Application of positron annihilation and Raman spectroscopies to the study of perovskite type materials

摘要

Defect properties of perovskite type materials, Ba_3B'Nb_2O_9 (where B'=Mg, Zn, or Co), with near-stoichiometric compositions were studied by positron annihilation and Raman spectroscopies. Theoretical simulations of stoichiometric perovskites revealed a dependence of the positron bulk lifetime on the degree of ordering. In Ba_3MgNb_2O_9 (BMN) the positron bulk lifetime for a completely disordered structure is 195 ps versus 237 ps for a completely ordered one. The predicted bulk lifetimes for Ba_3ZnNb_2O_9 (BZN) and Ba_3CoNb_2O_9 (BCN), with Pm3m symmetries are 193 ps and 194 ps, respectively. It was found that deviation from stoichiometry results in the appearance of secondary Ba- and Nb-rich phases, which according to theoretical simulations have bulk lifetimes much longer than that of the host material. Positron lifetime spectroscopy was used to monitor changes in the concentration of these second phases. The difference between predicted defect lifetimes and the bulk values for the studied perovskites was less than 70 ps. This and the likely small concentrations made it impossible to discern the presence of point defects in the samples. Raman measurements demonstrated the presence of a particular mode that could be attributed to the formation of a 1:1 phase, the size of which is limited by requirements for charge compensation. The existence of an internal electric field between charged 1:1 nanoregions and the rest of material creates conditions for preferential positron annihilation that influence the obtained positron lifetime values. For BZN type materials it was found that the degree of 1:2 cation ordering decreases by increasing the sintering temperature to above 1400 ℃.