International Journal of High Energy Physics
2014; X(X): XX-XX
Published online MM DD 2014 (http://www.sciencepublishinggroup.com/j/ijhep)
doi: 10.11648/j.XXXX.2014XXXX.XX
The nature of the color centers in KDP crystals
Baltabekov A. S., Koketai T. A., Tagayeva B. S., Tussupbekova A. K.
Physical and technical faculty, Academician Y. A. Buketov Karaganda State University, Karaganda, Kazakhstan
Email address:
[email protected] , [email protected], [email protected], [email protected]
Abstract: In work studying of absorption spectra of KDP crystals irradiated by x-rays is spent. It is shown that
absorption bands at 2.26 eV and 3.17 eV are connected with B-radicals, at 5.75 eV - with interstitial hydrogen atoms. In
area 110-120К migration of holes (B-radicals) is activated. Activation of atoms hydrogen migration gives of
thermoluminescence with maximum of peak at 180К.
Keywords: KDP, Defect, Color Center, Recombination, Luminescence, Absorption Spectra
1. Introduction
Crystals of potassium dihydrogen phosphate (KH2PO4 or
KDP) are a group of compounds with nonlinear optical
properties and are widely used in laser technology. Analysis
of published data showed that in the last decade has
significantly increased the number of studies of
radiation-stimulated processes in these crystals. This is due
to the fact that there is a dramatic deterioration in the
performance of this material is due to the formation of
crystal defects. At generation of harmonics with wave length
less of 180 nm it are colored. These color centers are
connected with formation in a crystal lattice of radiation
defects. Now in crystals KDP establishes following radiating
defects by the EPR method: A-radicals (HPO4)-, B-radicals
(H2PO4)0, interstitial of hydrogen atoms (Hi)0 and ions
(PO3)2- [1-4]. B-radicals are the self-trapped holes.
A-radicals are the hole centers with hydrogen vacancy.
The purpose of this work is establishment of mechanisms
of recombination luminescence in KDP crystals.
In work [5] by methods of quantum chemistry it is shown
that at capture of electron by proton of hydrogen bond the
hydrogen atom is unstable and it is displaced in interstitial of
crystal lattice. It is obvious that B-radicals result from
ionization of anions. In work [6] it is discussed two possible
mechanisms of creation for A-radicals: the ionization of the
L-defects and the second – at generation by radiation in a
crystal zonal electrons and holes. The electrons are captured
by protons. The hole localization on the nearest to vacancy
of oxygen atom leads to occurrence of A-radicals. The first
and second mechanisms of A-radicals formation are realized
only at a radiation stage, i.e. in the course of crystals
excitation by ionizing radiation. It contradicts the work [7].
It is shown by method EPR that the quantity of A-radicals
grows at heating of irradiated crystal KDP. Hence, the part of
A-radicals is created at a post-radiation stage. In work [8]
the assumption about their creation by excitons is made. The
excitons are formed by recombination of the hole and
electronic.
In work [7] after an irradiation irradiation by x-rays in
crystal DKDP observed the radiation induced absorption
bands at 230 nm, 390 nm and 550 nm. In KDP absorption
bands at 390 nm and 550 nm were observed in work [9]. The
absorption band at 215 nm was observed in work [10].
Authors of work [9] have assumed that absorption bands
with maximum at 390 nm and 550 nm are connected with
B-radicals. The correlation between optical density in
absorption bands at 390 nm and 550 nm and intensity of a
EPR spectrum for B-radicals has been established. In work
[10] it is established that a absorption band of with a
maximum at 215 nm it is not connected with presence in
crystals KDP of impurity, i.e. it is caused by own radiation
defects of a matrix. The nature of the given absorption band
hasn't been earlier.
Thus, from three found out radiation induced absorption
bands of two it is attributed the B-radical, and the third is not
established. The purpose of the given work is definition of
the nature of these optical bands by independent method.
2. Technical Preparatory Section
Monocrystals KDP have been grown up from the sated
water solutions at 40оС. At the expiration of 15-20 days
were obtained single crystals of sufficient size for the optical
measurements (30*10*5mm), habitus which coincided with
the literature. To measure the absorption spectra, as well
thermoluminescence were cooked wedges of potassium
dihydrogen phosphate, excluding orientation of planes with
respect to the crystallographic axes. Thickness of the plates
was about 0.3 mm. The initial material was high chemical
cleanliness. For additional clearing it was exposed three
times of recrystallization.
