Dielectric properties 

The dielectric constant of a medium is a measure of the electrostatic polarization, that reduces the forces among charges. Two distinct techniques contribute to the dielectric properties of a solid according to the time-scale involved. The static dielectric constant relies on the displacement of ions from their regular positions in an applied electric field. It is appropriate for frequencies or static fields of electromagnetic radiation up into the the range of microwave. The high-frequency dielectric constant is calculated at frequencies faster than the vibrational motion of ions. It is appropriate in the visible region of the spectrum, and determines the refractive index, that governs the transmission of light in transparent media.

As supposed, ionic substances have higher static dielectric constants than nonionic ones. Particularly large values arise when ions can be simply displaced from their positions in the regular structure. For instance, barium titanate BaTiO₃ has a very high dielectric constant that changes according to the temperature. In the perovksite structure the large Ba²⁺ ion imposes a comparatively large O-O distance so that Ti⁴⁺ can move simply out of the center of its octahedral site. Below 120°C a permanent distortion sets in, that provides each unit cell a dipole moment. The behaviour of this type is called ferroelectric and has significant applications, for instance, in capacitors for electronic circuits.

Large high-frequency dielectric constants (and hence refractive indices) depend not on ionic motion but on electronic polarizability. Large ions contribute towards this, and glasses containing Pb²⁺ are traditionally used for lenses where a high refractive index is essential. Electronic polarizability can be large in compounds with small bandgaps. A gap outside the visible spectrum is essential for a colorless material in optical applications. TiO₂ is employed as a white pigment since it has the right optical properties combined with cheapness, non-toxicity and chemical stability. The bandgap is only now in the UV and the refractive index in the visible spectrum is high. Every grain is highly reflective and a powdered sample appears white since light is reflected in random directions.