Crystals and glasses - reference information


Components from the above optical materials can be made with an anti-reflection optical coating in the visible and infrared (IR) ranges.

Crystallographic characteristics

Crystals are solids that have an ordered three-dimensional periodic spatial atomic structure. The use of crystalline materials in optics is determined by their high (in comparison with glasses) transparency in the ultraviolet and infrared spectral ranges, as well as by a variety of dispersion properties.

The listed crystallographic data includes crystal system, symmetry class, lattice parameters and cleavage.

Syngony characterizes the type of symmetry of the crystal unit cell. The symmetry class of a crystal is called the complete set of its possible symmetric transformations.

The lattice parameters are its three elementary translations: a, b and c. Cleavage is the property of a crystal to form cracks along certain crystallographic planes. To indicate cleavage, the crystallographic symbol of the plane of easy cleavage is indicated. Qualitatively, cleavage is characterized as “highly perfect”, “perfect” or “imperfect”.

A crystal can consist of one integral block, and then it is called a monocrystal. There are also polycrystals - aggregates of chaotically oriented monocrystal grains of different sizes. The properties of polycrystals are determined by the properties of the grains from which they are formed, as well as by their size, mutual arrangement and the forces of interaction between them.

Optical performance

The optical characteristics of materials are represented by data on refractive index, relative temperature coefficient of refractive index and transmission for various wavelengths; transmission spectra are given for samples 10 mm thick.

Refractive index n denotes the ratio of the speed of electromagnetic radiation in vacuum to the speed of radiation in the material.

The relative temperature coefficient of the refractive index is determined by the following formula: b (t, l) = dn (l) / dt, °Ń-1, where t is the temperature. For anisotropic and optically uniaxial crystals of magnesium fluoride and sapphire, the values of the refractive indices and the relative temperature coefficient of the refractive index are given for ordinary nî and extraordinary ne rays.

The transmission t (l) is the ratio of the monochromatic radiation flux passing through the material sample to the incident radiation flux.

In some cases, instead of the transmission, the value of the attenuation coefficient is indicated, which is calculated using the following formula:


where ti (l) is the internal transmittance, which is equal to the ratio of the monochromatic radiation flux that has reached the exit surface of the sample to the radiation flux that has passed through its entrance surface, S is the thickness of the sample, measured in centimeters.

ARadiation attenuation is caused by absorption and scattering within the material, but does not include reflection loss, which can be determined by the formula: Reflection loss = (n-1)2 / (n + 1)2

Thermal performance

The temperature coefficient of linear expansion, thermal conductivity, specific heat, heat resistance, and melting point are explained.

The temperature coefficient of linear expansion at, °Ń-1, characterizes the relative change in the length of the sample when its temperature changes by 1 °Ń and is determined by the formula:


where l is the sample length, t is the temperature.

Thermal conductivity, W/(m•°C), characterizes the ability of a material to conduct heat and is determined by the amount of heat transferred through a unit of area per unit of time at a unit of temperature gradient.

For anisotropic crystals of magnesium fluoride and sapphire, the values ​​of the temperature coefficient of linear expansion and thermal conductivity are given in directions parallel and perpendicular to the optical axis.

Specific heat, J/(kg•°C), characterizes the energy required to heat the material and is determined by the amount of heat required to heat a unit mass of the material by one degree.

Heat resistance, °C, characterizes the sample's ability to withstand abrupt temperature changes without destruction. The measure of heat resistance is the maximum temperature difference during rapid change, maintained by the sample without destruction.

Mechanical characteristics

Mechanical characteristics are described by the values of density, Mohs hardness, Vickers microhardness, elastic compliance constants, elastic modulus, shear modulus, and Poisson's ratio.

Density, g/cm3, is determined by the ratio of the mass of the material to its volume.

Mohs hardness characterizes the ability of a material to be scratched by another material. Reference numbers of hardness are given according to the conventional Mohs scale, in which 10 standard minerals are arranged in a row according to the degree of increasing hardness.

Vickers microhardness, Pa, characterizes the indentation resistivity of the material surface to the indentor in the form of a four-sided diamond pyramid at a certain indenter load.

The reference values of microhardness at a load of 1 N. The elastic compliance constants S11, S12, S44, Pa-1 are the coefficients of proportionality between the stress and strain components.

Elastic modulus (Young's modulus) E, Pa - normal stress, which doubles the linear dimension of the body. Shear modulus G, Pa - shear stress causing a relative shear equal to one.

The transverse strain ratio (Poisson's ratio) is the ratio of the relative transverse compression to its relative elongation.

Photoelastic characteristics

Photoelastic properties are represented by optical stress coefficients, photoelastic and piezo-optical constants.

Optical stress coefficients B1, B2, Pa-1 reflect the relationship between birefringence and the stress that causes it:


where Δn12 is the birefringence caused by the shear stress σ12.

The photoelastic constants C1, C2, Pa-1 characterize the function of the change in the refractive index Δn1 and Δn2 of the material under the effect of normal stress σ applied along the main crystallographic directions.


The piezo-optical constants p11, p12, p44, Pa-1 are the proportionality coefficients between the stress components and refractive index.

Chemical resistance

Chemical resistance of materials characterizes their resistivity to aggressive media: water, acids and organic compounds. The solubility of crystalline materials in water at a temperature of 20 °C, g/100 cm3, as well as their ability to dissolve in acids and organic compounds is given.