BISMUTH TUNGSTATE AND LAYERED FERROELECTRICS

These include crystals of Bi2WO6, Bi4Ti3O12, Bi6Ti3WO18, Bi10Ti3W3O30 and some other newly synthesized pBi2WO6 · qBi4Ti3O12 compounds with p:q = 7:2, 5:1, 6:1, 9:1, and 15:1. All they have layered crystal structures in which Bi2O2 layers alternate with perovskite-like fragments of various thickness (one octahedron in Bi2WO6, two octahedra in Bi6Ti3WO18, three octahedra in Bi4Ti3O12, etc). At the same time in the structure of Bi10Ti3W3O30 and other numerous mixed-layered compounds there are ordered alternations of perovskite-like fragments of different thickness. The general questions of symmetry and its changes at ferroelectric phase transitions in such the crystals have been analyzed for the first time by present authors. It was found also that bismuth- containing layered crystals exhibit a high ionic conductivity caused by fast migration of oxygen anions.

Grown single crystals of Bi2WO6 are of very high perfection and seem to exhibit unusual behavior on heating - spontaneous deformation of their lattice rises with temperature until the crystals loss stability and undergo reconstructive phase transition but not ordinary ferroelectric one. These crystals are not divided into domains.


SYMMETRY OF THE Bi-CONTAINING LAYERED COMPOUNDS AND ITS CHANGES AT FERROELECTRIC PHASE TRANSITIONS


(mh - mirror plane in perovskite-like fragment; i - inversion center in Bi2O2 layers; the digits are the number of MeO6 octahedra in perovskite-like fragments of the structure, * - with doubling of the c-parameter)
Symmetry elements and digital symbols of paraelectric phase Space group of the paraelectric phase and its changes at ferroelectric phase transitions in dependence on symmetry of digital symbols Examples of structure types and compounds

mh + i

(symbols are symmetric about digits and intervals)
                      B2cb   (symmetry about odd
                                              digits)
  I4/mmm

                      A21am   (symmetry about even
                                              digits)
11 (Bi2WO6);
33 (Bi4Ti3012);
122122, etc.

22 (Bi3TiNbO9);
44; 211211, etc.


Mh

(symbols are symmetric about digits only)
                      P2an   (symmetry about odd
                                              digits)

  P4/mmm     P21am   (symmetry about even
                                              digits)

                      I2cm*  (symmetry about both
                                    odd and even digits)
13 (Bi6Ti3WO18);
15; 123132, etc.

24; 26; 211411;
etc.

12 (Bi5Nb3O15);
23; 1222; etc.

i

(symbols are symmetric about intervals)

  P4/nmm       P21ab       (in all the cases)

1221; 132231; etc.


None

(symbols are not symmetric)
                      Aa         (symbols of the
                                    {pqr...}2 - types)

  P4mm           Pa         (the sum of digits
                                        is even)

                      Ia*         (the sum of digits
                                        is odd)
123123 = {123}2;
etc.

1234; etc.


1235; etc.

NOTE: Symmetry of real Bi4Ti3012 crystals seems to be more low because of some disorder in the perovskite-like fragments thickness.


Publications:

  1. Structure and properties of ferroelectric Bi2WO6 crystals.
    V.K.Yanovskii, V.I.Voronkova, A.L.Aleksandrovskii, and V.A.D'yakov. Doklady AN SSSR, 1975, v.222, p.94.
  2. Growth of Bi2WO6 single crystals.
    V.I.Voronkova and V.K.Yanovskii. Kristallografiya, 1977, v.22, p. 429.
  3. Dielectric properties, electrical conductivity and relaxation phenomena in ferroelectric Bi2WO6.
    V.I.Utkin, Yu.E.Roginskaya, V.I.Voronkova, V.K.Yanovskii, B.Sh.Galyamov, and Yu.N.Venevtsev. Phys. Stat. Solidi (a) , 1980, v.59, p. 75.
  4. Fast anionic transport in the Bi2WO6 crystals.
    V.K.Yanovskii, V.I.Voronkova, Yu.E.Roginskaya, and Yu.N.Venevtsev. Fiz. tverdogo tela, 1982, v.24, p.2829.
  5. Polymorphism and properties of Bi2WO6 and Bi2MoO6.
    V.K.Yanovskii and V.I.Voronkova. Phys. stat. sol. (a) , 1986, v.93, p.57.
  6. The structure and ferroelectric properties of layered compounds in the system Bi2WO6 - Bi4Ti3O12.
    V.K.Yanovskii and V.I.Voronkova. Phys. stat. sol. (a) , 1987, v.101, p.45.
  7. Structure, polymorphism and ferroelectric properties of mixed-layered bismuth containing compounds.
    V.K.Yanovskii and V.I.Voronkova. Neorg. Mater., 1986, v.22, p. 2029.
  8. Ferroelectric bismuth-containing compounds with the mixed-layered perovskite-like structure.
    V.K.Yanovskii and V.I.Voronkova. Kristallografiya, 1988, v.23, p. 1278.
  9. Structure and electrophysical properties of the mixed layered phases in the system Bi5Nb3O15 - Bi5Ti1.5W1.5O15.
    V.K.Yanovskii, V.I.Voronkova, and I.N.Leont'eva. Neorg. Mater., 1989, v.25, p.834.



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