S. Dacko

Characterization of ferroelectric phase transition in GPI crystal

The dielectric behaviour of a glycine phosphite [GPI: Gly.H 3 PO 3 ] crystal is investigated around its ferroelectric phase transition. The temperature dependence of the dielectric constant is measured for dc bias fields up to 200 kV/m. The dielectric constant is also measured in the paralectric phase as a function of electric field at constant temperatures. The results are discussed on the basis of the classical state equation and the equation coefficients are estimated. The obtained results lead to the conclusion that the ferroelectric phase transition in GPI has an intermediate character between critical and tricritical one.

Ferroelectric phase transition in hydrogen-bonded 2-aminopyridine phosphate (NC4H4NH2)·H3PO4

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.

Ferroelectricity in partially deuterated glycine phosphite crystals

Abstract Crystals of Gly·H3PO3 with various degree of deuteration were grown and their physical properties were investigated. Anomalies of permittivity related to para- ferroelectric phase transition were observed in the whole range of deuterium content. Ferroelectric character of the observed phase transitions was confirmed from hysteresis loops observation and pyroelectric measurement. The dependence of Tc on deuterium content was found to be linear.

Dielectric properties and phase transitions in [(CH3)2NH2]5Cd3Cl11 single crystals

Abstract Single crystals of [(CH 3 ) 2 NH 2 ] 3 Cd 3 Cl 11 have been grown and their properties have been studied. X-ray investigations have shown that crystals belong to the space group Cmcm . Dielectric measurements revealed a possibility of successive phase transitions at 260 K, 180 K and 127 K.

Structural phase transitions in tetra(isopropylammonium) decachlorotricadmate(II), [(CH3)2CHNH3]4Cd3Cl10, crystal with a two‐dimensional cadmium(II) halide network

Single crystals of tetra(isopropylammonium) decachlorotricadmate(II) as a rare example of a two-dimensional cadmium(II) halide network of [Cd(3)Cl(10)](n)(4-) have been synthesized and characterized by means of calorimetry and X-ray diffraction. The crystals exhibit polymorphism in a relatively narrow temperature range (three phase transitions at 353, 294 and 259 K). Our main focus was to establish the mechanism of these successive transformations. The crystal structure was solved and refined in the space group Cmce at 375 K (Phase I), Pbca at 320 K (Phase II) and P2(1)2(1)2(1) (Phase III) at 275 K in the same unit-cell metric. The structure is composed of face-sharing polyanionic [Cd(3)Cl(10)](4-) units which are interconnected at the bridging Cl atom into four-membered rings forming a unique two-dimensional network of [Cd(3)Cl(10)](n)(4-). The interstitial voids within the network are large enough to accommodate isopropylammonium cations and permit thermally activated rotations. While in Phase I isopropylammonium tetrahedra rotate almost freely about the C-N bond, the low-temperature phases are the playground of competition between the thermally activated disorder of isopropylammonium cations and stabilizing N-H···Cl hydrogen-bond interactions. The transition from Phase I to II is dominated by a displacive mechanism that leads to significant rearrangement of the polyanionic units. Cation order-disorder phenomena become prominent at lower temperatures.

Dielectric relaxation in high temperature phases of (NH3C2H5)2CuCl4 crystals

On the basis of dielectric investigations of (NH 3 C 2 H 5 ) 2 CuCl 4 crystals the existence of phase transitions at T 1 = 364 K and T 2 = 356 K (in a heating run) was confirmed. In the temperature region of 300-370 K Debye relaxation was observed. The change of the dielectric dispersion character around T 3 = 330 K is related to the phase transition. The determined values of the activation energy and the times of the dielectric relaxation for all phases are characteristic of the combined reorientation motions of the alkylammonium cation. The conclusions concerning the dynamics of the hydrogen bonds were confirmed by IR spectroscopy.

Dielectric and Ferroelectric Properties of [(CH3)2NH2]3CuCl5 Single Crystal

The detailed studies of dielectric properties of [(CH3)2NH2]3CuCl5 single crystal were performed. Distinct anomalies of permittivity along three crystallographic direction were observed at 277.5 K and 252.7 K on cooling. The observed permittivity behaviour evidenced the first order character of both phase transitions. The hysteresis loops characteristic of a ferroelectric phase were observed in the range of temperature 277.5–252.7 K. An appearance and disappearance of spontaneous polarization confirms the first order phase transitions in this crystal. The permittivity measured along the ferroelectric a-axis under influence of external electric field in paraelectric phase showed clearly a nonlinear increase with increase of electric field intensity applied to the crystalline sample.

Domain structure and ferroelastic phase transition in (NH4)2SbF5 single crystal

Abstract Crystal of the (NH4) SbF5 has been grown and optical observations along the b-axis have been done in the temperature range 310 - 100 K. An appearance of stripe- and needle-like domains with domain walls parallel to (100) and (001) planes of the room temperature phase (above 292 K) was found at 292 K. No changes of domain structure was observed down to 100 K including the phase transition point at 169 K. The changes of domain pattern and monodomain state may be achieved by application of stresses to Y-45° cut samples. The results indicate that the phase transition at 292 K is a ferroelastic one.

Structure and phase transition in pyridazine perchlorate

Single crystals of pyridazine perchlorate [(C4H4N2)HClO4] have been synthesized and characterized by x-ray diffraction, dielectric measurements and optical studies. At room temperature the crystal is monoclinic, space group P 21/n. The temperature dependences of the electric permittivity and birefringence exhibit anomalous behaviour, characteristic of the first-order phase transitions at 343 and 339 K on heating and cooling, respectively. Optical investigations under a polarizing microscope show that the crystal symmetry changes from the biaxial room temperature phase II to the optically uniaxial high temperature phase I. The domain pattern arising after cooling from phase I reflects observed symmetry changes at phase transition. The fully disordered structure of phase I can be described in the obverse hexagonal space group .

The isolated point on the concentration-temperature phase diagram of K2Cd2XMn2(l-X)(SO4)3 crystals

Abstract The isolated point of second order phase transition (xi=0.8; Ti,=358K) has been found on the phase x, T-diagram in the line of the first order ferroelastic phase transitions of the K2Cd2xMn2(1−X)(SO4)3 solid solutions. The concentration-temperature phase diagram of the K2Cd,2xMn2(1−x)(SO4)3 ferroelastic solid solutions was described and discussed based on the thermodynamic potential. It was shown that isolated point can occur in ferroelastic langbeinites when both coefficients near third order term γ1 and γ2 simultaneously are equal to zero.