A study of the Li 2 O-Bi 2 O 3-TiO 2 ternary system evidenced a Bi 2 Ti 2 O 7 pyrochlore phase in a large range of composition, centered on (Li 1/2 Bi 1/2)TiO 3 (LBT). This composition led to a pyrochlore phase stable up to 1100°C, while pure Bi 2 Ti 2 O 7 phase has been reported as metastable, with decomposition temperature around 600°C. According to previous studies, this pyrochlore phase was not obtained without lithium and here only a few percent of lithium was necessary for stabilization. The structural study, performed by neutron diffraction, evidenced a cubic Fd3 m structure (a = 10.317 Å), similar to the one of "pure" Bi 2 Ti 2 O 7 pyrochlore phase. The Li-ion is probably located on the Bi-site and is responsible for the stabilization of the pyrochlore structure. The electrical properties of the LBT composition were studied and appeared as very similar to the one of "pure" Bi 2 Ti 2 O 7. Because of its large thermal stability range, this pyrochlore phase appears to be a promising material for dielectric or tunability applications.

The structure of NBZ was studied by neutron diffraction. NBZ is monoclinic, (SG #11 P21/m) am = 5.696(8) Å, bm = 8.166(9) Å, cm = 5.790(4) Å and βm = 89.97(4)°. Being centrosymmetric, NBZ is neither piezoelectric nor ferroelectric. No phase transition was evidenced between 100 K and 923 K. NBZ presents a relaxor behavior below room temperature attributed to the mixed occupancy of the A-site by both Na + and Bi 3+ ions. The relative permittivity of NBZ appears noticeably stable (for 1 kHz-2.9% at-55 °C / 218K and +5.3% at 85 °C / 358 K compared to the 25 °C / 298 K value). NBZ appears thus as promising for the elaboration of capacitors requiring high thermal stability.

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