Local Structure of Triphenyl Phosphite
J. Phys. Chem. B, Vol. 108, No. 52, 2004 20081
is a TPP molecule cluster which has a low local free energy,
because it can assume an optimal molecular conformation. The
second type of LPS is a TPP molecule cluster linked by two
intermolecular hydrogen bonds, which slightly alters the mo-
lecular conformation.
More recently, Tanaka2 has suggested that the formation of
the glacial state has two different mechanisms: nucleation-
growth type when T > 215.5 K and spinodal-decomposition
type when T < 215.5 K. From the data presented here and
discussion above, our data are consistent with this interpretation
but do not provide verification. To address this issue more
carefully, we have performed additional time dependence
measurements on TPP, to observe the structural evolution of
the glacial state using both neutron and high-energy X-ray
diffraction techniques. These results and an assessment of
changes in molecular conformation in the different forms will
be the subject of our future publications.
Figure 11. Hydrogen-related correlation functions for TPP in different
forms, corresponding to the Fourier transforms of the curves in Figure
4 with Qmax ) 30 Å-1 and Lorch function applied. (a) crystal, diamonds
(offset by +4). (b) supercooled liquid, solid line. (c) glass, circles.
Conclusions
Total and hydrogen/deuterium isotopic substitution neutron
scattering techniques together with high-energy X-ray diffraction
experiments have been performed to investigate the intramo-
lecular and nearest-neighbor interactions for TPP in the crystal-
line, glacial, glassy, and liquid phases. The combined diffraction
data have been used to interpret structural characteristics of the
metastable glacial state and investigate the existence of “apparent
polyamorphism” in TPP. The neutron results show that the most
significant differences in structure between the glacial and
crystalline states appear at 3.0 and 4.5 Å. A first-order neutron
difference method applied to the crystalline, liquid, and glass
data show that hydrogen correlations at distances of 3.0 and
3.4 Å appear only in the crystalline spectra. It is suggested that
these features are due to inter-phenyl ring C-H and/or H-H
interactions, most probably associated in part with the formation
of weak intermolecular hydrogen bonds observed in Raman
measurements. The X-ray data show the opposite trend in the
same region: namely, a decrease in intensity at 3.12 Å for the
crystal compared to the glacial state. This change is attributed
to variations in molecular conformation, probably due to C-O/P
interactions, between the glacial and crystalline states. It is
argued that the glacial state is not a simple mixture of
nanocrystalline and supercooled liquid components, but rather,
the abortive crystallization attempt arises through competition
between the formation weak intermolecular hydrogen bonds and
optimum molecular conformation.
Figure 12. Total neutron correlation functions for the fully deuterated
sample in the early glacial stage at 215 K, later glacial stage at 225 K,
and crystal at 250 K. The data were truncated at Qmax ) 25 Å-1
.
that additional inter-ring hydrogen correlations must be present
in the crystal at 3.4 Å. The glassy spectra show two additional
hydrogen-related features: a peak at 2.80 and a shoulder at 3.2
Å, compared to the supercooled liquid. Significant local
structural differences are observed between the glass spectra
and the supercooled liquid (and crystalline spectra) around 3.8
Å. However, these features require sophisticated modeling
techniques to be properly interpreted.
Total neutron correlation functions for the glacial state formed
at 215 and 225 K and the crystal at 250 K for the fully deuterated
TPP sample are shown in Figure 12. These data show that
significant structural rearrangements in the glacial state have
to occur in the 2.8-3.0 and 4.5 Å regions before longer-range
crystallization can take place. In addition, because the 225 K
glacial-state data cannot be simply reproduced by a linear
combination of the 215 K glacial state and the crystalline
structures, the results suggest that the glacial state is not a simple
two-component mixture of the nanocrystalline and supercooled
liquid phases. On the basis of the neutron and X-ray results,
we suggest that slight variations in molecular conformation as
well as changes in the next-nearest-neighbor interactions are
responsible for the longer range structural differences between
the glacial and crystalline states. Therefore, the glacial state may
well originate from a frustration effect caused by competition
between optimal molecular conformation (in the glacial form)
and weak intermolecular hydrogen bonding effects (in the
crystal).
Acknowledgment. This work was supported by the U.S.
Department of Energy (W-31-109-ENG-38), the Department of
Defense (ARO-190310143), and the National Science Founda-
tion (CHE-0094202 and CHE-0313661). The authors would like
to gratefully acknowledge the help and assistance of Joan
Siewenie during data collection on GLAD and Yang Ren for
help with the cryostat on 11-ID-C. Ju¨rgen Senker is thanked
for providing the TPP crystal structure for modeling.
References and Notes
(1) Ha, A.; Cohen, I.; Zhao, X.; Lee, M.; Kivelson, D. J. Phys. Chem.
1996, 100, 1.
(2) Tanaka, H.; Kurita, R.; Mataki, H. Phys. ReV. Lett. 2004, 92, 25701.
(3) Kivelson, S. A.; Zhao, X.; Kivelson, D.; Fischer, T. M.; Knobler,
C. M. J. Chem. Phys. 1994, 101, 2391.
(4) Kivelson, D.; Kivelson, S. A.; Zhao, X.; Nussinov, Z.; Tarjus, G.
Physica A 1995, 219, 27.
(5) Cohen, I.; Ha, A.; Zhao, X.; Lee, M.; Fischer, T.; Strouse, M. J.;
Kivelson, D. J. Phys. Chem. 1996, 100, 8518.
The concept of “locally preferred structure” (LPS) has been
proposed by Kivelson and co-workers to interpret the structural
changes and the metastable glacial phase.6,31 A LPS is loosely
defined as an arrangement of molecules which, in a given region
of the pressure-temperature phase diagram, minimizes some
local free energy.31 It is assumed that there are (at least) two
different LPSs in TPP. In the suggested scenario, the first LPS