Физика/2. Физика твердого тела

Koketay T.A., Тussupbekova А.К., Abdykadyrova K.Zh.,

Kipshakov M.S., Amenova A.

Karaganda State University named after academician Y.A. Buketov, Kazakhstan

The study of holes centers in insulating crystal

The centers created by X-ray irradiation of the single crystals of potassium dihydrogen phosphate at room and a liquid nitrogen temperatures were studied by the termoactivation method. Today, in crystal of dihydrogen phosphate with different methods settled the chain of radiation damages. However, motions of their generation and recombination aren’t almost discussed. The only mechanism that is studied is the election capture by bridging protons with subsequent non-activate leaving of hydrogen machines in the interstitial site. Obviously, B-radicals, which are auto-localized holes (H₂PO₄)⁰ occur as a result of ionization of anions [1]. The generation of A-radicals connected with the ionization of L-defects, which have an up-radiation nature, or arise from capture of electrons by protons. These processes are implemented under X-ray irradiation of crystals. However, it is known [2] that A-radicals are formed in the post-radiation stage. These processes are discussed in literature quite not enough. For potassium dihydrogen phosphate crystals there’s no science-based visualization about mechanisms of recombination processes, except general statements about recombination of electrons and holes without specifying the nature of its carrier.

For crystals with hydrogen bonds characteristic with an up-radiation disorders of the crystal structure are Bierum’s defects. Bierum’s defects constitute a vacancy or excess proton in the hydrogen bond subsystem. In potassium dihydrogen phosphate crystals, the existence of these defects is shown with the help of electro-physical methods. Besides the luminescence of excitons, in these crystals at the temperature of liquid nitrogen discovered the bands of radiation with four maxima in the range of 2.5 - 3.6 eV [3].

It is concluded that the observed luminescence bands appear as the result of presence of lattice of potassium dihydrogen phosphate structural Bierum’s defects, based on the influence of impurity ions of divalent manganese and copper. The ionization radiation-induced or to radiation Bierum’s defects can lead to the formation of A-radicals. The realisation of this mechanism assumes that the insertion of potassium dihydrogen phosphate heterovalent impurity ions into the crystal lattice, which lead to the emergence of additional hydrogen vacancies, should lead to an increased probability of the formation of A-radicals. The dominance of this mechanism contradicts to the experimental fact of increasing of the corresponding electron paramagnetic resonance (EPR) signal while heating of the irradiated potassium dihydrogen phosphate crystals. Most likely there will be a variety of canals of formation of the hole center.

Thus, the comparison of the results from different articles shows the poor comparability of obtained results in measuring radiation-induced optical absorption. It is figured out that more reliable results are data that are presented in works [1, 4]. The nature of the optical absorption band with a maximum at 230 nm is not discussed in literature. As it was mentioned above, the nature of the origin of the absorption band with a maximum at 2.69 eV is just assumed.

According to the literature, a decrease in the electron paramagnetic resonance signal from B radicals and interstitial hydrogen compounds is observed in the temperature range below 140 K. The change in the intensity of electron paramagnetic resonance with increasing temperature due to thermal annealing of paramagnetic centers or because of the increase in spin-lattice relaxation. (Н₂РО₄)⁰ (B-radical) and (H_i)⁰ (interstitial hydrogen atom). These defects constitute a complementary pair, since the formation of one will lead to the appearance of the other. The B-radical arises as a result of ionization of the anion and is an autolocalized hole. The electron is captured by a bridge proton, which forms the hydrogen bond O-H-O. In this case, the hydrogen atom is pushed into the interstitial, which is shown by quantum chemical calculations [1, 2].

The peak of the thermally stimulated luminescence (TSL) in the region 110-130 K has a radiation spectrum consisting of two bands with maxima at 2.6 eV and 4.8 eV. The emission band at 4.8 eV is associated with the luminescence of excitons. It can occur when the migration of both holes and electrons is thermally activated. We have established that when the crystal lattice of potassium dihydrogen phosphate is rearranged as a result of the polymorphic phase transition, an abnormally sharp ignition of the recombination luminescence occurs, which can be explained by a decrease in the activation energy of the migration of any of the carriers [5].

This review shows a number of contradictory opinions about the nature, properties of radiation defects in potassium dihydrogen phosphate and the mechanisms of their formation.

References:

1. Voronov A. P., Babenko G. N. Puzikov V. M., Roshal A. D. Salo, V. I. Doping of KDP single crystals with cerium: Growth and optical properties// Crystallography Reports. - 2008. - Vol. 53, №4. - P. 708-712.

2. Byberg J.L. O‾ detected by EPR as a primary electron-excess defect in X-irradiated K₂SO₄ // Chem. Phys. – 1967. – Vol. 84, № 11. – P. 6083-6085.

3. Тагаева Б.С., Кокетайтеги Т.А., Ким Л.М., Балтабеков А.С., Тулегулов А.Д. Стабилизация В-радикалов ионами меди в кристаллах KDP // Материалы ХІ Междунар. конф. по физике твердого тела. – Ust-Kamenogorsk, 2010. – P. 620.

4. Koketai T.A., Тagayeva B.S., Tussupbekova A.K., Mussenova E.K. A mechanism of formation of A-radicals in KDP crystal (KH₂PO₄) // Book of abstracts. International conference of luminescence and optical spectroscopy of condensed matter. - Wroclaw, Poland. – 2014. – P.18.

5. Koketai T.A., Тagayeva B.S., Tussupbekova A.K., Mussenova E.K. Study of the types of hole centers in KDP crystals // L. N. Gumilyov Eurasian national University Scientific Journal Herald. - 2014. - V.4, №101. - P. 258-262.