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The following are several criteria necessary to consider when selecting a crystal material for a specific application:
Chemical and Physical Properties – for obvious reasons, the crystal must be chemically and physically compatible with the sample. Some crystal materials may dissolve or react with samples. Physical considerations are equally important as some crystals are more susceptible to scratching or indentation from hard samples.
Spectral Range – all crystals have different spectral ranges for transmission sampling. Specifically, in mid-IR the cutoff at low wavenumbers varies from approximately 896 cm-1 for CaF2 to 4 cm-1 for polyethylene. To a certain extent, the cutoff values are also affected by the thickness of the crystal. Useful spectral ranges of the most popular crystal materials are listed in the table below.
| Properties of Select Infrared Transmitting Materials for Transmission Spectroscopy | ||||||||
|---|---|---|---|---|---|---|---|---|
| Material | Comments | SWL cm-1 | LWL cm-1 | RI | Solubility g/100g | Hardness kg/mm2 | MP °C |
pH Range |
| AMTIR | SeAsGe glass, brittle | 11000 | 593 | 2.5 | 0 | 170 | 370 | 1-9 |
| BaF2 | Barium Fluoride | 66600 | 691 | 1.45 | 0.17 | 82 | 1280 | 5-8 |
| Ca F2 | Calcium Fluoride | 79500 | 896 | 1.4 | 0.0017 | 158 | 1360 | 5-8 |
| CsI | Cesium Iodide, very hygroscopic, Somewhat Toxic | 42000 | 172 | 1.73 | 44 | 20 | 621 | NA |
| Diamond | Type IIa, strong IR absorbance between 2700-1800 cm-1, costly | 30000 | <2 | 2.4 | 0 | 5700 | 550 fp | 1-14 |
| Ge | Germanium, brittle, becomes opaque at elevated temperatures | 5500 | 432 | 4 | 0 | 780 | 936 | 1-14 |
| KBr | Potassium Bromide, most widely used for mid-IR applications | 48800 | 345 | 1.52 | 53 | 6 | 730 | NA |
| KCl | Potassium Chloride | 55600 | 385 | 1.45 | 35 | 7 | 776 | NA |
| KRS-5 | Thallium Bromide / Thallium Iodide, Extremely Toxic! | 17900 | 204 | 2.37 | 0.05 | 40 | 414 | 5-8 |
| NaCl | Sodium Chloride | 52600 | 457 | 1.49 | 36 | 18 | 801 | NA |
| Polyethylene | For Far-IR, swells with some organic solvents | 625 | <4 | 1.52 | 0 | 110 | 1.5-14 | |
| SiO2 | Silicon Dioxide | 50000 | 2315 | 1.53 | 0 | 460 | 1713 | 1-14 |
| Si | Silicon, strong IR absorbance between 624-590 cm-1 |
8900 | 624,30 | 3.41 | 0 | 1150 | 1420 | 1-12 |
| ZnS | Zinc Sulfide | 17000 | 690 | 2.2 | 0 | 240 | 1830 | 5-9 |
| ZnSe | Zinc Selenide | 15000 | 461 | 2.4 | 0 | 120 | 1526 | 5-9 |
| Notes: The above table is meant to be a general guide – brief and concise. For more information about these materials, consult appropriate reference books and Material Safety Data Sheets (MSDS). | ||||||||
| SWL – Shortest wavelength for transmission, 1mm, 50% transmission | ||||||||
| LWL – Longest wavelength for transmission, 1mm, 50% transmission | ||||||||
| RI – Refractive Index, at relevant wavelength | ||||||||
| MP – Melting point | ||||||||
| pH – negative log of hydrogen ion concentration | ||||||||