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 (Tm)∼-80 °C] and BBTNO15 (Tm∼-100 °C) exhibited relaxorlike dielectric behavior. In contrast, BNTNO15 (Tm∼165 °C), BSTNO15 (Tm∼250 °C), and BGTNO15 (Tm∼320 °C) were classic ferroelectrics. Tm 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≈12.4 Å and c≈4 Å. However, electron diffraction revealed that the relaxor phases, BLTNO15 and BBTNO15, exhibited an incommensurate modulation, whereas the classic ferroelectric BNTNO15, BSTNO15, and BGTNO15 featured an orthorhombic superstructure with lattice parameters a≈b≈[Square_Root]2aTTB and c≈2cTTB. [ABSTRACT FROM AUTHOR]