Experimental evidence for a continuous phase transition in a multidimensional ferroelectric

Critical Slowing Down in Ferroelectric Pyridinium Tetrafluoroborate

“…238X7 K C2 ÀÀÀ3 204 K P2, and the ferroelectric phase transition is of the second order which is exceptional for multidirectional ferroelectrics [9]. The room-temperature structure of PyBF 4 is shown in Fig.…”

“…A characteristic feature of this new ferroelectric family is the occurrence of a sequence of two-phase transitions; in PyBF 4 there are the ferroelectric phase transition at T 1 238X7 K and another transition at T 2 204 K. Earlier NMR studies of pyridinium salts [4 to 7] proved the occurrence of reorientation of both pyridinium cation and BF 4 anion in the symmetric high-temperature phases. According to structural and dielectric studies [8,9], PyBF 4 undergoes a sequence of structural phase transitions R " 3m ÀÀÀ À3…”

Partial order among the 14 Bravais types of lattices: basics and applications

“…(ii) Pyridinium tetrafluoroborate [C 5 H 6 N] + BF 4 À has been investigated by Czarnecki et al (1998). It is paraelectric at room temperature with space group R3m and undergoes at T = 238.7 K a continuous transition to a ferroelectric phase with space group C2.…”

“…It is paraelectric at room temperature with space group R3m and undergoes at T = 238.7 K a continuous transition to a ferroelectric phase with space group C2. It follows that the lattice type is hR at room temperature and changes to mC at T. Using high-resolution neutron powder diffraction, Czarnecki et al (1998) found for the conventional primitive rhombohedral cell a 0 , b 0 , c 0 at 293 K: |a 0 | = |b 0 | = |c 0 | = a 0 = 567.074 (7) pm and 0 = 0 = 0 = 97.305 (1) . The entry at the bottom of our Table 3 tells us 1/2 À a = 2.07 pm, showing that 8.7 K below the transition temperature T the lattice already differs considerably from a rhombohedral one, in contrast to the first example.…”

Domain Structure and Birefringence in Pyridinium Perchlorate and Pyridinium Fluoroborate Single Crystal

“…According to Aizu [11] transitions I → II require an appearance of domain structure of ferroelastic type with domain walls predicted by Sapriel [12]. Resulting three orientational ferroelastic states were described by Czarnecki et al [13]. So far, there are no experimental data related to microscopic observations of domain structure.…”