CHARGE-DENSITY STUDY OF THE NON-LINEAR OPTICAL MATERIAL: ZINC
(TRIS)THIOUREA SULPHATE
Jacqueline M. Cole,a Shamus L. G. Husheer,a Dima S. Yufit,b Judith A. K. Howardb and Garry J. McIntyrec
aDepartment of Chemistry, University of
Cambridge, CB2 1EW. UK; bDepartment of Chemistry, University of Durham,
DH1 3LE, UK; cInstitut Laue Langevin, B. P. 156, 38042 Grenoble
Cedex 9, France (jmc61@cam.ac.uk)
A
charge-density study of the non-linear optical (NLO) material, zinc
(tris)thiourea sulphate is presented. The interest lies in the efficiency of
the first-order non-linear term, second harmonic generation (SHG), where the
laser penetration through the material affects the frequency doubling of light.
This work represents one in a series of charge-density studies that we have
undertaken in order to unravel and understand the relationships between
structure and optical properties [1,2]. In these studies charge-density
analysis was used to (i) conduct topologically-based bond-length-alternation
type calculations using values of ellipticity, (ii) perform polarization
mapping to investigate the nature of intramolecular charge-transfer, and (iii)
evaluate the solid-state molecular dipole moment and relate this to the crystal
field forces. All of these features play an important role in governing the SHG
activity and so structure / property relationships have been built up from this
work.
All of
these previous investigations have concentrated on organic materials. However,
with the subject study, we now extend this type of analysis our investigations
to an organometallic compound, zinc (tris)thiourea sulphate, which represents
one of few examples of promising organometallic SHG-active materials, many
otherwise viable organometallic candidates suffering from reactivity towards
air. There is much impetus to study organometallic SHG-active materials since
they possess both the major optical advantages of organic materials (eg. very
fast optical response and ease of molecular design) and those of inorganic materials
(principally thermal stability which is the single-most common problem with
organics). The results derive from a suitable combination of complementary 100K
X-ray and neutron diffraction data, the latter being important for locating the
hydrogen atoms precisely. The X-ray data were collected in the laboratory at
Durham, UK, on a Bruker SMART diffractometer, whilst the neutron data was collected
on the Laue Diffractometer, VIVALDI, at the ILL, Grenoble, France. The results
pertaining to the aforementioned analysis methods (i)-(iii) are presented.
References
1 Cole, J. M., Goeta, A. E., McIntyre, G. J. and Howard, J. A. K. (2002) Acta Crystallogr. B 58, 690-700.
2 Cole, J. M., Copley, R. C. B., McIntyre, G. J., Howard, J. A. K., Szablewski, M. and Cross, G. H. (2002). Phys. Rev. B, 65, 125107(1-11).