  We report on the synthesis of complex lead-free perovskite-type
(1-x)(Ba$_{0.8}$Ca$_{0.2}$)TiO$_3$-xBi(Mg$_{0.75}$W$_{0.25}$)O$_3$ (BCT-xBMW)
and (1-x)(Ba$_{0.8}$Ca$_{0.2}$)TiO$_3$-xBi(Mg$_{0.75}$Mo$_{0.25}$)O$_3$
(BCT-xBMM) solid solutions via conventional solid-state reaction route. The
sintering temperature was adjusted as a function of composition x to obtain
dense samples (relative densities over 95%) at the same time minimizing bismuth
evaporation. X-ray diffraction analysis shows formation of single-phase
perovskites for $0 \le x \le 0.10$ in the BCT-xBMW series and increasing
concentrations of impurity phases for $x \ge 0.15$ and for $x \ge 0.05$ in
BCT-xBMM. A transition from a tetragonal to pseudo-cubic perovskite structure
is observed in BCT-xBMW and BCT-xBMM at $x = 0.05$. The dielectric response has
been characterized between -60 $^\circ$C and 300 $^\circ$C for BCT-xBMW, and
between 30 $^\circ$C and 300 $^\circ$C for BCT-xBMM using impedance
spectroscopy, showing a transition from ferroelectric to relaxor-like behavior
at $x \ge 0.05$. Additional polarization and Raman spectroscopy measurements
reveal the occurrence of highly disordered systems. Analysis of the Raman
spectra indicates structural phase changes and lattice modifications caused by
chemical substitution. For the composition
0.8Ba$_{0.8}$Ca$_{0.2}$TiO$_3$-0.2Bi(Mg$_{0.75}$W$_{0.25}$)O$_3$ a
temperature-stable permittivity of about 600 ($\pm 15$% between 60 $^\circ$C
and 300 $^\circ$C) and small losses of $\tan\delta < 0.02$ for $T \le 230$
$^\circ$C at 1 kHz are observed, making it a suitable dielectric material for
high temperature capacitors.
