Ba6CoNb9O30 and Ba6FeNb9O30: Two new tungsten-bronze-type ferroelectrics. Centrosymmetry of Ba5.2K0.8U2.4Nb7.6O30 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), correspondign to a spontaneous polarization of 1.2(5) x 10(-2)C 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) x 10(-2) C m(-2); it too is demonstrably a new ferroelectric. Although Ba5.2K0.8U2.4Nb7.6O30 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.