Germanium ( Ge )

Crystal properties
Crystal Type:	cubic, diamond structure
Transmission Range	1.8 ... 23 µm
Refractive Index	4.0034 @ 9.72 µm, 4.0028 @ 10.6 µm
Reflective Loss	52.9 % @ 10 µm
Density	5.323 g/cm3
Melting Point	956, 959, 942, 938 °C
Molecular Weight	72.59
Thermal Conductivity	58.61 W/(m K)
Specific Heat	310 J/(kg K) @ 25 °C
Thermal Expansion	5.5×10–6 / K, 6.1×10–6 / K
Hardness (Knoop)	780
Young’s Modulus	102.7 <100>, 155.3 <111> GPa
Shear Modulus	67 <100> GPa
Bulk Modulus	77.2 GPa
Rupture Modulus	93 MPa
Elastic Coefficient:	C11 = 129 / C12 = 48.3 / C44 = 67.1 GPa
Dielectric Constant	16.6 @ f = 9.37 GHz
Solubility in Water	Insoluble
Application	Electronics lenses, band pass filters, thermography
Remarks	laser damage threshold: 2.9 ...3.3 GW / cm2
Germanium substrates

List of Optical Crystals
Materials	Refractive Index	Transmission Range (mm)	Density (g/cm³)	Thermal Expansion Coefficient (10-6/K)
BK7 glass	1.5164 (588 nm)	0.330 – 2.1	2.51	7.5
SF11 glass	1.78472 (588 nm)	0.370 – 2.5	4.74	6.8
F2 glass	1.62004 (588 nm)	0.420 – 2.0	3.61	8.2
Fused Silica	1.4858 (308 nm)	0.185 – 2.5	2.20	0.55
CaF2	1.399 (5.0 mm)	0.170 – 7.8	3.18	18.85
BaF2	1.460 (3.0 um)	0.15 – 12.0	4.88	18.4
Sapphire	1.755 (1.0mm)	0.180 – 4.5	3.98	8.4
Silicon	3.4179 (10mm)	1.200 – 7.0	2.33	4.15
Ge	4.003 (10 mm)	1.900 – 16	5.33	6.1
ZnSe	2.40 (10 mm)	0.630 – 18	5.27	7.8
ZnS	2.2 (10mm)	0.380 –14	4.09	6.5
LiF	1.39 (500 nm)	0.150 – 5.2	2.64	37
KBr	1.526 (10 mm)	0.280 – 22	2.75	43
MgF2	no = 1.3836 ne = 1.3957 (405 nm)	0.130 – 7.0	7.37	a: 13.7 b: 8.48
YVO4	no = 1.9500 ne = 2.1554 (1.3 mm)	0.400 – 5.0	4.22	a: 4.46 b: 11.37
Calcite	no = 1.6557 ne = 1.4852 (633 nm)	0.210 – 2.3	2.75	a: 24.39 b: 5.68
Quartz	no = 1.5427 ne = 1.5518 (633 nm)	0.200 – 2.3	2.65	7.07
?-BBO	no = 1.6749 ne = 1.5555 (532 nm)	0.190 – 3.5	3.85	a: 4.0 c: 36
LiNbO3	no = 2.2863 ne = 2.2027 (633 nm)	0.370 – 4.5	4.64	a: 16.7 c: 2.0