Among the family of temperature-compensated microwave dielectric ceramics, BaMg1/3Ta2/3O3 shows the lowest dielectric loss and remains a material of choice for state-of-the-art airborne and land-based communication systems. We report on the compositional stability range, microwave dielectric properties, and the degree of atomic order of the title compound within the BaO–MgO–Ta2O5 ternary diagram. In most cases an atomic order is robust to the deviation from stoichiometry with an exception of Ba-rich and/or Ta-deficient samples which favor (partial) disorder. We further demonstrate that the dense, atomically ordered BaMg1/3Ta2/3O3 ceramic shows large variation of dielectric loss within a single phase composition region – a clear message that the dielectric loss in practical ceramics is dominated by extrinsic sources and that the cation order alone is insufficient to achieve a minimum dielectric loss in BaMg1/3Ta2/3O3. The low-temperature dielectric relaxation studies suggest that the extrinsic dielectric loss in the title compound is due to the ‘rattling’ of the off-centered Mg2+ ions misplaced at the Ba sites. Controlled deviation from the BaMg1/3Ta2/3O3 stoichiometry toward the Mg-deficient region leads to suppression of the extrinsic dielectric loss as a result of the reduced chemical activity of Mg ion.

We report on structural, dielectric, magnetic and Raman data of the title compounds. Analysis of the single crystal diffraction data reveals that the ground state of the title compounds can be characterized within the Cm space group. The Ba4MTa10O30 compounds show typical features of incipient ferroelectrics, i.e., their dielectric constant gradually increases on cooling and levels off or slightly decreases below 10 K. Nb solubility limit, x, for Ba4MNbxTa10–xO30 compositions equilibrated at 1450–1600 °C was determined by powder XRD and by electron probe microanalysis. Substitution of Nb for Ta leads to the low-temperature dielectric anomaly having a relaxor-type signature. The Co- and Ni-based compounds show paramagnetic behavior in the 2–300 K range with no indication of the magnetic long-range order.Keywords: dielectric loss; incipient ferroelectrics; paramagnetism;

We report on the phase relation in the BaO–MgO–Ta2O5 system equilibrated at 1450 °C. We found three ternary phases, namely 3:1:1 perovskite Ba3MgTa2O9, 9:1:7 TTB (tetragonal tungsten bronze)-type Ba9MgTa14O45 and 4:1:5 Ba4MgTa10O30 closely related to the tungsten bronze structure. Another ternary phase, Ba10Mg0.25Ta7.9O30, with a 10 layer hexagonal perovskite structure can be only obtained after equilibration at 1600 °C. MgO solubility in the TTB-like Ba4Ta10O29, BaTa4O11 and Ba6.63Ta34.95O95 does not exceed 2.0 mol%. Only the hexagonal polymorph of the BaTa2O6 phase was found along the Mg4Ta2O9–BaTa2O6 join. Measurements of dielectric properties in the 20 Hz–2 MHz range revealed that Ba9MgTa14O45 has a dielectric constant of 99 with no frequency dispersion and low dielectric loss. We have also clarified several contradictory reports related to the ternary and binary phases in the BaO–MgO–Ta2O5 composition field.

We report on the phase relation in the BaO–MgO–Ta2O5 system equilibrated at 1450 °C. We found three ternary phases, namely 3:1:1 perovskite Ba3MgTa2O9, 9:1:7 TTB (tetragonal tungsten bronze)-type Ba9MgTa14O45 and 4:1:5 Ba4MgTa10O30 closely related to the tungsten bronze structure. Another ternary phase, Ba10Mg0.25Ta7.9O30, with a 10 layer hexagonal perovskite structure can be only obtained after equilibration at 1600 °C. MgO solubility in the TTB-like Ba4Ta10O29, BaTa4O11 and Ba6.63Ta34.95O95 does not exceed 2.0 mol%. Only the hexagonal polymorph of the BaTa2O6 phase was found along the Mg4Ta2O9–BaTa2O6 join. Measurements of dielectric properties in the 20 Hz–2 MHz range revealed that Ba9MgTa14O45 has a dielectric constant of 99 with no frequency dispersion and low dielectric loss. We have also clarified several contradictory reports related to the ternary and binary phases in the BaO–MgO–Ta2O5 composition field.