- and b-modification.

Chemical Structure and Space Filling Model:

Lattice parameters for the PTCDA a - and b - polymorph: [M. Möbus, N. Karl, and T. Kobayashi, J. Crys. Growth, 116, 495 (1992).]

	a-phase	b-phase
a / Å	3.74	3.78(01)
b / Å	11.96	19.30(03)
c / Å	17.34	10.77(02)
b / 0	98.8	83.6(1)
space group	P21/c	P21/c
volume / Å3	766.5	780.8
-direction / Å (a* of (102)-plane)	11.96	19.30
-direction / Å (b* of (102)-plane)	19.91	12.45
area of unit mesh in (102)-plane	238.12	240.3
d(102) / Å	3.22	3.25
angle between a-axis and (102)-plane / 0	30.6	30.7

Arrangement of the molecules in a -PTCDA:

bc-plane (top), ab-plane (right), and ac-plane (below)

Arrangement of the PTCDA molecules in two neighboring (102) planes:

a-modification	b-modification

The arrows indicate the lateral shift of the unit mesh between the two planes.