Transition metal compounds ferroelectricity

These selected examples, and a number of others, suggest that for the transition-metal ions with empty d shells, the contact radius decreases (and instability to distortion increases) in the approximate order Hf4+, Zr4+, Ti4+, Ta6+, Nb5+, W6+, Mo6+, V6+, Cr6+, Mn7+ (Table XV). Ferroelectrics are largely restricted to compounds containing the group of ions of intermediate size, Ti4+, Ta6+, Nb6+, WB+. We can see, in a qualitative way, why this should be so. Larger ions are too stable in regular octahedral environ-... 

As for the transition metal elements, M and M, we will confine ourselves to compounds containing Fe, mainly because studies have been concentrated on them. The magnetism of YMnOj will not be referred to, although this is interesting because of the coexistence of magnetic ordering and ferroelectricity (Bertaut et al. 1963a,b, Bertaut and Mercier 1963, Kohn and Tasaki 1965). 

Some ferroelectric crystals are known which do not exhibit a Curie point because they melt before leaving the polarized state. Examples are the BaMp4 compounds, where M can be either a divalent M transition metal ion (Mn, Fe, Co, or Ni) or a nonmagnetic divalent ion (Mg or Zn). Others, like Rochelle salt, NaK(C4H406)-4H2O, and SrTe03, exhibit two Curie points, i.e., they are only ferroelectric between an upper and a low temperature. 

Magnetite, Fe30>4, is a unique material. It is a mixed-valency compound (Fe " and Fe " on crystallographic sites of the same symmetry) with a low electrical conductivity below 120K, and a nearly metallic conductivity above this temperature. Between 120 and 770 K, magnetite is an inverse spinel, (Fe )[Fe Fe ]04, and all the transition phenomena occur in the octahedral sites. For T > 770 K, the redistribution of Fe on tetrahedral sites becomes non-negligible (Wu Mason, 1981). The reversible, sharp discontinuity in conductivity was first observed by Okamura (1931). Ferroelectric features have also been observed at low temperatures (Rado Ferrari, 1975 Kato et al, 1983). 

---