If you only remember one sentence: zinc selenide is a 1:1 II–VI semiconductor, cubic zincblende, grown polycrystalline by CVD. The amber color is the band edge. Everything else on the RFQ is a consequence of that lattice.
We get drawings that say “ZnSe” the way a machine shop says “aluminum.” That is not how this material behaves. The formula is ZnSe — one zinc atom per selenium atom — in the sphalerite (zincblende) structure, space group F-43m. Each Zn sits in a tetrahedron of four Se, and each Se in a tetrahedron of four Zn. The cubic lattice constant is about 5.667 Å. Formula weight 144.35 g/mol. Density we quote at 5.27 g/cm³. If your CAD volume times 5.27 g/cm³ does not match the mass on the packing list within a few percent, something is wrong with the blank, not with the periodic table.
Why it looks like honey
The room-temperature bandgap is about 2.67 eV. Photons bluer than that get absorbed; the rest of the visible leaks through as yellow-amber. That is not a dye and it is not “impurity color” in a healthy boule. It is the crystal telling you it is a semiconductor, not a glass. The same edge is why a CO2 resonator can carry a 633 nm HeNe alignment beam through the same ZnSe window that is doing 10.6 µm. Germanium cannot do that. Silicon cannot do that. That visible path is half the reason this crystal still owns high-power CO2 beam delivery.
Numbers we actually use
| Property | Typical value | Why it shows up on a print |
|---|---|---|
| Useful transmission | ~0.6–16 µm (bulk farther; coatings and scatter cut the ends) | Window vs. Ge / Si / ZnS choice |
| Refractive index at 10.6 µm | ~2.403 | AR design, Fresnel loss ~17% per uncoated face |
| Absorption (laser grade, 10.6 µm) | aimed at the 10−4 cm−1 class | Whether the optic survives kilowatts |
| Thermal conductivity | ~18 W/m·K | Lens heating, focus shift |
| dn/dT | ~+61 × 10−6 /K | Thermal lensing in CO2 |
| CTE | ~7.1 × 10−6 /K | Mounts, solder, epoxy |
| Knoop hardness | ~105–120 | Why we shout about gloves |
| Young’s modulus | ~67 GPa | Deflection of thin windows |
CVD ZnSe is polycrystalline. Grain size and grain-boundary chemistry dominate scatter and, in the wrong boule, 10.6 µm absorption. We do not sell research-grade single-crystal Bridgman ZnSe for quantum-well work. If you need that, you are not on this RFQ.
Next in this series: the zinc half of the lattice, then selenium and H2Se, then how the Gwangju reactor actually grows the plate you core.