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Hg2Cl2 , calomel
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Elastic constants cij : (1011 dyn/cm2 or 1010 Pa):
1.8925 1.7192 1.563 0 0 0
1.7192 1.8925 1.563 0 0 0
1.563 1.563 8.037 0 0 0
0 0 0 0.8456 0 0
0 0 0 0 0.8456 0
0 0 0 0 0 1.225

Photoelastic constants pij:
0.551 0.44 0.256 0 0 0
0.44 0.551 0.256 0 0 0
0.137 0.137 0.01 0 0 0
0 0 0   0 0
0 0 0 0   0
0 0 0 0 0 0.047

Acousto-optic figure of merit:
sound dir. sound pol. light dir. light pol. M (10-18 s3/g)
[100] [100], L   [100] 506
[100] [100], L   [010] 340
[110] [110]   [110] 273
[110] [110]   [1-10] 185
[110] [1-10]   arb. 640
[001] [001], L   [001] 0.17
Sound velocity:
direction polarization v (105 cm/s)
[100] [100] 1.622
[101] [101] 2.641
[100] [100] 2.054
[001] [001] 3.343
[001] [010] 1.084
[100] [010] 1.305
[110] [001] 1.084
[110] [1-10] 0.347
[101] [101] 1.132
[101] [010] 1.206

Refraction indices:
l (mkm) nx ny nz
0.6328 1.96 1.96 2.62

NOTE: It should be taken into account that nx=ny=nz=n for optically isotropic material and nx=ny=no and nz=ne for uniaxial crystal		 Crystal symmetry: 4/mmm
Cell Dimensions: a = 4.45, c = 10.89, Z = 2;
V = 215.65 Den(Calc)= 7.27
X Ray Diffraction:
by intensity (I/Io): 3.164(1), 4.141(0.97), 1.97(0.38),
Density: 7.19 g/cm3
Optical spectral range: 0.38 - 28 mkm

  CALOMEL is extremely suitable for the construction of acousto-optic devices. In comparison, CALOMEL's figure of merit is comparable with tellurium dioxide TeO2 and more than one order better than lead molybdenate PbMoO4. The simplest application for CALOMEL would be as an integral light beam modulator. Because of the high value of the figure of merit, M2, the required power consumption is low and diffraction efficiency is high. It follows that due to the low value of shear wave velocity, dimensions can be reduced and the modulation bandwidth is slightly reduced.The transparency of CALOMEL (0.38 to 20 µm) enables the use of these modulators in a broad spectral range (compared with TeO2). The high value of optical damage in the infrared region could be efficiently exploited in industrial laser beam manufacturing (drilling, annealing, etc.). In addition, these properties are valuable in laser surgery, because CALOMEL has a broad band of transparency and is thereby suitable as a visible light beam for infra-red beam pointing. A more sophisticated acousto-optic application would be a spatial light beam modulator - optical processor. This acousto-optic unit is used to convert an electric signal into an optical image which can be processed by optical data processing techniques.
  Due to a very slow shear acoustic wave, high resolution (about 5000 pts) and a large time window (about 150 µs) can be achieved. These parameters are excellent comparable to TeO2 and the time window is superior. This type of acousto-optic unit is used for optical spectrum analyzers, Kalman filters, convolvers, correlators, matrix multipliers, etc. Because of high speed parallel optical processing these systems are used in aircraft, space and military applications where real time processing of large amounts of data is fundamental.
NOTE: In those cases when different researchers give distinct values of constants, avaraged data are presented in Tables.

Data source: Dept. of Physics of MSU