Z. Czapla

Ferroelectric phase transition in [(CH3)3 NH]3 Sb2Cl9 (TMACA)

Abstract The ferroelectric first order phase transition at 367 K has been discovered in TMACA crystals, which is presumably related to a motion of trimethylammonium cations.

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.

Phase situation in ferroelectric NH4HSeO4 crystals with various degrees of deuteration

Abstract NH4HSeO4 crystals of varying deuterium content have been studied. The high-temperature phase transition at 408 K is observed both for crystals of symmetry I2 grown from the solutions containing 0–40% D and those of symmetry P212121 i.e. grown solutions containing above 50% D. This phase transition practically does not depend on %D. For the crystals of symmetry P212121 an additional dielectric anomaly at 338 K (only slightly dependent on %D) has been found which most probably should be assigned to the P212121-I2 transition. The results obtained and our earlier data available so far enabled to construct more detailed phase diagram for NH4HSeO4 with %D as a parameter.

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.

Crystal Structure of NH3CH2COOHH2PO3 in the Ferroelectric Phase

X-ray investigation was carried out to examine the ferroelectric phase transition in glycinium phosphite, NH 3 CH 2 COOHH 2 PO 3 , from the structural point of view. The result indicates that the crystal belongs to the space group P 2 1 in the ferroelectric phase and the unit cell parameters obtained at 198 K are a =9.758(2) A, b =8.510(1) A, c =7.375(1) A and β=100.47(1)°. The crystal structure in ferroelectric phase was determined using an automatic four-circle diffractometer and refined to R =0.032 and R w =0.023 for 2685 independent reflections. Structure analysis clarified that in the ferroelectric phase, the hydrogen atoms between neighboring phosphite tetrahedra are in the ordered state and phosphite tetrahedra deform from those in the paraelectric phase.

Local order-disorder behaviour in Cr3+-doped dimethylammonium aluminium sulphate hexahydrate (DMAAS) studied by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) investigations of the ordering behaviour in single crystals of Cr3+-doped dimethylammonium aluminium sulphate hexahydrate (DMAAS) are presented at the transition from the ferroelastic to the ferroelectric phase. The measurements on chromium-doped DMAAS show that the Cr3+ probe substitutes at aluminium sites. Remarkable line-shape changes and line splittings are observed in the EPR experiment in contrast to the findings of former 27Al nuclear magnetic resonance studies. The EPR measurements confirm the order-disorder character of the ferroelectric phase transition at 151 K on a microscopic level and give information about the local order parameter of the dimethylammonium reorientation and the relation of the dimethylammonium reorientational motion to the critical dynamics.

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.

New material with strong electro‐optic effect: Rubidium hydrogen selenate (RbHSeO4)

A very large electro‐optic Pockels coefficient is reported in a crystal of rubidium hydrogen selenate. This new material is found to be promising for modulation applications because of its very low half‐wave voltage (∼27 V) which is determined.