Silicon( Si) is a good IR material , it can be used from 1.2 to 7 microns, its peak transmission range is
in 3-5 micron. Silicon can be used in some circumstances where weight is a concern as that the density of Silicon is similar to optical glass. Silicon material is widely used in security and military applications.  

• Transmission Range: 1200 nm~7000 nm
• hermal Expansion Coefficient: 2.55x10^-6/K
• Density: 2.33g/cm³