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Investigation of optical spectra of dielectric crystals

Optical spectra of crystals oxyhalides of bismuth are investigated in this work. When measuring the transmission spectra of single crystals of oxyhalides of bismuth in the direction perpendicular to the layers, a large number of peaks associated with the occurrence of the interference pattern when light is reflected from the layers. The intense absorption band at 4.0 eV for BiOCl crystal has a doublet structure, in the case of the BiOBr crystal splitting is less pronounced. Upon excitation in the fundamental absorption region there photoluminescence spectra are investigated. The spectrum of luminescence of BiOCl crystal is not elementary and it consists of two bands of radiation. The presence in the excitation spectra and luminescence shortwave bands strongly depends on the degree of purity of the bismuth oxide is used in the synthesis oxyhalides of bismuth crystal. Thus, it allows to link the short-wave emission band with uncontrolled impurity, and long-wave – with its own glow crystal substrate.


Introduction
Two mechanisms are known for the formation of radiation defects in dielectric materials: electron and shock. The electronic mechanism of formation of radiation defects was developed in the 60's of the last century in the works [1,2]. In oxide crystals (MgO, CaO, A12O3, etc.), the electronic mechanism, as a rule, does not take place. In these materials, radiation defects are formed by the impact mechanism, which explains their high radiation resistance [3].
In complex sulfates, the mechanism of formation of radiation defects was considered in the works [4][5][6]. One of the first studies on the spectroscopy of impurity ions in crystals of ammonium halides and oxyhalides of bismuth and antimony was carried out at the Karaganda State University named after academician Y.A. Buketov under the guidance of T.A. Kuketayev and V. Yurov.
The Bi 3+ and Sb 3+ ions have an electronic configuration of the mercury atom and are related to mercury-like ions. In the homologous series, upon transition from Cl and Br, the absorption and emission spectra shift to the region of lower energies. This pattern is inherent in all mercury-like ions and is associated with an increase in the spin-orbit interaction upon transition to a heavier halogen. The oxyhalides of bismuth and antimony have tetragonal symmetry. The absorption spectrum should consist of three groups of bands corresponding to electronic transitions 1 S0→ 3 P1, 1 S0→ 3 P2 and 1 S0→ 1 P1. It is precisely this situation that is observed in our experiments. The splitting of the levels of the 3P1 state in a crystal field gives the experimentally observed doublet in the long-wave absorption spectrum [7].
At present, the optical properties of compounds containing ions with fn-configuration are widely studied in connection with the problem of stimulated emission. Questions arising in connection with optical studies of crystals with f-ions are grouped around the following three directions: 1) the physicochemical conditions for the formation of activated crystals and the structure of activator centers; 2) energy schemes of activating ions in crystals; 3) processes occurring in the excited state [8].
A large number of studies around the world have been devoted to the study of calcium sulfatebased TLD. Calcium sulphate is used in many countries as a basis for thermoluminescent dosimetry (TLD) of the population. TLDs based on this compound have a record sensitivity at the moment to the level of background radiation.
Analysis of literature data showed that there is no clarity in the mechanisms of degradation processes of TLD based on calcium sulphate, therefore, we investigated the processes of recombination of charge carriers in such model dielectrics as potassium sulfate, ammonium halides, bismuth halide and antimony. From a brief review of earlier work and the current state of the issue, the first section outlines the tasks of the dissertation study [9].

Research Method
Growth of monocrystals of antimony oxyhalides was carried out by sublimation-condensation in closed vacuumed quartz ampoules. As a temperature sensor, platinum-platinum-rhodium thermocouples were used.  The temperature of the evaporation zone was from 500 to 550°C for oxychlorides and from 520 to 580°C for osbromide antimony. The temperature difference between the sublimation and growth zones was (20-25)°C. The growth time reached 5-6 days. Reducing the temperature difference to 20°C led to a decrease in the total number of crystals, an increase in their length to 3-5 mm, improved cutting and optical quality. Figure 1 shows the absorption spectra of BiOCl and BiOBr. In the range 3.0-6.0 eV, in both cases, there are three absorption bands. The shape of these absorption bands is not elementary. The intense absorption band at 4.0 eV in BiOCl has a doublet structure. For a BiOBr crystal, the splitting of the absorption band at 4.0 eV is less pronounced.

Result and Discussion
When excited in the region of fundamental absorption of BiOCl and BiOBr crystals, photoluminescence arises [10]. The luminescence spectrum of BiOCl is non-elementary and consists of three bands. A shortwave band with a maximum of 3.1 eV does not disappear with an increase in temperature to 300-400 K, while the 2.4 eV band decays already at temperatures of 200-220 K. The excitation spectrum of the 2.4 eV band has a pronounced maximum at 3.75 eV. A short-wave band is excited in the region of 5 eV.
The presence in the excitation and luminescence spectra of the short-wave band depends strongly on the degree of purity of the bismuth oxide. Bismuth oxide is used in the synthesis of oxyhalides. In specially purified samples at low temperature, only a longwave band with a maximum of 2.4 eV is observed. Thus, the appearance of a short-wave luminescence band is associated with an uncontrolled impurity, and the long-wavelength luminescence band is associated with the intrinsic luminescence of the crystalline base.
The absorption spectrum of Sb4O5Cl2 crystals is shown in figure 2. As in the case of bismuth oxyhalides in the energy range 3.0-6.0 eV, three non-elementary absorption bands are observed here.
On the decline of the long-wave absorption band, luminescence with a maximum of 2.54 eV and a maximum excitation band of 3.5 eV is effectively excited. The luminescence of Sb4O5Cl2 crystals is already extinguished at T=80 K and completely damped at T=180-200 K.

Conclusion
Irradiation of bismuth oxyhalides with UV or X-ray quanta results in the appearance of a broad band in the decay of exciton absorption bands. Photostimulation in the band of induced absorption leads to the appearance of a luminescence band in the long-wavelength region of the spectrum. A change in temperature leads to broadening of the luminescence spectrum, however, the shift of the maximum of the band is insignificant.