Crystal is solid material with ordered three-dimensional periodic spatial atomic structure. The use of crystal material in optics is explained by its high (in comparison with glass) transmittance in ultra-violet and infra-red spectral range and also by wide variety of dispersion properties.
Adduced crystallographic data includes syngony, symmetry class, lattice constants and cleavability.
The syngony describes the symmetry type of crystal unit cell.
The symmetry class is the total sum of its possible symmetric transformations.
The lattice constants are its three elementary translations a, b and c.
The cleavability is the property of crystal to crack along definite crystallographic planes. For denoting cleavability the crystallographic symbol of a plane of easy cleavage is indicated. Qualitatively, the cleavability is characterized as "highly perfect", "perfect" or "imperfect".
Crystal can consist of one entire block; then it is called monocrystal. There are also polycrystals – aggregates of random orientated monocrystalline grains of different sizes.
The properties of polycrystals are determined by the properties of grains, which formed them, and also by their size, mutual location and interaction forces between them.
Optical characteristics are represented by data on refractive index, thermal coefficient of refractive index and transmittance coefficient for various wavelengths; transmittance spectrums for samples with 10 mm thickness are adduced.
The refractive index, n, denotes the ratio between the velocity of electromagnetic radiation in vacuum and the velocity of radiation in material.
The thermal coefficient of the refractive index is determined by the formula as follows: b (t,l ) = dn(l)/dt, deg C-1, where t is the temperature. For anisotropic and optically uniaxial Magnesium Fluoride and Sapphire crystal the refractive index and thermal coefficient of the refractive index are given both for the ordinary no and for the extraordinary ne rays.
The transmittance coefficient t(l) is the ratio between the flux of monochromatic radiation that has passed through the sample of the material and the incident flux of radiation. In some cases the attenuation factor is indicated instead of the transmittance coefficient. It is calculated by the formula:
where ti (l) – is the internal transmittance that is equal to the ratio between the flux of monochromatic radiation that has reached the exit surface of the sample and the flux of radiation that has passed its entry surface. S is the thickness of the sample measured in cm.
Attenuation of radiation is due to absorption and scattering inside material, but it does not include reflection loss from a surface, which can be calculated by the formula: Reflection loss = (n-1)2 / (n+1)2
Reference values are presented for thermal linear expansion coefficient, thermal conductivity, specific thermal capacity, thermal stability and temperature of melting points. Thermal linear expansion coefficient at, °Ñ-1, characterizes the relative change in length of the sample at a change in temperature of one deg C. It is determined by the formula:
where l is the length of the sample and t is the temperature.
Thermal conductivity, W/(m• °C), characterizes the capacity of the material to transmit heat and is determined by the amount of thermal energy that has gone through a unit area in a unit time at a unit temperature gradient.
For anizotropic Magnesium Fluoride and Sapphire crystal thermal linear expansion coefficients are given for directions parallel and perpendicular to optical axis.
Specific heat capacity, J/(kg•°C), characterizes the energy necessary for heating the material and is determined by the amount of heat needed for warming the material by one degree.
Thermal stability, °C, characterizes the capacity of the material to resist sharp temperature changes without destruction. The measure of thermal stability is the maximum difference in temperature in an abrupt change of the latter, which the sample can withstand without destruction.
Mechanical characteristics are described by the values of density, Mohs hardness, Vickers microhardness, constants of elastic compliance, elastic modulus, shear modulus and Poisson coefficient.
Density, g/cm3 is determined by the ratio between the mass of the sample and its volume.
Mohs hardness shows the capacity of material to resist being scratched by another material. Reference values are presented for hardness according to the conditional Moh scale, in which 10 standard minerals are arranged in the order of increasing hardness.
Vickers microhardness, Pa, is characterized by the resistance of the surface of the material to impression by the indentor in the form of a four-faced diamond pyramid at indentor load of 1 Newton.
The constants of elastic compliance are proportionality coefficients between stress and deformation components.
Elastic modulus (Young modulus), Pa, is normal stress that changes linear dimension two times as much.
Shear modulus, Pa, is tangent stress that causes a relative shift equal to one.
Coefficient of transversal deformation (Poisson coefficient) is a ratio between specific cross compression and specific elongation.
Photoelastic characterictics are presented by stress optical coefficients, photoelastic and piesooptical constants.
Stress optical coefficients Â1, Â2, Pà-1 reflects correlation between birefringence (double refraction) and stress that causes it:
where Dn12 – birefringence caused by shear stress s12
Photoelastic constants Ñ1, Ñ2, Pà-1 defines the dependence of material’s refractive indexes Dn1 è Dn2 alteration under the force of normal stress s, applied along the main crystallographic axises.
Piesooptical constants p11, p12, p44 are proportionality coefficients between stress and refractive index components.
Chemical stability of the materials is characterized by their resistivity to effect of aggressive agents – water, acid and organic substances. Solubility of the materials (gram/cm3) in water at 20 deg,C and also their capacity to dissolve in acids and organic compounds are presented.