THORIUM FLUORIDE, ThF₄
FOR OPTICAL COATING

Introduction
Thorium fluoride, ThF₄, is the currently preferred low index material which is transparent from the UV to the IR. Evaporated films are durable and chemically stable and exhibit low stress in thicknesses up to 2 µm. Common applications are for AR coatings in the 8 to 12 µm region, including high power CO₂ laser coatings.

Thorium fluoride is a naturally radioactive material and must be handled and wastes disposed of in accordance with state and/or federal regulations. In spite of special precautionary measures needed with the use of ThF₄, it still is the premier coating material for lasers in the 8-12 µm region.

Deposition Suggestions
Thorium fluoride evaporates at 800°C. E-beam or resistance-heated sources can be used. When using E-beam, slow, gentle heating is advised. A molybdenum or tantalum hearth is suggested. The recommended substrate temperature is 175°C, with a chamber pressure below 10^-5 Torr. A deposition rate of 10 Å/sec. minimum is suggested. A greater rate is possible provided no spitting occurs.

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Optical Properties
Films at least 1000 nm thick are transparent from approximately 250 nm to less than 11 µm. The refractive index in the visible (550 nm) is 1.50. At 10 µm, the index is 1.35. Low substrate temperatures and low deposition rates can cause water absorption bands to appear near 3 µm and 6 µm in the IR.

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Applications in Multilayers
Thorium fluoride can be used as a component in multilayer coatings for AR, bandpass and dichroic filters. The low tensile stress of ThF₄ and its good adhesion make it compatible with ZnS, ZnSe, MgF₂ and other fluoride compounds. The microstructure of the deposited coating is amorphous, and the films are relatively soft.

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A Note on Thorium Oxyfluoride
ThOF₂ is a well characterized compound, along with ThF₄. However, it has been shown that ThOF₂ does not evaporate stoichiometrically, and all resulting films are ThF₄.^*

*" Production and Properties of Vacuum Evaporated Films of Thorium Fluoride," W. Heitmann & E. Ritter, Applied Optics, Vol. 7, p. 307 (February1968).

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Production Process, Identification & Purity
Thorium fluoride powder is prepared chemically in a number of carefully controlled steps and melted under inert or fluoride-based atmosphere to yield completely anhydrous ingots. The unequivocal identification of ThF₄ is made by X-ray diffraction analysis, where patterns correspond to JCPDS Card No. 23-1426. The following table shows typical spectrographic analysis results of ThF₄.

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Radioactivity & Safety
Thorium fluoride and thorium oxyfluoride are compounds containing naturally radioactive thorium. These radiochemicals emit ionizing radiation, primarily in the form of alpha particles. Thorium and its decay products, called thorium daughters, also emit some beta particles and gamma radiation.

There is low risk of external radiation exposure when handling generally licensed quantities of thorium compounds (150 lbs or less per year and a possession limit of 15 lbs). Radiation exposure to personnel should, however, be kept as low as reasonably achievable. Inhalation or ingestion of all alpha emitters should be avoided. The use of filter masks can be beneficial when the possibility of airborne particulate radiation exists.
 

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Physical Data

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