Crystal structure and microwave dielectric properties of A- and B-site co-substituted NdNbO4 ceramics

The microwave dielectric properties of NdNbO4 (NN) ceramics were optimized by substituting Ca2+ at the A-site and W6+ at the B-site. (1-x)NdNbO4-xCaWO4 (NNCWx) (x = 0-0.3) ceramics crystallize in the monoclinic fer-gusonite structure (space group I2/a), whereas NNCW-0.4 exhibits a tetragonal scheelite structure (space group I41/a), suggesting that Ca2+/W6+ doping effectively contributed to the ferroelastic phase transition. A near-zero temperature coefficient of resonant frequency (tau f) of +7.8 ppm/degrees C, a high Qxf value of 59,190 GHz, and a low permittivity (epsilon r) of 17.51 were obtained for the NNCW-0.3 ceramic. With the increase of x, the permittivity decreases gradually, while tau f shows the opposite trend. The permittivity is mainly affected by the polarizability per unit molar volume (alpha/Vm) and the rattling effect of A-site cations (Nd3+ and Ca2+). The Qxf values of NNCWx ceramics are mainly affected by relative density, packing fraction, and lattice energy (U). The influencing mechanism of tau f is a combination of an increase in the temperature coefficient of ion polarizability (tau alpha m), lattice strain, and phase transition.