Pair distribution function analysis of neutron-scattering data and of ab initio molecular dynamics results have been employed to study short-range structural correlations and their temperature dependence in a heavily disordered dielectric material ${\mathrm{Sr}}_x{\mathrm{Ba}}_{1-x}{\mathrm{Nb}}_2{\mathrm{O}}_6(x=0.35,0.5$, and 0.61). Intrinsic disorder caused by a partial occupation of the cationic sites by differently sized Sr and Ba atoms and their vacancies introduces important local strains to the structure and directly influences the Nb-${\mathrm{O}}_6$ octahedra tilting. The resulting complex system of tilts is found to be both temperature and Sr-doping sensitive with the biggest tilt magnitudes reached at low temperatures and high strontium contents, where ferroelectric relaxor behavior appears. We find evidence for two Nb-${\mathrm{O}}_6$ subsystems with different variations of niobium-oxygen bond lengths, distinct dynamics, and disparate levels of deviation from macroscopic polarization direction. These findings establish a detailed picture of the local structure of ${\mathrm{Sr}}_x{\mathrm{Ba}}_{1-x}{\mathrm{Nb}}_2{\mathrm{O}}_6$ and provide a deeper insight into the origins of the materials dielectric properties.

• Received 22 November 2018
• Revised 11 February 2019

DOI:https://doi.org/10.1103/PhysRevB.99.104102