Ba₆CoNb₉O₃₀ and Ba₆FeNb₉O₃₀: Two New Tungsten-Bronze-Type Ferroelectrics. Centrosymmetry of Ba_5.2K_0.8U_2.4Nb_7.6O₃₀ at 300 K

Ba₆CoNb₉O₃₀ and Ba₆FeNb₉O₃₀ in space group P4bm are shown to satisfy the structural criteria for ferroelectricity. Ba₆CoNb₉O₃₀ undergoes a diffuse phase transition at 660 (11) K, as observed calorimetrically, in addition to a dielectric permittivity anomaly with an onset temperature of 685 (10) K. The demonstration of dielectric hysteresis at room temperature under the application of a varied DC field reaching a maximum of ± 300 kV m^-1, corresponding to a spontaneous polarization of 1.2 (5) × 10^-2C m^-2, provides unambiguous verification that it is a new ferroelectric. Ba₆FeNb₉O₃₀ also undergoes a diffuse phase transition at 605 (16) K, with a dielectric anomaly at 583 (5) K, and exhibits dielectric hysteresis at room temperature under a varied DC field ranging to ± 310 kV m^-1 corresponding to a spontaneous polarization of 2.2(5) × 10^-2Cm^-2; it too is demonstrably a new ferroelectric. Although Ba_5.2K_0.8U_2.4Nb_7.6O₃₀ has also been reported in space group P4bm, all atomic displacements from the corresponding centrosymmetric positions are less than their refined root-mean-square thermal or static amplitudes. Such an arrangement is likely to be thermodynamically unstable. Either its space group has been incorrectly assigned, and reinvestigation will show the space group is P4/mbm, or the structural refinement is incomplete.