Is there any kind of material which when placed between the plates of capacitor reduces its capacitance ?

Is there any kind of material that when inserted between the plates of a capacitor reduces its capacitance.?

A sheet of aluminium foil of negligible thickness is placed between the plates of a capacitor of capacitance C as shown in the figure then capacitance of capacitor becomes

When a conducting slab fills the space between the two plates of a capacitor, its capacitance

A dielectric slab is placed between the plates of a parallel plate capacitor. Its capacitance

A dielectric slab is placed between the plates of a parallel plate capacitor. Its capacitance.

When a sheet of dielectric is inserted between the two plates of a capacitor, its capacitance