PHASE STUDY AND ELECTRICAL PROPERTIES OF DIVALENT DOPED NON-STOICHIOMETRIC BISMUTH ZINC NIOBATE CUBIC PYROCHLORE

Abstract

A study of extrinsic doping is essential to elucidate relative ability of the pyrochlore materials to accommodate various dopants in forming new solid solutions and to search for improvement of electrical properties. Much reported stoichiometric cubic pyrochlore Bi₃Zn₂Nb₃O₁₄ appears to contain ZnO as secondary phase. Reducing Zn content in Bi₃Zn₂Nb₃O₁₄ resulted in Bi₃Zn_2-xNb₃O_14-x solid solutions where 0.04 < x < 0.31. The phase pure material of composition Bi₃Zn_1.84Nb₃O_13.84 has been indexed on a cubic cell, space group FD3M, with cell constant a = 10.5579 (4) Ǻ. Incorporation of divalent ions Cu²⁺, Mg²⁺, Ca²⁺, Co²⁺, Pb²⁺, Cd²⁺ and Ni²⁺ in non-stoichiometric cubic pyrochlore Bi₃Zn_1.84Nb₃O_13.84 has been studied. Extensive solid solutions were observed when copper and cadmium were introduced as dopants where solid solution limits were obtained at x = 0.5 and 0.4, respectively in Bi₃Zn_1.84-xNb₃M_xO_13.84. Other divalent dopants showed rather narrow solid solutions in the range of x = 0.15 to 0.25.  Conductivity Arrhenius plots of all the doped materials are linear and reversible on heat-cool cycles with activation energy of 1.4-1.6 eV (x = 0.1). At the frequency of 100 kHz, these materials exhibit high relative permittivity (e’) and low dielectric loss (tan d) with values in range of 50-100 and 0.001-0.015, respectively at 28 ^oC. A slight increase in relative permittivity was observed in the doped materials except where Cd²⁺ and Ni²⁺ were dopants. Dielectric loss generally increases with increasing dopant content. Copper doped solid solutions exhibit higher conductivity which could be associated with lowering of activation energy (1.4-0.8 eV).