K₃Nb₃B₂O₁₂ FAMILY

K₃Nb₃B₂O₁₂ single crystal
 

K₃Nb₃B₂O₁₂ single crystals are of great interest as materials with unique combination of ferroelectric, ferroelastic and superionic properties. These properties are determined by peculiarities of their atomic structure consisting of rigid framework from NbO₆ octahedra and plane BO₃ groups with wide channels occupied by labile K ions.

The K₃Nb₃B₂O₁₂ single crystals and their solid solutions with a various degree of substitutions K⁺ by Na⁺ or Rb⁺, Nb⁵⁺ by Ta⁵⁺ or Sb⁵⁺, and B³⁺ by Si⁴⁺ are grown and their phase transitions and physical properties are investigated.

It is shown that the above crystals undergo to 7 phase transitions (the temperature is indicated in ^oC for the pure K₃Nb₃B₂O₁₂):

which are accompanied by intensive anomalies of dielectric permeability e₃₃ and by changes in domain structure. The high temperature phase a is hexagonal one with the point group -62m, phase b has symmetry 3m or -62m, other low-temperature phases are monoclinic or triclinic. The b « a phase transition is anti- or ferroelectric, whereas other transitions have mixed ferroelectric-ferroelastic nature.

K₃Nb₃B₂O₁₂ and solid solutions demonstrate superionic properties and high conductivity caused by high mobility of K⁺ cations. Electric conductivity s₃₃ of pure K₃Nb₃B₂O₁₂ crystals is near to 10^-5 Ohm^-1cm^-1 at 300^oC with a sharp drop at d « g transition by an order of magnitude.

     The full substitutions of B by Si is possible and K₃Nb₃Si₂O₁₃ single crystals are grown and studied. They have hexagonal growth form but at the room temperature have lower symmetry and are divided into domains. These crystals are characterized by high dielectric permeability e₃₃ which is about 4000 at room temperature and rises with temperature.

Domain structure of K₃Nb₃B₂O₁₂ single crystal at room temperature under polarized light

 

1. V. I. Voronkova, V. K. Yanovskii, I. N. Leont’eva, N. I. Sorokina, V. I. Simonov. Growth, Structure, Polymorphism, and Physical Properties of KNbB₂O₆ and K₃Nb₃B₂O₁₂ Crystals // Crystallography Reports. 1996. V. 41. No. 5. P. 837 - 840.

2. E.P. Haritonova, V.I. Voronkova, V.K. Yanovskii, S.Yu. Stefanovich, A.V. Mosunov. Phase transitions and domain structure of ferroelectric-ferroelastic-superionic crystals K₃Nb₃B₂O₁₂ and their solid solutions. // Ferroelectrics. 1999. V. 221. p. 73 - 77.

3. T.Yu. Losevskaya, E.P. Kharitonova, V.I. Voronkova, V.K. Yanovskii, S.Yu. Stefanovich, N.I. Sorokina, V.I. Simonov. Superionic transitions in the K_1-xTi_1-xNb_xOPO₄ and K₃Nb₃B₂O₁₂ crystals. // Crystallography Reports. 1999. V. 44. No 1. p. 95 - 97.

4. V.I. Voronkova, E.P. Kharitonova, V.K. Yanovskii, S.Yu. Stefanovich, A.V. Mosunov, N.I. Sorokina. Growth, structure and properties of ferroelectric-ferroelastic-superionic K₃Nb₃B₂O₁₂ and K_3-xNa_xNb₃B₂O₁₂ crystals. // Crystallography Reports. 2000. V. 45. No 5. p. 816 - 820.

5. E.P.Kharitonova, V.I.Voronkova, V.K.Yanovskii, and S.Yu.Stefanovich. Polymorphism of ferroelectric, ferroelastic, superionic crystals K₃Nb₃B₂O₁₂ and K_3-xNa_xNb₃B₂O₁₂. // Inorganic Materials. 2002. V.38. No.8. p.819 - 824.

6. E.P. Kharitonova, V.I. Voronkova, V.K. Yanovskii. Phase transitions and ferroelectric and superionic properties of K₃Nb₃B₂O₁₂ - K₃Nb₃Si₂O₁₃ single crystals. // Inorganic Materials. 2003. V. 39. No. 2. p. 127 - 132.

7. E.P. Kharitonova, V.I. Voronkova, V.K. Yanovskii. Phase transitions and physical properties of K₃Nb₃B₂O₁₂ and K₃Nb_3-xM_xB₂O₁₂ (M = Ta, Sb) crystals. // Inorganic Materials. 2003. V.39. No.6. p.710 - 715.