Structural and Transport Properties of Doped Bismuth Titanates
Mixed Bi titanates with pyrochlore-type structure are promising materials for insulating layer of advanced MOS transistors and for design of storage capacitors in DRAM due to their high dielectric constant and low leakage current. The current study aims at studying structure and transport features of doped Bi titanates.
The mixed oxide Bi1.6Co0.24Ti2O7-δ with the pyrochlore type structure was prepared by the modified polymeric precursor method using bismuth and cobalt nitrates, titanium dioxide (anatase) and citric acid as starting materials. Stoichiometric mixture of crystallohydrates was heated to melting, then TiO2 powder and citric acid were added in the 1:1 molar ratio. After heating and stirring a gel-like mixture was formed and then burned out giving a powder. Powder was pressed into pellets and calcined at 650 °С (5 h), 850 °C (5 h), 950 °C (5 h) with intermediate grinding and pelleting. The structure of samples was studied by XRD, the local composition - by EDS microanalysis, conductivity - by using LCR meter MT-4090 (1-200kHz, 25 – 750°С) in the air. The oxygen mobility and surface reactivity were studied by isotope exchange with C18O2 in a closed reactor in isothermal and temperature-programmed modes.
The content of Co2TiO4 impurity in Bi1.6Co0.24Ti2O7-δ was <2%. Pycnometric density of this sample is 6.46±0.11 g/cm3, which corresponds to the formula [Bi1.57Co0.15][Co0.04Ti1.96]O7-δ and shows the distribution of 80 and 20 % of Co atoms in the Bi-sites and in the Ti-sites of the pyrochlore type structure, respectively. This cations distribution is confirmed by the X-ray structural analysis. The conductivity of Bi1.6Co0.24Ti2O7 δ is 3.6·10 3 Ω 1cm 1 at 750 °C (Ea = 0.38 eV). The Seebeck coefficient sign is 0.109±0.026 mV/K (100-270 °C) and indicates p-type conductivity increasing with p(O2) in the broad temperature range 25 - 750°С. The oxygen mobility and surface reactivity is comparable to other doped Bi titanates studied earlier , with Do reaching values up to 10-10 cm2/s at 700 K .
Support by Russian Science Foundation (Project 16-13-00112) and Russian Fund of Basic Research (project No. 15-03-09173 A) is gratefully acknowledged.
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