Preparation and crystal structure of H-BaTa2O6-type K1.83Ba4.17Nb12.18O36 and dielectric properties of the related compounds

Single crystal of a novel compound, K_1.83Ba_4.17Nb_12.18O₃₆, has been synthesized in the course of investigation on the K₂O-BaO-Nb₂O₅ system. The crystal structure was determined by single crystal X-ray diffraction data. The space group of this compound was found to be P6/mmm (#191) with the lattice parameters of a = 21.109(4) and c = 3.967(1) Å. The final R-factors were R = 0.039 and Rw=0.042 for unique 508 reflections. The crystal structure had the same tunnel structure as that of hexagonal BaTa₂O₆ (H-BaTa₂O₆), which is the high temperature form in three modifications of BaTa₂O₆. The chemical composition of K_1.83Ba_4.17Nb_12.18O₃₆ was close to that of the tetragonal tungsten bronze (TTB) type KBa₂Nb₅O₁₅ and the powder samples within this composition were prepared so as to clarify the boundary between H-BaTa₂O₆ and TTB-type structures. The H-BaTa₂O₆-type structure appears in (K + Ba)/Nb ≤ 0.500 and the TTB-type structure is in (K + Ba)/Nb ≥ 0.575. The dielectric constants of these samples were measured from room temperature to 500 °C for sintered body. The TTB-type compounds exhibited ferroelectric temperature dependence with the Curie points of 284-372 °C and the H-BaTa₂O₆-type compounds were not ferroelectrics as predicted from the crystal structure analysis.