Dielectric and structural studies of Ba2MTi2Nb3O15 (BMTNO15, M=Bi3+,La3+,Nd3+,Sm3+,Gd3+) tetragonal tungsten bronze-structured ceramics

The structure and dielectric properties of a new family of tetragonal tungsten bronze (TTB) ceramics with the general formula, Ba2MTi2Nb3O15 where M=Bi3+,La3+,Nd3+,Sm3+,Gd3+, have been investigated. Hereafter the compositions will be referred to by the abbreviation BMTNO15 where M=B(Bi3+), L(La3+), N(Nd3+), S(Sm3+) or G(Gd3+). BLTNO15 [permittivity maximum (T-m)similar to-80 degrees C] and BBTNO15 (T-m similar to-100 degrees C) exhibited relaxorlike dielectric behavior. In contrast, BNTNO15 (T-m similar to 165 degrees C), BSTNO15 (T-m similar to 250 degrees C), and BGTNO(15) (T-m similar to 320 degrees C) were classic ferroelectrics. T-m increased with decreasing radius of the M ion. Room temperature x-ray powder diffraction (XRD) patterns of all the compounds indexed on a prototype P4/mbm (or P4bm) space group with lattice parameters a approximate to 12.4 angstrom and c approximate to 4 angstrom. However, electron diffraction revealed that the relaxor phases, BLTNO15 and BBTNO15, exhibited an incommensurate modulation, whereas the classic ferroelectric BNTNO15, BSTNO15, and BGTNO(15) featured an orthorhombic superstructure with lattice parameters a approximate to b approximate to root 2a(TTB) and c approximate to 2c(TTB). (c) 2007 American Institute of Physics.