BA6CONB9O30 AND BA6FENB9O30 - 2 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 t he structural criteria for ferroelectricity. Ba6CoNb9O30 undergoes a d iffuse phase transition at 660 (11) K, as observed calorimetrically, i n addition to a dielectric permittivity anomaly with an onset temperat ure of 685 (10) K. The demonstration of dielectric hysteresis at room temperature under the application of a varied DC field reaching a maxi mum of +/- 300 kV m(-1), correspondign to a spontaneous polarization o f 1.2(5) x 10(-2)C m(-2), provides unambiguous Verification that it is a new ferroelectric. Ba6FeNb9O30 also undergoes a diffuse phase trans ition at 605(16)K, with a dielectric anomaly at 583(5)K, and exhibits dielectric hysteresis at room temperature under a varied DC field rang ing 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. A lthough Ba5.2K0.8U2.4Nb7.6O30 has also been reported in space group P4 bm, all atomic displacements from the corresponding centrosymmetric po sitions are less than their refined root-mean-square thermal or static amplitudes. Such an arrangement is likely to be thermodynamically uns table. Either its space group has been incorrectly assigned, and reinv estigation will show the space group is P4/mbm, or the structural refi nement is incomplete.