Crystal structure and dielectric relaxation studies of the [N(CH3)(3)H](3)Sb2(1-x)Bi2xCl9 mixed crystals

The influence on ferroelectric properties of a partial substitution of anti mony by bismuth in the [N(CH3)(3)H](3)Sb2Cl9 (TMACA) crystals are investiga ted by dielectric and x-ray diffraction methods. The dielectric relaxation in the [N(CH3)(3)H](3)Sb2(1-x)Bi2xCl9 (TMACAB) mixed crystals (x = 0, 0.07, 0.33, 0.38, and 1) is studied in the frequency range between 20 Hz and 1 G Hz and temperature range between 170 and 375 K. The ferroelectric propertie s of mixed crystals are preserved for the Bi concentration less than 0.33. The increase of the Bi concentration distinctly influences the phase transi tion temperature but only weakly the dynamical properties of those crystals . The dielectric response of the ferroelectric TMACAB crystals is character ized by a presence of two relaxation modes. One of them behaves critically and the other one is only thermally activated. The disappearance of ferroel ectric properties is connected with the essential change of crystal structu re of mixed compounds. The anionic sublattice of nonferroelectric TMACAB (x > 0.33) (<R(3)over bar>c space group) instead of two-dimensional layers is composed of the isolated Sb2(1-x)Bi2xCl9 bioctahedral units. (C) 2000 Amer ican Institute of Physics. [S0021-8979(00)02214-3].