Structure and dielectric properties of yttrium-doped Ca_0.28Ba_0.72Nb₂O₆ ceramics.

An unfilled tungsten bronze-structured ferroelectric ceramic, Ca _0.28Ba _0.72Nb ₂O ₆ (CBN28), has been doped with Y ³⁺ to produce ceramics with a nominal composition, (Ca _0.28Ba _0.72) _1–3w/2Y _wNb ₂O ₆ [0 ≤ w ≤ 0.05]. The substitution of Y ³⁺ for Ca ²⁺/Ba ²⁺, and consequent additional vacancy formation, is assumed to occur on the A1/A2 sites. This resulted in a minor reduction of the c lattice parameter, and unit cell volume. For undoped CBN28, there was a slightly diffuse relative permittivity-temperature (ε _r-T) peak at 268 ⁰C. The peak became much broader for sample compositions w = 0.04 and 0.05 and the peak temperature showed a level of frequency dependence consistent with weak relaxor behaviour. The polarisation-electric field loops became narrower for samples w = 0.04 and 0.05, corresponding to a reduction in remnant polarisation value, from 2.4 to 0.8 µC cm ⁻² (30 kV cm ⁻¹). The Y doped ceramics exhibited stable relative permittivity over a wide temperature range, the variation being within± 15% of the median value from 36 ⁰C to 218 ⁰C for w = 0.05, when measured at 1 kHz. Consequently, we suggest that A site donor-doping and aliovalent B site doping of CBN holds potential for industry standard, temperature stable, high temperature dielectrics (ɛ _r ≥ 500 ± 15% from - 55–250 + ⁰C).