The dielectric properties of glycinium phosphite (GPI) crystals as a function
of temperature and electric field magnitude are investigated. The electric field
E
is applied perpendicularly to the ferroelectric
b-axis in the direction of hydrogen-bonded phosphite chains in the crystal (the
c-axis). The shift of the paraelectric–ferroelectric phase transition to lower temperatures proportionally
to Ec2
(where Ec
is an effective field in the sample) is observed. Strong anomalies in the field dependence of
the permittivity in the temperature region are revealed. It is shown that the observed jump-like changes of are caused by the phase transition from the ferro- to paraelectric phase
induced by the electric field. Such a transition is connected with the
rearrangement of protons on hydrogen bonds and the reversal of the corresponding
dipole moments, at which the compensation of their components along the
b-axis takes place. The theoretical description of the observed dielectric anomalies, given on
the basis of the phenomenological Landau free energy approach, is in good agreement with
the experimental data.
A new crystal of 2-aminopyridine phosphate (NC4H4NH2)·H3PO4
has been grown and its x-ray structure and physical properties were
studied. At room temperature the crystals are monoclinic, space group
C2/c.
The flat 2-aminopyridine cations are hydrogen bonded to the anionic [PO4 ]
groups. The interesting feature of the crystal structure is the
three-dimensional network of hydrogen bonds including, among
others, two strong, symmetrical O · · · H, H · · · O
interactions with disordered proton locations. Symmetrically related PO4
anions linked through these protons form infinite (PO4)∞ chains along
the crystal a-axis.
The anomalies in the temperature dependence of the electric permittivity showed that
the crystal undergoes ferroelectric phase transition at Tc = 103.5 K.
The spontaneous polarization takes place along the crystal
a-axis,
being parallel to the chains of the hydrogen-bonded PO4.
The disordered protons, thermally activated at room temperature, can be frozen
at their positions in the ferroelectric phase. The order–disorder continuous type of
the transition has been evidenced on the basis of the temperature dependences of
electric permittivity and spontaneous polarization measurements.
A microscopic model based on the consideration of the proton ordering is proposed for describing the H-bonded ferroelectric crystalline systems with a complex structure of the hydrogen bond network. The model has been used for the investigation of thermodynamics and dielectric properties of the GPI crystal. The symmetry analysis of the order parameters responsible for the mixed (ferro-and antiferroelectric) nature of ordering is performed within the model. The phase transition into the ferroelectric state is described. Changes in the dielectric susceptibility of the crystal are studied in the presence of the transverse external electric field acting along the caxis. The results of measurements of temperature and field dependences of dielectric permittivity ε c in the paraelectric phase are presented. The microscopic mechanism of the observed effects is discussed based on the comparison of theoretical results and experimental data. A conclusion is made about the significant role of the ionic groups connected by hydrogen bonds in the charge transfer. So they make an important contribution into the polarizability of the GPI crystal along the direction of H-bonded chains.
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