  Neutron pair distribution function analysis and first principles calculations
have been employed to study short-range correlations in heavily disordered
dielectric material Sr$_x$Ba$_{1-x}$Nb$_2$O$_6$ ($x=0.35, 0.5$ and 0.61). The
combination of methods has been fruitful in pinpointing main local-structure
features, their temperature behaviour and interrelation. A rather complex
system of tilts is found to be both temperature and Sr-content sensitive with
the biggest tilt magnitudes reached at low temperatures and high $x$. Relative
Nb-O$_6$ displacements, directly responsible for material's ferroelectric
properties, are shown to be distinct in two octahedra sub-systems with
different freezing temperatures and disparate levels of deviation from
macroscopic polarization direction. Intrinsic disorder caused by Sr, Ba and
vacancy distribution is found to introduce local strain to the structure and
directly influence octahedra tilting. These findings establish a new atomistic
picture of the local structure -- property relationship in
Sr$_x$Ba$_{1-x}$Nb$_2$O$_6$.
