Abstract: Microwave dielectric ceramics are the key basic materials of 5G/6G communication technology, with particular emphasis on the materials that exhibit a high quality factor (Q×f), low dielectric constant (ε_r) and near-zero temperature coefficient of resonant frequency (τ_f). However, most low-ε_r materials tend to have a significantly negative τ_f value. This paper provides a systematic overview of the classical ionic polarizability dilution mechanism and phase transition mechanism, along with the structural factors affecting τ_f, such as unit cell volume mechanism, oxygen polyhedron distortion, bond energy and bond ionicity, and bond valence. Subsequently, the anomalous changes in τ_f in the cubic normal and inverse garnet system without phase transition are described in detail. The “Rattling” effect is introduced as a novel mechanism affecting the τ_f of microwave dielectric ceramics. Cations involved in “Rattling”, characterized by high coordination and weak chemical bonds, are the primary factors affecting the overall microwave dielectric polarization and loss of the material. This phenomenon results in an increase in ionic polarizability and ε_r, a forward shift in τ_f and a reduction in Q×f value, which has been verified and applied in many different material systems. Furthermore, the introduction of a weighted function for total ion polarization deviation serves to evaluate the impact of the entire molecule's “Rattling” and “Compressed” effects on ε_r. A novel concept of temperature coefficient of ionic polarizability τ_αm is also proposed, allowing for quantitatively calculation. This simplifies the factors that affect the positive and negative of dielectric constant temperature coefficient τ_ε, by relating it to ε_r, τ_αm and linear expansion coefficient α_L.

Key words: microwave dielectric ceramics, Rattling effect, τ_f influence mechanism, temperature coefficient of ionic polarizability, weighting function of the total ion polarizability deviation