Ferroelectric domains in the RbNbWO6 crystals under polarized microscope.

There is a broad family of compounds A(B,W)₃O₉ where A = K, Rb, Cs, Tl, whereas B are cations of different elements from Li⁺ to Nb⁵⁺ and Ta⁵⁺. All these compounds have the structure of hexagonal tungsten bronze type with wide channels along c axis in which cations A⁺ are situated. It was found that many of these compounds undergo ferroelectric transitions. Single crystals some of them, KNbW₂O₉, RbNbW₂O₉, TlNbW₂O₉, and K₃AlW₈O₂₇, were grown from fluxed melts and their structural features, phase transitions, and physical properties were studied in details. All the crystals are characterized by complicated polymorphism in the temperature range from room temperature to 800^o C with the formation polar and nonpolar phases and with rearrangements of domain structures. In polar state all the crystals show a high nonlinear optical susceptibility.

Another group of new potential ferroelectrics consists of compounds A(B,W)₂O₆ having the structure of "defect" pyrochlore type with incompletely filled voids in a rigid framework of (B,W)O₆ octahedra. Here elements A are K, Rb, Tl, and elements B are Al and Nb. Some of these crystals exhibit diffuse ferroelectric phase transitions in the range of 50-240^o C. It was noted that voids in these crystals can be reversible occupied by water molecules with the formation of hydrated forms A(B,W)₂O₆ · nH₂O. Ferroelectric transitions in these crystals are apparently related to collective interactions of A⁺ cations. Strongly polarized molecules of absorbed water can be involved in these processes considerably affecting the dielectric and nonlinear optical properties.

Among recently studied new ferroelectrics are the K₃Nb₃B₂O₁₂ crystals and their solid solutions. Numerous KTP - family crystals and mixed-layered Bi-bearing compounds are also typical examples of such materials.

1. Ferroelectrics with the structure of hexagonal tungsten bronze type.
   V.K.Yanovskii, V.I.Voronkova, and I.P.Klimova. Ferroelectrics, 1983, v.48, p.239.
2. New compounds AB_nW_3-nO₉ with the structure of hexagonal tungsten bronze.
   I.P.Klimova, V.I.Voronkova, and V.K.Yanovskii. Neorg. mater. , 1995, v.31, p.262.
3. Peculiarities of crystal chemistry, polymorphism and dielectric properties of the K(M_xW_1-x)₂O₆ tungstates.
   A.A.Bosenko, T.P.Petrovskaya, and V.K.Yanovskii. Neorg. mater., 1980, v.16, p.1076.
4. Neutronographic refining of the atomic structures of RbNbWO₆ and TlNbWO₆ crystals.
   N.N.Bydanov, T.S.Chernaya, L.A.Muradyan, V.A.Sarin, E.E.Ryder, A.A.Bosenko, and V.K.Yanovskii. Kristallographiya, 1987, v.32, p.623.
5. Growth, structure and physical properties of KNbB₂O₆ and K₃Nb₃B₂O₁₂ crystals.
   V.I.Voronkova, V.K.Yanovskii, I.N.Leont'eva, N.I.Sorokina, V.I.Simonov. Kristallographiya, 1996, v.41, p. 879.
6. Phase transitions and domain structure of ferroelectric - ferroelastic - superionic crystals K₃Nb₃B₂O₁₂ and their solid solutions.
   E.P.Haritonova, V.I.Voronkova, V.K.Yanovskii, S.Yu.Stefanovich, and A.V.Mosunov. Ferroelectrics, 1999, v.221, p.73.
7. Growth, structure and properties of ferroelectric- ferroelastic-superionic K₃Nb₃B₂O₁₂ and K_3-xNa_xNb₃B₂O₁₂ crystals. V.I.Voronkova, E.P. Kharitonova, V.K.Yanovskii, S.Yu.Stefanovich, A.V.Mosunov, N.I. Sorokina.Crystallography Reports. 2000, V.45, 816.
8. Crystal growth and physical properties of Cs₂Nb₄O₁₁ and Rb₂Nb₄O₁₁ single crystals. E.P.Kharitonova, V.I.Voronkova, V.K.Yanovskii, S.Yu.Stefanovich. J. Crystal Growth , 2002, v.237-239, pp.703-706.
9. Polymorphism of ferroelectric, ferroelastic, superionic crystals K₃Nb₃B₂O₁₂ and K_3-xNa_xNb₃B₂O₁₂. E.P.Kharitonova, V.I.Voronkova, V.K.Yanovskii, and S.Yu.Stefanovich. Inorganic Materials , 2002, v.38, No.8, pp.819-824.