Structural characterization and frequency response of sol–gel derived Bi_3/2MgNb_3/2O₇ thin films

The Bi_3/2MgNb_3/2O₇ (BMN) thin films were prepared via a modified sol–gel process on glass substrates at various post-annealing temperatures. The crystalline structure, morphology and frequency response have been investigated systematically. The X-ray diffraction results indicated that the BMN thin films had different orientations depending on post-annealing temperature. Thin films annealed above 650 °C presented well crystallized cubic pyrochlore structure with (222) orientation, and (400) preferentially oriented were observed when they were annealed below 600 °C. The surface morphology images of the BMN thin films revealed different grain size and grain size distribution, and the average grain size increased from 28.3 to 37.0 nm as the post-annealing temperature increasing. The low frequency dielectric properties of the BMN thin films were closely correlated with the (222) orientation, which was favorable to enhanced dielectric constant and tunability. The high-frequency optical measurements revealed an average transmittance (T _av ) varying between 76.6 and 82.2 % and band gap energy (E _g ) ranging from 3.40 to 3.44 as a function of the temperature and the crystallite size. Thin film annealed at 700 °C possessed the best crystallinity and highest (222) orientation, and showed the best electrical properties, with a dielectric constant of 105 at 1 MHz, dielectric tunability of 25.8 %, and an average optical transmittance of 82.2 % in the visible range (400–800 nm), making it promising for optical/electronic tunable devices applications.