Li_{4: X /3}Co_{2-2 x}Ti_{1+2 x /3}O₄ spinel solid solutions: Order and disorder phase transition, cations distribution and adjustable microwave dielectric properties

A series of spinel solid solutions Li_4x/3Co_2-2xTi_1+2x/3O₄ (LCT, 0.2 ≤ x ≤ 0.8) were synthesized and their structures and microwave dielectric properties were characterized in detail. The distribution of cations at A and B sites in the lattice has been analyzed by using Rietveld refinement. The microstructure and dielectric properties were studied by scanning electron microscope and microwave network analyzer. The solid solutions undergo a discontinuous B-site, Li/Ti order-disorder phase transition from a disordered cubic phase to an ordered cubic phase with increasing x values from 0.4 to 0.6. With increasing the A-site lithium content, the LCT ceramics exhibit improved dielectric permittivity ϵ_r (20.3-26.5), high Q × f value (≥29400 GHz) and a tailored temperature coefficient of resonant frequency τ_f from -40 to 10 ppm per °C. Optimized microwave dielectric properties were achieved for the composition with x = 0.8: ϵ_r = 26.5, Q × f ≈ 29400 GHz and τ_f ≈ 10 ppm per °C. A correlation between the cation distribution and microwave dielectric loss is discussed in detail.