Anomalies of dielectric permeability at the ferroelectric phase transitions in the crystals KTiOPO4 (1), RbTiOPO4 (2) and TlTiOPO4 (3).

KTiOPO₄ (KTP) and other crystals of this family are of great interest as new effective materials for nonlinear optics and are intensively studied in the last time. Their high nonlinear optical susceptibility suggests existence of ferroelectric properties and, indeed, the second order ferroelectric phase transitions were observed for the first time in our laboratory for KTiOPO₄, RbTiOPO₄ and TlTiOPO₄ crystals. The most effective methods to study these transitions are measurements of dielectric permeability and thermooptical studies. Similar transitions were thereafter observed in many other crystals of this family. They can lead to formation of ferroelectric domains and to accidental variations of physical properties. At the same time ferroelectric properties of the KTP - family crystals can be used in practical purposes such as production of optical elements with a specified domain configuration.

One of peculiarities of KTP-family crystals, especially of flux grown ones, is also their anomalous high ionic conductivity due to presence of movable K or other alkaline cations which can be regulated in wide limits by means of heterovalent substitutions. Hence these crystals may be regarded as representatives of a new type of solid state materials: ferroelectrics - superionic conductors. Recently we observed also in the KTiOPO₄ crystals doped with 3-5 at.% of niobium jumps of electrical conductivity up to 2-3 orders of magnitude which is typical for some other superionics.

1. Ferroelectric phase transitions and properties of crystals of the KTiOPO₄ family.
   V.K.Yanovskii and V.I.Voronkova. Phys. stat. sol. (a), 1986, v. 93, p. 665.
2. Morphology of KTiOPO₄ crystals.
   V.I.Voronkova and V.K.Yanovskii. Kristallographiya, 1986, v.31, p.207.
3. Ferroelectric domains in the KTiOPO₄ and RbTiOPO₄ crystals.
   V.I.Voronkova, R.S.Gvozdover, and V.K.Yanovskii. Pis'ma JTF, 1987, v.13, p.934.
4. Flux growth and properties of the KTiOPO₄ family crystals.
   V.I.Voronkova and V.K.Yanovskii. Neorg. mater. , 1988, v.24, p.273.
5. Piezoelectric, elastic and dielectric properties of RbTiOPO₄ crystals.
   I.M.Sil'verstova, Yu.V.Pisarevskii, V.I.Voronkova, and V.K.Yanovskii. Kristallographiya, 1990, v.35, p.229.
6. Ferroelectric and physicochemical properties of the K_{1- x}Na_xTiOPO₄, K_{1- x}Tl_xTiOPO₄, and Rb_1-xCs_xTiOPO₄ solid solutions.
   V.I.Voronkova, E.S.Schubentsova, and V.K.Yanovskii. Neorg. mater. , 1990, v.26, p.143.
7. Thermooptical study of KTiOPO₄ family crystals.
   R.V.Pisarev, P.A.Markovin,B.N.Shermatov, V.I.Voronkova, and V.K.Yanovskii. Ferroelectrics, 1989, v.96, p.181.
8. Ferroelectric phase transition and atomic structure of KGeOPO₄ crystals.
   V.I.Voronkova, V.K.Yanovskii, N.I.Sorokina, I.A.Verin, and V.I.Simonov. Kristallographiya, 1993, v.38, p.147.
9. Crystal structures and properties of compounds in the KTiOPO₄ - TlTiOPO₄, and RbTiOPO₄ - TlTiOPO₄ systems.
   N.I.Sorokina, V.I.Voronkova, Av.k.Yanovskii, D.Y.Lee, M.Yannin, C.Kolinsky, G.Godefroy, B.Yannot, V.I.Simonov. Crystallography Reports, 1997, v.42, p.39.
10. Superionic transitions in the K_1-xTi_{1- x}Nb_xOPO₄ and K₃Nb₃B₂O₁₂ crystals.
    T.Yu.Losevskaya, E.P.Kharitonova, V.I.Voronkova, V.K.Yanovskii, S.Yu.Stefanovich, N.I.Sorokina, and V.I.Simonov. Crystallography Reports, 1999, v.44, p.90.
11. "Synthesis, atomic structure, and properties of crystals in the RbTiOPO₄ - CsTiPO₅ system." Liu Wen, V.I.Voronkova, V.K.Yanovskii, N.I.Sorokina, I.A.Verin, and V.I.Simonov. Crystallography Reports, 2000, Vol.45, No.3, pp.380-385.
12. "Crystal structure of KTi_0.93Sn_0.07OPO₄." Liu Wen, N.I.Sorokina, V.I.Voronkova, V.K.Yanovskii, I.A.Verin, A.G.Vigdorchik, and V.I.Simonov. Crystallography Reports, 2000, Vol.45, No.3, pp.386-388.
13. "Crystal structure and properties of the niobium doped potassium titanyl-phosphate crystals." T.Yu.Losevskaya, O.A.Alekseeva, V.K.Yanovskii, V.I.Voronkova, N.I.Sorokina, V.I.Simonov, S.Yu.Stefanovich, S.A.Ivanov, S.Ericsson, and S.A.Zverkov. Crystallography, 2000, Vol.45, No.5, pp.809-813.
14. Growth and properties of KTi_1-xSn_xOPO₄. Liu Wen, V.I. Voronkova, V.K. Yanovskii, S.Yu.Stefanovich,N.I. Sorokina, I.A. Verin. Inorganic materials .2001, V. 37, 290.
15. 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.
16. Growth and physical properties of K_1-xTi_1-xSb_xOPO₄ crystals. T.Yu.Losevskaya, V.K.Yanovskii, V.I.Voronkova, and S.Yu.Stefanovich. Inorganic Materials, 2002, v.38, No.8, pp.1164-1167.