Crystal is solid material with ordered three-dimensional periodic spatial atomic structure. The use of crystal materials in optics is determined by thier high (in comparison with glass) transmittance in ultra-violet and infra-red spectral range, as well as by wide variety of dispersion properties.
The presented crystallographic data includes syngony, symmetry class, lattice constants and cleavability.
The syngony characterizes the symmetry type of crystal unit cell.
The symmetry class is the complete set of its possible symmetric transformations.
The lattice parameters are its three elementary translations – a, b and c.
The cleavage is the property of crystal to form cracks across certain crystallographic planes.
To indicate cleavage, the crystallographic symbol of a plane of easy cleavage is indicated. Qualitatively, the cleavage is characterized as “highly perfect”, “perfect” or “imperfect”.
A crystal can consist of one integral block; and in this case it is called monocrystal. There are also polycrystals – aggregates of randomly orientated monocrystalline grains of different sizes.
The properties of polycrystals are determined by the properties of grains, from which they are formed. as well as by their size, mutual dislocation and interaction forces among them.
The optical characteristics of materials are represented by data on refractive index, relative thermal coefficient for refractive index and transmittance coefficient for various wavelengths; transmittance spectrums are given for samples with 10 mm thickness.
The refractive index, n, denotes the ratio between the speed of electromagnetic radiation in vacuum and the speed 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 t,(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, °С-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 В1, В2, Pа-1 reflects correlation between birefringence (double refraction) and stress that causes it:
where Dn12 – birefringence caused by shear stress s12
Photoelastic constants С1, С2, Pа-1 defines the dependence of material’s refractive indexes Dn1 и 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.