Druck-Materie 20b.qxd - JUWEL - Forschungszentrum Jülich

ABSTRACT
ACoM 6
6 th Meeting of the Collaboration on Advanced Cold Moderators
Jülich, 11 – 13 September 2002
Structural phase transitions and dynamics of solid mesitylene investigated by
diffraction and inelastic incoherent neutron scattering methods
I. Natkaniec
Frank Laboratory of Neutron Physics, JINR, 141980 Dubna, Russia
H. Niewodniczański Institute of Nuclear Physics, 31-342 Kraków, Poland
K. Hołderna-Natkaniec
Institute of Physics, A. Mickiewicz University, 61-614 Poznań, Poland
The results of simultaneous investigations of neutron powder diffraction (NPD) and
inelastic incoherent neutron scattering (IINS) performed at the IBR-2 pulsed reactor of the
JINR in Dubna are presented. It is shown that solid mesitylene can exist in different crystallographic
structures depending on the cooling rate and thermal procedure. The phase I, which
can be obtained by annealing of solid mesitylene at (200–220)K, is stable from the melting
point at 227K down to liquid He temperatures. The phase II, obtained when freezing overcooled
liquid, can pass to the phase III at about 90K. Generalized density of phonon states of
these three crystallographic phases have been obtained from their IINS spectra at 20K. The
frequencies of methyl librations in phase III are determined at 19.2 and 23.3 meV. These
modes in phases I and II are shifted down to the lattice mode frequencies below 15 meV.
1. INTRODUCTION
Mesitylene, or 1,3,5-trimethylbenzene, C6H3(CH3)3, is a well known organic solvent
characterized by the relatively low freezing (227K) and high boiling (437K) temperatures.
Because of the high content of hydrogen and the assumed weakly hindered rotation of methyl
groups in the solid phase, which can remove energy from neutrons, this compound has been
recommended as a neutron moderator [1], and used for the construction of the TCNS cold
neutron source at the TRIGA Mark II pulsed reactor of the NETL in Austin [2]. However, the
structure and dynamics of solid mesitylene has not been well investigated until recently.
The temperature dependence of the IINS spectra has confirmed the occurrence of the
rotational freedom of the methyl groups in solid mesitylene at T=100K, but at T=20K the
rotational jumps seem to be frozen out [3]. The NDP spectra, measured in this experiment for
lattice spacings up to 0.6 nm, did not indicate any diffraction peaks.
The Raman spectra of C6H3(CH3)3 indicate some changes in the temperature dependence
of the lattice modes and certain internal modes at 95K and 195K [4]. The DSC thermographs
reported in [4] show two endothermic peaks for solid mesitylene: a strong one at 91K
and a very weak one at 188K, respectively. The following three endothermic peaks at 220K,
222K and 227K, correspond to the melting of the samples at different heating rates. This result
was interpreted as the manifestation of three different structural modifications: Iα, Iβ and
Iγ of the high-temperature solid phase.
Our recent results of the NPD and IINS investigations of mesitylene-D3, C6D3(CH3)3,
and mesitylene-D12, C6D3(CD3)3, show up that solid mesitylene can exist in various crystallographic
structures, depending on the cooling rate. At a cooling rate of 2K/min, the
undercooled liquid was freezing in the structure of phase II, and the first order structural
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