New high permittivity tetragonal tungsten bronze dielectrics Ba2LaMNb4O15: M=Mn, Fe

The new phases Ba2LaMNb4O15: M=Mn, Fe were prepared by solid state reaction at 1100 °C. They have the tetragonal tungsten bronze structure, space group P4/mbm, at room temperature. The two octahedral sites show partial order of M and Nb with preferential occupancy of the smaller B(1) sites by M. Both phases have high permittivities 90±15 over the range 10–320 K. Ba2LaFeNb4O15 is highly insulating with bulk conductivity ⪡10−8 ohm−1 cm−1 at 25 °C and tan δ⪡0.001 over the range 100–320 K and at 105 Hz. Solid solutions between these new phases and the compositionally and structurally related relaxor ferroelectric Ba2LaTi2Nb3O15 show gradual loss of ferroelectric behaviour attributed to replacement of polarisable Ti4+ by a mixture of (Mn, Fe)3+ and Nb5+.