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The X-ray powder diffraction (XRPD) pattern
of [Mn(NH
3
)
6
](ClO
4
)
2
at room temperature can be indexed
in the regular (cubic) system (Fm
3m space group) with
lattice cell parameter
a
=
11.5729 A
̊
and with four mole-
cules per unit cell. The XRPD pattern at 100 K is evidently
different, and it indicates lowering of the crystal structure.
One phase transition at
T
h
C1
=
143.5 K (on heating) and at
T
c
C1
=
137.6 K (on cooling) was detected by DSC for
[Mn(NH
3
)
6
](ClO
4
)
2
in the range of 120–300 K. The fol-
lowing thermodynamic parameters for phase I
$
phase II
transition were obtained:
D
H
=
3.1
±
0.2 kJ mol
-
1
and
D
S
=
21.6
±
0.3 J mol
-
1
K
-
1
. The large value of entropy
changes (
D
S
) indicates considerable configurational disor-
der in the high temperature phase. The presence of 5.9 K
hysteresis of the phase transition temperature at
T
C
and
sharpness of the heat flow anomaly attest that the detected
phase transition is of the first-order type. Appearing of the
two new bands (splitting of
d
s
(HNH)
F
1u
mode) in FT-MIR
spectra at the vicinity of 1200 cm
-
1
suggests that the phase
transition undergoes a change of the crystal structure.