Preparation and crystal structure of H-BaTa 2O 6-type K 1.83Ba 4.17Nb 12.18O 36 and dielectric properties of the related compounds

Abstract

Single crystal of a novel compound, K 1.83Ba 4.17Nb 12.18O 36, has been synthesized in the course of investigation on the K 2O–BaO–Nb 2O 5 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 BaTa 2O 6 (H-BaTa 2O 6), which is the high temperature form in three modifications of BaTa 2O 6. The chemical composition of K 1.83Ba 4.17Nb 12.18O 36 was close to that of the tetragonal tungsten bronze (TTB) type KBa 2Nb 5O 15 and the powder samples within this composition were prepared so as to clarify the boundary between H-BaTa 2O 6 and TTB-type structures. The H-BaTa 2O 6-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-BaTa 2O 6-type compounds were not ferroelectrics as predicted from the crystal structure analysis.