CALOMEL (Hg2Cl2) single crystals

Basic Physical Properties

CALOMEL single crystals exhibit a wide range of optical transmission, high indexes of refraction, extremely high values for acoustooptic figures of merit, low velocity of elastic wave propagation (sound), and a high value of birefringence (four times higher than calcite). Because birefringence is positive, the ordinary light beam can be used as the functional beam resulting in a higher quality beam than is possible when using the extraordinary beam. As shown in Table I., the outstanding physical properties of this material make CALOMEL an important discovery.

Basic Properties of CALOMEL (TABLE I)

Chemical formula Hg2Cl2
Chemical name mercurous chloride
Mineralogical name CALOMEL
Molecular weight 472.086
Crystallographical system tetragonal(D174h)
Specific weight [g/cm3] 7.18
Vapour tension [Torr] 10-8 at 27 °C
10-4 at 80 °C
Optical transmission [µm] 0.38 to 20
Index of refraction (for λ=589 nm) n0 = 1.973 ne = 2.656
Birefringence (for λ=589 nm) + 0.683
CALOMEL is transparent and birefringent in the microwave region
Velocity of acoustic wave 347 [m/s]
Acoustooptical figure of merit M2= 640 x 10-15 kg/sec3 (for λ=632.8 nm)
Mechanical properties soft, existing cleavage planes
Mohs hardness 1.5
Chemical properties:
almost insoluble in water (2.2 x 10-4g/100g at 20°C)
highly reactive with metals except Sn, Pb, Ag, Au

In addition, CALOMEL is a very suitable material for high-power laser requirements due to its high optical load resistance (Table II).

TABLE II: Optical Load Resistance of CALOMEL

Ar-laser: λ= 0.5145 µm CW mode: >10 5
YAG-laser: λ=1.062 µm pulsed mode
(f=25 Hz; t=20 ns)
>5.10 7
CO2 -laser: λ=10.6 µm CW mode: 5.10 2

CALOMEL has many advantageous properties for applications in microwave systems. All physical properties - optical, elasto-optic, mechanical, dielectric, etc.) - exhibit outstanding anisotropy.

Growing calomel crystal
Growing calomel crystal

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