Study On The A₂BB'O₆-type Microwave Dielectric Ceramics

A₂BB?O₆-type ceramics have exhibited promising applications in wireless communication technology owing to the diversified microwave dielectric properties.High-quality Mg₂NdNbO₆?Re=La,Nd,Sm?were prepared by solid-state method,and their sintering temperatures,crystal structures,micro-structures,microwave dielectric properties were investigated;first-principles calculations were performed to study the structural,elastic,electronic,and lattice dynamical properties of Ba₂InMO₆?M=Ta,Nb?under hydrostatic pressure.The main work and obtained results are listed as follows:?1?High-quality A₂BB?O₆-type microwave dielectric ceramics Mg₂ReNbO₆?Re=La,Nd,Sm?were prepared by conventional solid-state method,which were synthesized for the first time,and the mechanism of microwave dielectric properties had been systematically studied.The relationship between phases,micro-structures,microwave dielectric properties and sintering temperatures had been investigated.Based on the experimental results,it was found that Mg₂La NbO₆,Mg₂Nd NbO₆,Mg₂SmNbO₆ ceramics sintered at 1500°C,1375°C,1500°C,respectively,had promising excellent microwave dielectric properties:Q×f⁹5,000 GHz,?_r¹6.23,?_f².81 ppm/°C;Q×f¹16,000 GHz,?_r¹6.22,?_f^-30.96 ppm/°C;Q×f¹56,000 GHz,?_r¹6.03,?_f^-39.52 ppm/°C.?2?The structural,elastic,electronic,and lattice dynamical properties of double perovskite Ba₂In MO₆?M=Ta,Nb?under hydrostatic pressure were investigated using local density approximation?LDA?method.It was found that the pressure was an efficient way to adjust physical properties,and some functional relationship existed among them.As the pressure increased,the elastic constants C_ij,bulk modulus B_H,shear modulus G_H and Young modulus E_H exhibited a linear increasing trend;band-gap values exhibited a non-monotonous,monotonous pressure-induced trend,respectively;phonon frequencies of Raman?IR?-active modes at the zone center showed red?blue?shifts.When the pressure was beyond 102?11.5?GPa,Ba₂InTaO₆?Ba₂InNbO₆?was dynamically instable,which could be confirmed by the fact that the T_1gg silent modes would soften to zero frequency.