Probing local structure of the morphotropic phase boundary composition of Na_0.5Bi_0.5TiO₃–BaTiO₃ using rare-earth photoluminescence as a technique

Abstract

(1-y)Na_0.5Bi_0.5TiO₃-(y)BaTiO₃ (NBT-BT) is one of the most investigated lead-free piezoelectric system in the recent past. Unlike the conventional piezoelectric alloys wherein the morphotropic phase boundary (MPB) compositions exhibiting maximum piezoelectric response, exhibit coexistence of ferroelectric phases with different symmetries on the global scale, the MPB composition (y = 0.06) of NBT-BT shows a cubic-like phase in the unpoled state. On poling it transforms to a non-cubic ferroelectric phase. Here we exploit the sensitivity of the photoluminescence (PL) property of doped rare-earth ions with regard to the local symmetry of the host crystal to investigate the local structure of the cubic-like phase of the MPB composition of NBT-BT. We performed a comparative study of the PL response by doping separately Er and Eu in very dilute concentration in the (i) MPB composition (y = 0.06) exhibiting a cubic-like phase, (ii) a sub-MPB composition (y = 0.03) exhibiting rhombohedral phase, and (iii) above the MPB composition (y = 0.10) exhibiting tetragonal phase. We found that both the Er and the Eu PL spectra of the MPB composition (y = 0.06) exhibits more number of Stark lines in its unpoled cubic-like phase as compared to that in the field-stabilized rhombohedral phase. The additional Stark lines in the cubic-like global phase are identified to be that of the tetragonal structure. Our study therefore confirms that what appears as a cubic-like phase on the global scale has intimately connected tetragonal and rhombohedral local structures. The success of our experiments suggests that rare-earth PL can be used as a simple yet powerful tool to investigate local structures in scenarios wherein the global structure need not comply with the local structure.