In this study, we have investigated the temperature dependence of atomic ordering at multiple length scales in a lead-free sodium niobate-based relaxor, i.e., 0.75 ${\mathrm{NaNbO}}_3$-0.25 ${\mathrm{Ba}}_{0.9}{\mathrm{Ca}}_{0.1}{\mathrm{TiO}}_3$ (NN-25BCT) via synchrotron x-ray diffraction, Raman spectroscopy, and pair distribution function analysis. High-resolution synchrotron x-ray powder diffraction (SXRD) measurements reveal a ferroelectric phase transition in the relaxor ferroelectric NN-25BCT below the Vogel-Fulcher freezing temperature ($T_{\text{VF}}\approx 270$ K). In addition, SXRD analysis demonstrates the competition between in-phase octahedral tilting and ferroelectric order at the long-range scale using mode crystallography. On the other hand, Raman spectroscopic analysis provides evidence of polar ordering for $T>T_{\text{VF}}$ (with tetragonal symmetry) persisting up to the Burns temperature ($T_{\text{B}}$). Furthermore, pair distribution function (PDF) analysis reveals the presence of a polar antiferrodistortive tetragonal phase with $P4\mathit{bm}$ space group at short ranges throughout the studied temperatures (i.e., $110\text{K}\le T\le 500$ K), irrespective of nonpolar long-range ordering above $T_{\text{VF}}$. Therefore, our measurements provide direct evidence for the presence of polar ordering at short ranges and their gradual transformation into long-range polar ordering using an integrated multiscale structural analysis. As a result of a transition from relaxor to a ferroelectric phase in the vicinity of room temperature, NN-25BCT can be exploited for applications in pyroelectric detectors, electrocaloric devices, and multilayered ceramic capacitors.

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• Dielectric properties
• Phase transitions
• Structural properties
• Dielectrics
• Ferroelectrics
• Paraelectrics
• Polycrystalline materials
• Smart materials
• Dielectric spectroscopy
• Raman spectroscopy
• X-ray diffraction