Titanium charge state drives antiferrodistortive phase transition in LaxS1-xTiO3

The structure of LaxSr1-xTiO3 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 (TC) 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 Ti3+-rich TiO6 octahedra below TC.