The 72 Families of Ferroelectrics

In the Landolt-Bdmstein data collection, ferroelectric and antiferroelectric substances are classified into 72 families according to their chemical composition and their crystallographic structure. Some substances which are in fact neither ferroelectric nor antiferroelectric but which are important in relation to ferroelectricity or anti-ferroelectricity, for instance as an end material of a solid solution, are also included in these families as related substances. This subsection surveys these 72 families of ferroelectrics presented in Landolt-Bornstein Vol. III/36 (LB III/36). Nineteen of these families concern oxides [5.1,2], 30 of them concern inorganic crystals other than oxides [5.3], and 23 of them concern organic crystals, liquid crystals, and polymers [5.4]. Table 4.5-1 lists these families and gives some information about each family. Substances classified in LB 111/36 as miscellaneous crystals (outside the families) are not included. 

In the following, remarks are made on 13 of the families, labeled by the letters a-m in Table 4.5-1. The corresponding family numbers are repeated in the headings. 

The pure compounds are divided into simple perovskite-type oxides and complex perovskite-t)fpe oxides. Simple perovskite-type oxides have the chemical formula A +B +03 or A +B +03. Complex perovskite-type oxides have chemical formulas expressed by (A +Af+)B03, A2+(B2+Bf+)03, 

A2+(bM+)03, a2+(B2+B 5+p A2+(B2+Bf+)03, A(B, B, B 003, or (A, A0(B, BOO3. Among the complex perovskite-type oxides, most of the Pb(B, BOO3-type oxides show a diffuse phase transition such that the transition point is smeared out over a relatively wide temperature range and exhibits a characteristic dielectric relaxation these materials therefore are called relaxors . 

Family Nr. Inorganic Crystals Oxides [5.1,2] Name Family Nr. Inoi anic Crystals other than Oxides [5.3] Name Orgi Family Nr. uiic Crystals, Liquid Crystals, and Polymers [5.4] Name 

5 Stibiotantalite family (7, 6 0) c 24 Sd2P2S6 family (2,2 0) 54 (CH3NH3)2AlCl5 6H2O 

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Table A..5-1 The 72 families of ferroelectric materials. The number assigned to each family corresponds to the number used in LB III/36. The numbers in parentheses (A sub>. f+a ) after the family name serve the purpose of conveying some information about the size and importance of the family. The numbers indicate the following A sub the number of pure substances (ferroelectric, antiferroelectric, and related substances) which are treated as members of this family in LB III/36 A f+A the number of ferroelectric and antiferroelectric substances which are treated as members of this family in LB III/36 n, the number of representative substances from this family whose properties are surveyed in Sect. 4.5.4. For some of these families, additional remarks are needed for instance, because the perovskite-type oxide family has many members and consists of several subfamilies because the liquid crystal and polymer families have very specific properties compared with crystalline ferroelectrics and because the traditional names of some families are apt to lead to misconceptions about their members. Such families are marked by letters a-m following the parentheses, and remarks on these families are given under the corresponding letter in the text in Sect. 4.5.3.1...

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