Chemical pressure effect in Sm and La substituted ferroelectric BiFeO₃ thin films: Insights from infrared spectroscopy

We investigate the effects of Sm and La substitution in ferroelectric BiFeO₃ thin films on the lattice dynamics by infrared reflection measurements at room temperature. The frequencies of the infrared-active phonon modes are studied as a function of Sm and La content in Bi_1–x(Sm,La)_xFeO₃ composition spread films in the range from x = 0 up to x = 0.25, grown on SrTiO₃ substrates by pulsed laser deposition. Substitution of the Bi³⁺ ions with small Sm³⁺ ions leads to the appearance of a new phase above x ≈ 0.09 coexisting with the ferroelectric BiFeO₃ phase up to x ≈ 0.19. In contrast, for the substitution of Bi³⁺ ions with La³⁺ ions of similar size a continuous transition from the original BiFeO₃ phase to a new phase takes place. In both cases, we assign the new phase to the paraelectric, orthorhombic phase. These findings are discussed in terms of the morphotropic phase boundary in Sm-doped BiFeO₃ around x = 0.14 with a phase coexistence, which was suggested as the origin for enhanced piezoelectric properties.