Antiferroelectric transition

Fig. 12 Temperature dependence of the average MAS peaks at 14.1 Tesla in the vicinity of the antiferroelectric transition at 372 K. Note that the value above the transition differs considerably from its value below the transition temperature, thus yielding clearer evidence for the role of the displacive component in the transition mechanism [26]...

Conclusions from the Measurements on the Role of the NH4 Sites in the Antiferroelectric Transition... 

Hydrogen positions have been found in the tetragonal phase of ammonium dihydrogen phosphate, by neutron diffraction [6). The interesting feature of the O—H 0 bonds here is that the hydrogens do not lie on a line connecting the two O s. H-bond participation in the mechanism of the antiferroelectric transition at 148°K [7] is discussed. 

Application of high external pressures influences the transition temperature to the antiferroelectric phase (Yasuda et al., 1979 Samara and Semmingsen, 1979). The Tq becomes lower as the applied pressure increases. Under an ultra-high pressure of about 3 GPa, the antiferroelectric transition itself disappears and the high dielectric constant of ca. 200 is maintained even at cryogenic temperatures (Moritomo et al., 1991). Since Raman diffraction measurements under 3-4.5 GPa revealed that squaric acid exists still as an alternating bond form, the tautomerization coupled with intermolecular proton transfer occurs even at low temperatures (Moritomo et al., 1990). 

As the crystal cell below the structural transition remains centrosymmetrical one, the dipole moments in the cell are antiparallel. Thus, the magnetostructural phase transition at T = Ts is simultaneously an antiferroelectric transition (Fig. 20). 

Figure 8.23. A. Typical K NMR powder patterns of KOH at temperatures near the antiferroelectric transition. B. Variation with temperature of the nuclear quadrupole coupling constant xq and the asymmetry parameter ti for KOH. Note the sharp change in t] at the transition temperature by contrast with the sluggish change in xq- From Bastow et a/. (1991) by permission of Elsevier Science.

Sodium and potassium formate, together with copper formate (anhydrous and as the tetrahydrate) have been used as model compounds for surface formates [56] and 7.4.3. Copper formate tetrahydrate undergoes an antiferroelectric transition at 235.5 K. The INS spectra clearly show that above this temperature the water molecules are disordered and below it they are ordered [57]. 

THERMAL EXPANSION OF CU/HCO2/2.4H20 IN A TEMPERATURE REGION COVERING THE ANTIFERROELECTRIC TRANSITION. 

Okada, K. Phenomenological theory of antiferroelectric transition. I. Second-order transition. J. Phys. Soc. Jpn. 27, 420-428 (1969)... 

Fig. 13.25 Geometry for discussion of the electric field-induced ferroelectric-antiferroelectric transition. Antiferroelectric structure (a) in the zero field and ferroelectric structure at the field exceeding the F-AF transition threshold (b)...

The v2 bending vibration is a quartet or, in a simplified picture, two Davydov doublets as a consequence of a site-symmetry-induced doublet (see Fig. 2.6).40 A system of particular interest is CO/NaCl(100) it is characterized by inclined molecular orientations with =25° and antiferroelectric ordering of chains at low temperatures (see Fig. 2.7) which is removed on the phase transition at T 25 K. This structural information is deduced from the observed Davydov splitting of the spectral line for the CO stretching vibrations at 2155 cm 1 and T<24 K (see Fig. 

A less well-documented effect is that of the phase-transition temperature of certain crystals which are very sensitive to deuteration. Some crystals of ferroelectric and antiferroelectric materials, and in particular dihydrogen phosphates and hydrogen selenites, which are extensively hydrogen bonded, display this effect (Blinc and Zeks, 1974). For some crystals, such as caesium... 

Figure 2 shows the schematic structure in the paraelectric (T > Tn) and an-tiferroelectric (T < Tn) phases, hi the paraelectric phase the time-averaged position of the H atoms hes in the middle of an O - H...0 bond, whereas in the antiferroelectric phase, the protons locahze close to one or the other O atom. Prior to the recent NMR work [20-25], the largely accepted model of the phase transition was that the phase transition involved only the ordering of the H atoms in the O - H...0 bonds, and no changes in the electronic structure of the C4 moieties were considered to take place. The NMR results show that, in addition to the order/disorder motion of the H atoms, the transition also involves a change in the electronic charge distribution and symmetry of the C4 squares. 

Fig. 7 CP-MAS spectra of squaric acid, peak assignment and temperature variation in the close vicinity of its paraelectric-antiferroelectric phase transition [24], Note the pronounced coexistence of the spectra from the two phases...

Fig. 18 Temperature dependence of the CPMAS spectra of NH4H2ASO4 around the antiferroelectric phase transition temperature. The peaks corresponding to the paraelec-tric and antiferroelectric phases are labelled P and AF, respectively...

Fig. 19 Temperature evolution of the paraelectric and antiferroelectric phases around the phase transition temperature of NH4H2ASO4, as obtained from Fig. 18...

The temperature dependence of 1/Ti for is shown in Fig. 21. The discontinuity in the l/Ti data near Tn 216 K (highhghted by the arrow) appears at the onset of the antiferroelectric phase transition. Below Tn l/Ti increases abruptly from about 370 ms to 700 ms. 

Pociecha D, Gorecka E, Cepic M, Vaupotic N, Gomola K, Mieczkowski J (2005) Paraelectric-antiferroelectric phase transition in achiral liquid crystals. Phys Rev E 72 060701R... 

All of structures (3) relate to antiferroelectric phase that is in an agreement with the available measurements evidencing just the same character of low-temperature phase in the M3D(A04)2 crystals [6], It is also evident that the doubling of b-parameter of the paraelectric A2/a cell under transition to the low-temperature D-ordered phase directly follows from proposed scheme (3). Recently, such fe-doubling is observed experimentally in [7]. This scheme is also consistent with the doubling of c-parameter of A2/a cell found in the cited paper. In particular, such fe-doubling takes place when the layer sequence (3) takes the form ... 

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