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

After melting phase I in the cryostat and cooling down the liquid sample from 240K, at
a rate of 2K/min, the undercooled liquid solidified in the structure of phase II, and at 20K we
got a pure structure of the low temperature phase III. The NPD spectra presented in Fig. 3
illustrate the structural transformations of solid mesitylene, when the heating procedure starts
at phase III. Transition III - II occurs just above 90K and is reversible with a small hysteresis
of about 2-5K, when phase II was heated up to no more than 180K. After a few hours of annealing
of the phase II at 200K, the NPD spectrum displayed some nucleation of phase I (see
spectrum 4 in Fig. 3). Cooling the sample at such a state of transformation, the phase II can be
undercooled down to 20K, without transition to phase III. Transition from phase II to phase I
is not reversible, it starts at approx. 190K, but the NDP spectra indicated a mixture of phase II
and phase I up to 210K. The pure phase I diffraction pattern was achieved only after annealing
of the sample at 220K. Phase I does not undergo any structural transition at cooling down
to 20 K (see spectra 5 and 6 in Fig.3), and can only be melted at Tm=227 K.
I(λ)/ΦΦ(λλ) [a.u.]*10 3
35
30
25
20
15
10
5
0
1,3,5-(CH 3 ) 3 C 6 D 3
T = 20 K
T = 220 K
T = 200 K
T = 120 K
T = 80 K
T = 20 K
CR = 2K/min
from 220K
CR = 2K/min
from 250K
0.3 0.4 0.5 0.6 0.7
d hkl [nm]
2θ = 56.6 o
Figure 3. The NPD spectra 1 - 5, indicate
structural phase transitions in mesitylene-D3 at
heating from the low temperature phase III,
spectra 5 and 6 confirmed the stability of phase I
from 220 K down to 20 K.
6
5
4
3
2
1
Intensity of scattered neutrons - I(λ)*10 3 counts/10 h
106
25
20
15
10
5
0
6
5
4
3
2
1
ph. I
ph. II
ph. III
(CH 3 ) 3 C 6 D 3
T = 20 K
T = 220 K
T = 200 K
T = 120 K
T = 80 K
T = 20 K
1 2 3 4 5 6
Incident neutrons wave-length [A]
Figure 4. The corresponding IINS spectra
measured by heating the phase III of solid
mesitylene-D3. Note the QNS broadening of the
elastic line for temperatures above the phase III to
phase II structural phase transition.
The temperature dependence of the IINS spectra measured by heating of phase III are
shown in Fig. 4. The time-of-flight spectra recorded at fifteen scattering angles between 20°
and 160° were summed up to improve the statistics. The summed-up spectra were normalized
to the same number of incident neutrons integrated over the range of neutron wavelengths
from 0.5 Å to 6 Å, and corrected for background by measuring the empty cryostat.
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