Effect of external electric field on crystalline structure and dielectric properties of Bi_(1.5)MgNb_(1.5)O_7 thin films

Bismuth-based cubic pyrochlore materials have attractive dielectric properties, especially dielectric tunability. The Bi_1.5MgNb_1.5O₇ ceramic samples were prepared by solid state reaction. The XRD results and SEM pictures prove the raw material are well mixed and co-fired and the BMN cubic pyrochlore is well crystallized, no second phase was found in the result. BMN thin film were fabricated by depositing BMN ceramic nanoparticles on the sapphire. The BMN thin film has a high dielectric tunability of 43% at a bias voltage of 1.5 MV/cm, with loss tangent lower than 0.009. A Raman study of BMN cubic pyrochlore reveals O'-A-O' and O-A-O bending modes contribute to 80% of dielectric permittivity, obstructing these modes such as applying external electric field can have apparent influence on dielectric constant. Berry Phase calculation results shows that A₂O' tetrahedrons are more easy to distort under an external field. The A-site Mg have the highest displacement (0.765028 Å), followed by A-site Bi cations (0.346317 Å). Compared to zero-bias thin film, the biased one with A-O and A-O' bonds being stretched and external coulomb force applied on cations and anions, the dielectric constant under bias field dramatically decreased.