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

mesitylene-D0, and the IINS spectra reflected dynamics of methyl groups. The typical NPD
spectra measured for a scattering angle of 56.6 o , after different freezing and cooling
procedures, are presented in Figures 1-3. The intensity of scattered neutrons - I(λ), measured
on a time-of-flight scale, is normalized to the flux of incident neutrons - Φ(λ) measured by the
same detectors on a standard vanadium sample. The time-of-flight scale has been transformed
to the lattice spacing scale using the Bragg diffraction formula. For a better visualization of
temperature effects, the NPD spectra are shifted by addition of some constants.
The liquid sample of mesitylene-D3, rapidly quenched in the liquid nitrogen bath, did
not show any structural phase transitions up to 200 K. Repeated cooling and heating at a rate
of 2K/min of this sample in the cryostat did not change the diffraction pattern in the temperature
range of 20 K – 200 K either (see spectra 1 – 4 in Fig. 1). It seems that rapidly frozen
samples of solid mesitylene present the same conglomerate of different crystallographic
structures, which is quite stable up to 200 K. However, after subsequent heating and annealing
of this sample at 220 K, the diffraction pattern of phase I has been recorded. Repeated cooling
and heating do not change the diffraction pattern of phase I in the temperature range from
220 K to 20 K, as one can see in the spectra 5 and 6 of Fig. 1.
I(λ)/Φ(λ) [a.u.]*10 3
18
16
14
12
10
8
6
4
2
1,3,5-(CH 3 ) 3 C 6 D 3
T = 20 K
T = 220 K
T = 20 K
T = 200 K
T = 80 K
T = 20 K
cooling rate (CR)
CR 2K/min
CR 2K/min
CR 100K/min
0.3 0.4 0.5 0.6 0.7
d hkl [nm]
2θ = 56.6 o
Figure 1. The NPD spectra of a rapidly frozen and
cooled sample of mesitylene-D3 recorded at
subsequent heating and cooling processes.
6
5
4
3
2
1
105
I(λ)/ΦΦ(λ) [a.u.]*10 3
50
45
40
35
30
25
20
15
10
5
0
T = 220 K
T = 200 K
T = 150 K
T = 100 K
T = 80 K
T = 20 K
1,3,5-(CH 3 ) 3 C 6 D 3
2θ = 56.6 o
CR 5K/min
0.3 0.4 0.5 0.6 0.7
d hkl [nm]
Figure 2. The NPD spectra of mesitylene-D3
cooled down at 5K/min and recorded at a
subsequent heating process
The same sample, placed into cold cryostat shaft at room temperature and cooled down
at a rate of 5 K/min, solidified as a mixture of phase I and phase II. The NPD spectra of such a
solid sample of mesitylene-D3, measured during a heating procedure are presented in Fig. 2.
Reversible structural phase transitions from phase III to II were detected at about 90 K, but
the initial amount of phase I was stable. One can see that in this case transition from phase II
to phase I occurs at 200K.
6
5
4
3
2
1