Iron sulfide is a chemical compound (different from a mixture of Iron and Sulfur, show by its different physical and chemical properties). It is made of iron and sulfide ions.
A dielectric material (dielectric for short) is an electrical insulator that can be polarized by an applied electric field. dielectric resonator oscillator (DRO) is an electronic component that exhibits resonance of the polarization response for a narrow range of frequencies, generally in the microwave band. It consists of a puck of ceramic that has a large dielectric constant and a low dissipation factor.
Electricity. the ratio of the flux density produced by an electric field in a given dielectric to the flux density produced by that field in a vacuum. Also called dielectric constant, relative permittivity, specific inductive capacity.