Application of Positron Annihilation Spectroscopy to the Study of Defects in Perovskite Type Materials

Abstract

<p>Formation of defects in ceramics <em>BaA_1/3Nb_2/3O₃ </em>(where A=Mg, Co or Zn) suitable for microwave applications was analyzed by using positron annihilation spectroscopy and X-ray diffraction techniques. The dependence of the microwave properties of perovskite ceramics on the defect structure of <em>ANb₂O₆</em> (A = Mg, Co or Zn ) that was used as a precursor during the sintering process prompted us to conduct positron analysis of<em> ANb₂O₆</em> columbites as well.</p> <p>Positron lifetime experiments revealed only one defect type - grain boundaries- and demonstrated that the lifetime component originating from the annihilation in defects depends on the size of the grains contained in the materials. For perovskite samples where the grain sizes are smaller than those of columbite materials, the intensity of the defects' component is larger and varies within 28-40%. Results of coincidence Doppler broadening measurements confirmed the presence of only one type of defect in both materials, i.e. grain boundaries.</p> <p>Theoretical calculations for the <em>ANb₂O₆</em> columbite system resulted in bulk lifetimes of <em>τ_b</em> = <em>177, 176, and 172psec</em>, respectively for magnesium, zinc and cobalt columbites and are in agreement with experimental values. The presence of second phases in nonstoichiometric <em>A</em>_1+x<em>Nb₂O₆</em> (A=Mg, Co, or Zn) columbites did not change the bulk lifetimes of the columbite material because of the small difference in bulk lifetimes of the host material and impurities.</p> <p>Ordering in perovskite type materials that influences the microwave properties in ceramics also changes the positron characteristics of perovskites. Theoretical simulations demonstrated that for the disordered cubic phase, <em>τ_b≈</em> 190<em> p</em>sec and for the completely ordered hexagonal phase, <em>τ_b≈</em> <em>240 p</em>sec. The bulk lifetime of the final perovskite material greatly depends on the presence of the second phases (Ba-rich and, possibly, Co-rich) as well as on the fractional part of the hexagonal phase. The influence of additional phases on rb is more significant than that of the "disorderedordered" transition and can be used to monitor changes in the concentration of the former ones.</p>