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Crystal Selection for Transmission Sampling

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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