Two
three-dimensional metal–organic compounds of the formula
Pyr
2
KM(CN)
6
, where M = Co, Fe and Pyr = pyrrolidinium
((CH
2
)
4
NH
2
+
), have been
found to crystallize at room temperature in a monoclinic structure,
space group
P
2
1
/
c
. They
are cyano-bridged compounds with an unprecedented type of architecture
containing pyrrolidinium cations in the voids. The materials have
been investigated by X-ray diffraction, dielectric, and spectroscopic
methods as a function of temperature in order to determine their properties
and the mechanism of the reversible phase transitions occurring at
ca. 345–370 K. The phase transitions in both crystals are first
order and are associated with a symmetry increase to a rhombohedral
structure (space group
R
3
®m
) as well as a significant disorder of organic cations above
T
c
. On the basis of Raman scattering and IR spectroscopy
it has been assumed that the phase transition in both crystals is
triggered by thermally induced pseudorotation of the organic cation
and large out-of-plane motions of its atoms followed by a “click-in”
of the cyanide bridges. The materials have been proposed as possible
switchable dielectrics due to their respective high differences in
dielectric permittivities across the phase transition.