Mechanisms of the effect of dopants and P(O₂) on the improper ferroelastic phase transition in SrTiO₃

Perovskite-structured SrTiO₃ undergoes a cubic (Pm3m) to tetragonal (I4/mcm) transition at ∼ 108 K (T_a) 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 T_a. of La³⁺ and Mn²⁺ A-site substitution and Mn⁴⁺ and Mg²⁺ B-site substitution in polycrystalline SrTiO₃ processed in air and sintering SrTi_0.95Mn _0.05O_3-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 Mg ²⁺ substitution on the Ti site. Sintering SrTi_0.95Mn _0.05O_3-δ in N₂ reduced T_a, but sintering in O₂ had a negligible effect compared to air. It is proposed that two mechanisms are responsible for the modification of T _a: (i) the creation of oxygen vacancies by acceptor doping (Mg ²⁺ ions on the Ti site) and sintering SrTi_0.95Mn _0.05O_3-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).