LiF Crystal

18 Dec 2020

Lithium fluoride (LiF) crystal has excellent VUV region transmittance. It is used for windows, prisms, and lenses in the visible and infrared in 0.104μm-7μm. LiF single crystal is sensitive to thermal shock and would be attacked by atmospheric moisture at 400°C. In addition irradiation produces color centers. Modest precautions should be taken against moisture and high energy radiation damage. Besides LiF softens at 600°C and is slightly plastic that can be bent into radius plates. The material can be cleaved along (100) plane and less commonly (110) plane. The optical characteristics are good and yet the structure is not perfect and cleavage is difficult. For good structure LiF is less commonly grown by the Kyropoulos method (air-grown) specifically for monochromator plates. High quality LiF is usually grown by modified Bridgman method.

Parameter

Material and Specifications

Orientation	[100]< ±0.5°
Angle Tolerance	< 0.5°
Parallelism	<20〞
Perpendicularity	5ˊ
Surface Quality	10-5 (MIL-O-13830A)
Wavefront Distortion	<λ/4@633 nm
Surface Flatness	<λ/8 @633 nm
Clear Aperture	>90%
Chamfer	Technological for protection from

	edge cracks
Thickness/Diameter Tolerance	±0.05 mm

Physical and Chemical Properties

Crystal Structure	Cubic (NaCl)
Lattice Constants	4.026
Density	2.639 g/cm3
Melting Point	870°C
Thermal Conductivity /(W·m-1·K-1@314k)	4.01
Specific Heat Capacity/ (J·kg-1·K-1)	1562
Thermal Expansion /（10-6·K-1@283k ）	37
Hardness (Knoop)	102-113@600g
Young`s Modulus /(1011dyne cm-2)	6.5-7.6
Elastic Coefficient	C11=112; C12=46; C44= 63.5
Apparent Elastic Limit/ MPa	11.2@1620 psi
Cleavage	-100

Optical characteristics

Transmission Range	0.105 … 6 µm
Refractive Index	no = 1.37327@2.5µm, 1.624@0.12 µm
Reflective Loss	4.4% @4.0 µm
Poisson Ratio	0.326
Absorption Coefficient/cm-`1	0.74×10-3@2.7µm