In this reactor zinc is the metal source, not a dopant. CRC melting point 419.53 °C, boiling point 907 °C. We do not melt ZnSe. We raise the metal until the vapor pressure is useful, meet it with H2Se on a hot mandrel, and the solid that sticks is the plate you later core.
Miles et al. wrote the reaction the industry still uses (DTIC ADA042147, 1977): zinc vapor is generated by heating the metal under reduced pressure and carrying it into the deposition zone. Hydrogen selenide meets it at the mandrel. Typical laser-quality work in that report sat near 750 °C. KM’s 2023.06 plant introduction lists Se 99.999% and H2 99.995% as the hydride feedstock; the zinc charge is the other half of the stoichiometry. We do not publish a “6N zinc” line on the quote unless your incoming lab actually assays the metal. What we publish is the absorption of the plate.
Why the cation is zinc, not cadmium
Swap zinc for cadmium and you have CdSe — different gap, different color, different toxicity rules, and not a CO2 window. Swap selenium for sulfur and you have CVD ZnS, which was industrialized in parallel with ZnSe in the mid-1970s (see Harris, SPIE, chapter on CVD ZnS/ZnSe). This USA desk quotes CVD ZnSe only. A ZnS RFQ is a different mill.
Impurity in the zinc charge partitions into the deposit. Transition metals and oxygen are the usual suspects when 10.6 µm absorption creeps up. At that wavelength extra absorption is heat. Heat plus dn/dT of +61×10−6 K−1 (published ZnSe data at 10.6 µm) is thermal lensing. Specify laser grade or optical grade, the wavelength, the size, and the absorption or scatter you will reject. We pick the plate from that, not from a purity slogan on a zinc ingot certificate.
The metal is also why the optic is soft
II–VI crystals are not silicates. Published Knoop for CVD ZnSe is 105–120 kg/mm². Published Knoop is 120 with a 50 g indenter. The 1977 DTIC table listed 100. Harris (SPIE, 1999) treats CVD ZnSe with the soft IR crystals — closer to a salt than to BK7. A fingerprint on an uncoated face is a generating tool. Steel tweezers leave a record. Gloves. No dry paper. I put the handling numbers and the NIOSH selenium limits in the SDS note.
Density of the finished plate is still 5.27 g/cm³. That is zinc plus selenium in the zincblende cell, not a coating. CAD volume × 5.27 should match the weighed blank. If it does not, stop before you load the spindle.
References. CRC Handbook of Chemistry and Physics (Zn melting 419.53 °C, boiling 907 °C). Miles, P. et al., DTIC ADA042147 (1977), zinc-vapor + H2Se CVD. Harris, D.C., Materials for Infrared Windows and Domes, SPIE (1999), ch. 6 on CVD ZnS/ZnSe. KM Innovation ZnSe datasheet and 2023.06 plant introduction (feedstock, grade on the quote).