BCZYx samples were prepared by a modified version of
citrate-nitrate combustion synthesis with subsequent
decomposition of organometallic complexes at high temperatures.
Prior to the synthesis, the starting materials (Ba(NO3)2,
Ce(NO3)3$6H2O, Y(NO3)3$6H2O, CuO, Co3O4 and citric acid, with
purity no less than 99.5%), were thoroughly mixed and dissolved
afterward in a solution containing stoichiometric required
amounts of zirconium oxynitrate and glycerin. Citric acid and
glycerin were chosen as chelating and complexing agents with
the appropriate molar ratio to metal nitrates (0.5:1.5:1, respectively).
The as-obtained solutions were heated and stirred at
100 C for 0.5 h with subsequent addition of ammonium hydroxide
to ensure pH at the level of 8e9. The final solution was
again heated at 250 C to evaporate the excess water and assist in
foaming of a viscous gel, which was eventually autoignited. The
obtained gray (for low Zr content) or black (for high Zr content)
powders were subsequently fired at 700 C (1 h) in air to remove
organic residue, and then the light-colored ones were calcined at
1150 C (5 h) to form single-phase products. The powders were
milled in a planetary mill (Fritsch Pulverisette 6) for 1 h at
250 rpm, uniaxially pressed into the pellets under 4 ton cm2
and then sintered at 1450 C for 5 h.