  Dielectric permittivity was studied in ceramics of relaxor ferroelectric
bismuth-sodium titanate Na$_{0.5}$Bi$_{0.5}$TiO$_{3}$. The measurements were
performed on as sintered and heat treated in vacuum samples. The diffuse
dielectric anomalies associated with the structural phase transitions were
observed in as sintered samples. The intense peak of permittivity ($
\varepsilon_{\text{max}} \sim 10^{4}$) appeared after heat treating in vacuum.
The anomaly of $\varepsilon(T)$ was contributed by slow polarization processes
($f<10$ kHz) and was non-stable, vanishing on heating in air up to $\sim 800$
K. Temperature and frequency dependencies of $\varepsilon$ were described by
using Cole-Cole model with accounting thermally stimulated decay of the
non-stable polarization. It is supposed that the dielectric anomaly is
determined by space charge polarization mechanism. Oxygen vacancies
V$_{\rm{O}}^{\bullet \bullet}$ and electrons localized on titanium ions
Ti$'_{\rm{Ti}}$
  are assumed to be responsible for the phenomenon observed.
