Abstract
The new phases Ba 2La MNb 4O 15: 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. Ba 2LaFeNb 4O 15 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 10 5 Hz. Solid solutions between these new phases and the compositionally and structurally related relaxor ferroelectric Ba 2LaTi 2Nb 3O 15 show gradual loss of ferroelectric behaviour attributed to replacement of polarisable Ti 4+ by a mixture of (Mn, Fe) 3+ and Nb 5+.
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