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, K1.83Ba4.17Nb12.18O36, 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 R w = 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 K1.83Ba4.17Nb12.18O36 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.