Titanium charge state drives antiferrodistortive phase transition in La _x Sr _1-x TiO ₃

Abstract The structure of La x Sr 1-x TiO 3 single-phase perovskites (0.10 ≤ x ≤ 0.30) was studied using synchrotron X-ray Powder Diffraction for Rietveld and Pair Distribution Function analysis. Electron Spin Resonance and X-ray Raman Scattering Spectroscopy confirmed that Sr substitution by La is charge compensated via electron injection into the conduction band. High-resolution diffraction (110–490 K) revealed phase transitions to a tetragonal phase with antiferrodistortive tilting ( a 0 a 0 c - , in Glazer notation). Fitting the Ginzburg-Landau mean field equation indicated a transition regime between second-order and tricritical. The critical temperature (T C ) and tilt order parameter ( Φ 0 ) increase with La content up to x = 0.20, then decrease at x = 0.30. This non-monotonic behavior reflects an emerging local symmetry lowering toward Cmcm , supported by increased octahedral tilting and Bond Valence Sum analysis. The enhanced tilting offsets compensate for reduced thermal contraction of Ti 3+ -rich TiO 6 octahedra below T C .