For an infinite ionic crystal, the relation ∊(0)/∊(∞) = ω_L²/ω_T², where ∊(0) is the crystal's static dielectric constant, ∊(∞) is the dielectric constant at a frequency at which electronic polarizability is effective but ionic polarizability is not, ω_L is the frequency of longitudinal optical phonons with zero wave vectors, and ω_T is the frequency of transverse optical phonons with large wave vector.