Mechanisms of the Effect of Dopants and P(O2) on the Improper Ferroelastic Phase Transition in SrTiO3

Abstract

Perovskite-structured SrTiO3 undergoes a cubic (Pm3m) to tetragonal (I4/mcm) transition at ∼108 K (Ta) associated with rotations of the O octahedra in the antiphase around the [001] direction. This phase transition gives rise to modes at the R point of the Brillouin zone in the Raman spectra and superlattice reflections at 1/2{odd–odd–odd}. The effect on Ta of La3+ and Mn2+ A-site substitution and Mn4+ and Mg2+ B-site substitution in polycrystalline SrTiO3 processed in air and sintering SrTi0.95Mn0.05O3–δ in different P(O2) has been studied using in situ Raman spectroscopy and electron diffraction. The transition temperature was raised when Mn2+ and La3+ were substituted for Sr2+ but lowered for Mn4+ and Mg2+ substitution on the Ti site. Sintering SrTi0.95Mn0.05O3–δ in N2 reduced Ta, but sintering in O2 had a negligible effect compared to air. It is proposed that two mechanisms are responsible for the modification of Ta: (i) the creation of oxygen vacancies by acceptor doping (Mg2+ ions on the Ti site) and sintering SrTi0.95Mn0.05O3–δ in low P(O2) and (ii) adjustment of the perovskite tolerance factor (t) when, e.g., La3+ (1.36 Å) and Mn2+ (1.27 Å) substitute for Sr2+ (1.44 Å, decrease in t) and Mn4+ (0.53 Å) substitutes for Ti4+ (0.605 Å, increase in t).