Section 3.1.5.3. Low-temperature ferroelectric transitions

It has historically been difficult to establish the nature of ferroelectric phase transitions at cryogenic temperatures. This is simply because the coercive fields for most crystals rise as the temperature is lowered, often becoming greater than the breakdown fields below ca. 100 K. As a result, it is difficult to demonstrate via traditional macroscopic engineering techniques (switching) that a material is really ferroelectric. Some authors have proposed (e.g. Tokunaga, 1987) on theoretical grounds the remarkable (and erroneous) conjecture that no crystals have Curie temperatures much below 100 K. A rebuttal of this speculation is given in Table 3.1.5.1 in the form of a list of counterexamples. References may be found in the 1990 Landolt–Börnstein Encyclopedia of Physics (Vol. 28a). The original work on pure cadmium titanate and on lead pyrochlore (Hulm, 1950, 1953) did not demonstrate switching, but on the basis of more recent studies on mixed crystals Ca_2−2xPb_2xNb₂O₇ and Ca_xCd_1−xTiO₃, it is clear that the pure crystals are ferroelectric at and below the stated temperatures.

Table 3.1.5.1 | top | pdf | Low-temperature ferroelectrics Formula Curie temperature (K) Curie constant C (K) Entropy change (cal mol−1 K−1) NH4Al(SO4)2·12H2O 71 ? ? NH4Fe(SO4)2·12H2O 88 400 0.15 (NH4)2Cd(SO4)3 95 ? ? CdTiO3 55 ? Pb2Nb2O7 15.3 ? ? LiTlC4H4O6·H2O 10.5 ? ? K3Li2Nb5O15 7 ? ?

Hence, in Table 3.1.5.1 we see examples where X-ray structural studies may establish the symmetries requisite for ferroelectricity without the macroscopic switching being demonstrated. This is the converse case to that primarily emphasized in this section (i.e. the use of techniques complementary to X-ray scattering to determine exact crystal symmetries); it is useful to see these reverse cases to demonstrate the full complementarity of X-ray crystallography and dynamic spectroscopic techniques.