Novel high dielectric constant and low-loss tetragonal tungsten bronze dielectrics Ba5LnMgNb9O30 (Ln=La, Nd, Sm, Gd and Yb)

Five novel dielectric ceramics Ba5LnMgNb9O30 (Ln=La, Nd, Sm, Gd and Yb) were synthesized using a conventional solid-state reaction method. The phase structure, microstructure and dielectric properties of the prepared ceramics were investigated. Results show that the ceramics exhibit tetragonal tungsten bronze structure with fully occupied A-sites. Only tungsten bronze structure can be observed in the ceramics of which Ln refers to La, Nd, Sm and Gd, while tungsten bronze structure and YbNbO4 were found in the ceramic of which Ln refers to Yb. All the ceramics exhibit high dielectric constant (εr=272~429), with the ceramics of which Ln refers to La, Nd, Sm and Yb possessing low dielectric loss with the order of 10−3 at 1 MHz. By decreasing the rare-earth ionic radius, the transition temperature increases from −114 °C to 45 °C and then decreases to −58 °C, while the maximum dielectric constant increases to 460 at Ln=Sm and subsequently decreases. The phase transition is a diffuse phase transition (DPT) in the ceramics, while the diffuse exponent γ increases gradually as the ionic radius of rare earth element decreases from that of La3+ to that of Gd3+, and decreases thereafter. The temperature coefficients of dielectric constant of the ceramics with Ln=La, Nd, Sm and Yb are −1229 ppm/°C, −1542 ppm/°C, −1171 ppm/°C and −1067 ppm/°C respectively in the range of 20–80 °C at 1 MHz.