Measurements of absorption spectra it was spent on
spectrophotometer SF-16 with the modified channel of
registration. For registration of a light signal photomultiplier
136 or 39 was used. The sample was located in a cryostat
with quartz windows. The irradiation was spent by x-ray
device URS-55a (Mo, U=35 kV, I=10 mA) through
beryllium window of cryostat. Irradiation doses were
supervised by the chemical dosimeter (Fricke). The
temperature was registered copper-constantan the
thermocouple. Speed of heating was supported by a constant
0,16 grad/sec.
3. Results and Discussion
In fig.1 absorption spectra of crystal KDP before and after
an irradiation by x-rays at 80К are resulted. In the field of
transparency three bands of the radiation induced absorption
with maxima at 2.26 eV, 3.17 eV and 5.75 eV are observed.
This result will well be coordinated with results in works
[7,9,10].
Figure 2. The curves of thermodecolouration absorption bands with
maxima at 2.26 eV (1), 3.17 eV (2) and 5.75 eV (3).
In figure 2 the curves of thermodecolouration for the
radiation induced absorption bands are shown. After a
crystal irradiation at 80К the absorption spectrum was
measured. Then the sample heated up to certain temperature
and was cooled to 80К for measurement of absorption
spectrum and etc. Thus all measurements are spent at one
temperature, but after heating to some level. It allows to
exclude all temperature effects. The values of optical density
give a thermodecolouration curve. From figure 1 it is visible
that absorption bands with a maximum at 2.26 eV and 3.17
eV are annealed simultaneously. Above 130К this optical
bands aren't observed. The absorption at 5.75 эВ is annealed
in temperature area 100-200К. Comparison to the data on
EPR from work [7] allows to assert that absorption bands
with maximum at 2.26 eV and 3.17 eV are connected with
B-radicals, and absorption band with maximum 5.75 eV hydrogen atoms in interstitial.
Figure 1. Absorption spectrum of crystal KDP at temperature of liquid
nitrogen before (1) and after (2) irradiations by x-rays with a dose 500 kGy.
By results of work [7] in crystals DKDP EPR signal from
B-radicals disappears at heating in area 140К, for hydrogen
atoms - in area 210К. EPR signal from A-radicals to 130К at
heating irradiated DKDP grows, in area 170-210К decreases
to zero. Reduction of EPR spectra at heating can be
connected with reduction of number of the paramagnetic
centers or increase of probability of spin-lattice relaxation.
Сomparison of temperature dependences for EPR spectra
and optical density in radiation induced absorption bands
allow to remove the these questions.
Figure 3. TL curve for KH2PO4. Doze irradiation is 40 kGy.
In figure 3 the typical thermoluminescence (TL) curve of
KDP after an irradiation by x-rays at 80К is shown. On TL a
curve there are three peaks of recombination luminescence
with maximum in area 110-120К, 180К and 290К. The
spectral structure of the first peak contains two bands of
emission with maximum at 2.6 eV and 4.8 eV. In work [6]
emission 4.8 eV is attributed to excitons, 2.6 eV - to
L-defects. In [6] it is shown that the luminescence at 2.6 eV
is effectively excitation at 7К by photons with energy 5.8 eV.
It will well be coordinated with the radiation induced
absorption band. However, the this absorption band and a
absorption band of the selftrapped holes (B-radicals) are
annealed together. The hole center can't recombination with
L-defect. From all known defects in KDP the role of the
electronic can carry out only interstitial hydrogen atoms and
D-defects. Last it is possible to present as (H3PO4)0.
We consider that in area 110-120К recombinate
B-radicals with interstitial hydrogen atoms. The proton is
thus formed, and it recombinate with L-defects. The small
lightsum in this TL peak is connected with temperature
suppression of luminescence by excitons in this temperature
range. In 110-120К there is a thermal activation of holes
migration. Full annealing of interstitial hydrogen atoms in
area 200К we connect with their recombination with
A-radicals. Thus occurrence of TL peak at 180К is caused by
thermal activation of hydrogen atoms. The spectral structure
of this TL peak consists of a single emission band with a
maximum at 3.55 eV.
The nature of TL peak at 290К remains unstated.
Thermoannealing of absorption bands at 140К allows to
reveal an additional absorption band with a maximum at 2.8
эВ. We assume that it is associated with PO32- ions.
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