Exploring the origin of zero thermal expansion in relaxors: A combined SXRD, Raman, and PDF study

Relaxor ferroelectrics are known for their structural complexity and intriguing physical properties. These materials can demonstrate negative thermal expansion (NTE)/zero thermal expansion (ZTE), offering scientific and technological advantages [1]. In the present work, distinct atomic orderings at various length scales (long/intermediate/short) have been determined in 0.90(Ba_0.9Sr_0.1TiO₃)−0.10(K_0.5Na_0.5NbO₃) (BKST10) ceramics using temperature-dependent Synchrotron X-ray diffraction (SXRD), Raman scattering, and pair distribution function (PDF) data. An average cubic structure (Space Group: Pm-3m) is found to be stable at long ranges for a wide temperature range (100 K ≤ T ≤ 500 K). Moreover, integrated analysis of temperature-dependent Raman scattering and PDF data reveals rhombohedral (Space Group: R3m) and monoclinic (Space Group: Pm) structures stable at intermediate and short ranges, respectively, throughout the studied temperature range. In addition, ZTE has been observed for BKST10 ceramics at low temperatures with a coefficient of thermal expansion ranging between 0.623-10.109 × 10^-6 K^-1 (100 K-500 K). The ZTE has been attributed to the enhanced correlations among PNRs at low temperatures [1], which have been confirmed using temperature-dependent P-E loops and amplitude of ferroelectric phonon mode (Γ₄^-).

[1]. A. Tripathi, A. Pandey, J. A. Alonso, R. Erasmus, M. T. Fernandez-Diaz, and S. Tripathi, Origin of zero thermal expansion in an average cubic structure in Pb-free relaxor ferroelectrics, Applied Physics Letters 125 (2024).