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G(ν) [a.u.]
8
6
4
2
Phase II
Phase I
Phase III
0
0 10 20 30 40
ν [meV]
110
1,3,5-(CH 3 ) 3 C 6 H 3
T = 20 K
Figure 9. Density of phonon states of the three structural modifications of solid mesitylene-D0
obtained from the IINS spectra at 20K
The density of the phonon states G(ν), obtained within the approximation of harmonic
dynamics and one-phonon scattering processes [8] from the IINS spectra of various phases of
solid mesitylene at T=20K are presented in Fig. 9 in the energy range up to 40 meV. The
intense bands seen in the spectrum of phase III at 19.2 meV and 23.3 meV, are interpreted as
corresponding to librations of the methyl groups. These bands disappear in the spectra of
phases II and I, which testifies a significant decrease of the barrier heights for rotation of the
methyl groups in these phases. As a consequence, the libration of methyl groups are shifted
into the energy range of lattice vibrations below 15 meV. The G(ν) of phase I at low energies
is proportional to ν 2 , according to the Debye model of dynamics of ordered crystals. The G(ν)
spectrum for phase II in the range of the lattice vibrations is characterized by broadened bands
of optical phonons and much greater density of states in the acoustic phonon range below
10 meV. This additional density of states is known as the so-called “boson peak” in the G(ν)
of disordered solids. The above evidence suggests that phase II could be classified as a
reorientational or proton glass. The internal vibrations of the mesitylene molecule above
30 meV practically do not depend on the crystalline structure.
5. CONCLUSIONS
The NPD investigations of solid mesitylene at different temperatures give clear evidence
of three different structural phases of crystalline mesitylene. The phase I, which can be
obtained as a single phase only after annealing of solid mesitylene close to the melting point,
is stable in the whole temperature range of solid phases. Much more often liquid mesitylene is
solidifying in the structure of phase II, which at 91 K transforms to the low temperature phase
III. This phase is characterized by the relatively high barriers for methyl rotations. The energy
of librational modes in phase III has values of 19.2 and 23.3 meV, which are too high for
effective slowing down of cold neutrons. In phases II and I the hindrance barriers of methyl