J. Phys.: Condens. Matter 12 (2000) 5765–5774. Printed in the UK PII: S0953-8984(00)12301-X
Chlorine-35 NQR and
1
H NMR study of guest structure and
dynamics in the thiourea–hexachloroethane inclusion
compound
A M Panich†§, Ju H Krieger‡, A R Semenov‡, S D Goren† and
G N Chekhova‡
† Department of Physics, Ben-Gurion University of the Negev, PO Box 653, Beer Sheva 84105,
Israel
‡ Institute of Inorganic Chemistry, Russian Academy of Sciences, Novosibirsk 630090, Russia
E-mail: pan@bgumail.bgu.ac.il
Received 28 February 2000
Abstract. Atom–atomic potential calculation of the channel non-stoichiometric thiourea–
hexachloroethane inclusion compound shows that the structure of the guest sublattice comprises
two types of finite molecular chain, having different structure and separated by domain walls.
In the present paper we present results of
35
Cl NQR and
1
H NMR measurements of thiourea–
hexachloroethane, [2.95(NH
2
)
2
CS]C
2
Cl
6
, in the temperature range from 7.5 to 90 K, which
confirm this model and show the existence of such a state at least below 60 K. Two resonances in
the NQR spectra were assigned to the two nearly commensurate regions, while the third resonance,
showing an anomalous behaviour, was attributed to the guest molecules in the domain wall. The
observed structure results from the different periodicity of the guest and host substructures and
shows a difference from conventional continuum models of the incommensurate state. Propagation
motion of the domain wall over the channel is discussed.
1. Introduction
In the past two decades, a large number of compounds showing incommensurate (IC) states
has been studied. In IC system the periodicity q
i
of at least one of the modulation waves cannot
be expressed as a rational fraction of the periodicity q
L
of the underlying lattice:
q
i
/q
L
= M/N M,N = 1, 2, 3, .... (1)
As a result, translational periodicity is lost in at least one dimension in spite of the existence of
perfect long range order. The modulation waves can be either of the plane wave (sinusoidal)
type or soliton-like, consisting of nearly commensurate (C) regions where the phase of the
order parameter is practically constant and other regions (phase solitons) where it changes
rapidly [1, 2]. It has also been shown that some of the IC phases represent in fact a sequence of
long-period commensurate (LPC) phases where the superstructure wave vector varies in steps
and ‘locks-in’ at an infinity of C phases, which may or may not be separated by an infinity of
true IC phases [1–6].
One of the interesting models of the IC state was suggested by Frank and Van der Merwe
[1, 7]. In this model, incommensurability is expected to result from the different periodicity of
§ Corresponding author.
0953-8984/00/265765+10$30.00 © 2000 IOP Publishing Ltd 5765