Abstract

Nonlinear optics is a new frontier of science and technology playing a major role in the emerging area of photonics. Photonics involves the application of photons for information and image processing and is branded to be the technology of the 21st century, wherein nonlinear optical (NLO) processes have applications in the vital functions such as frequency conversion and optical switching. These require materials exhibit second-order NLO effects and hence there is a great need for good quality single crystals for device fabrication. The inorganic materials were the first to be exploited for such applications. Further investigations on organic NLO materials have subsequently produced very good materials with highly desirable characteristics. The developments in the field of non-linear optics have gained important applications in optical information processing, telecommunications and integrated optics, because of the emergence of this field from solid state physics in which inorganic semiconductors, insulators and crystals constituted a major part of the scientific base, the early experimental and theoretical investigations were primarily concerned with materials from these classes. Single crystals of Potassium para-nitrophenolate dihydrate were grown by the slow evaporation technique. The crystal system and lattice parameters were found using single crystal X-ray diffraction. The optical parameters like optical band gap (E_g), refractive index (n), electric susceptibility (χ_c) and dielectric constants were calculated from UV-Vis-NIR Spectrum. The mechanical behaviour was analysed using the Vickers microhardness test. Using Wooster’s empirical relation the elastic stiffness constant (C₁₁) was calculated from the Vickers hardness values at different loads. The dielectric studies were carried out on the grown crystals to study the dielectric behaviour. The electrical properties such as plasma energy, Penn gap, Fermi energy and polarizability were calculated to analyze second harmonic generation (SHG). Photoconductivity measurements carried out on the grown crystal reveals the negative photoconducting nature of the crystals. The growth pattern was analysed by etching studies. Nonlinear optical properties were performed to confirm the SHG efficiency of the grown crystal. Hence, semi organic single crystal of potassium para-nitrophenolate dihydrate is an excellent NLO material with enhanced SHG efficiency required for important applications in the field of optoelectronic and photonics.

Keywords Solution growth, Single X-ray diffraction, Microhardness test, Dielectric studies and Photoconductivity measurement.

Author and Article Information

Author info
Department of Physics, Sree Sastha Institute of Engineering and Technology, India

RecievedFeb 28 2014　　AcceptedJul 16 2014　　PublishedJul 30 2014

CitationSagadevan S (2014) Growth, optical, mechanical and electrical studies of nonlinear optical single crystal: potassium para-nitrophenolate dihydrate. Science Postprint 1(1): e00026. doi: 10.14340/spp.2014.07A0001

Copyright©2014 The Authors. Science Postprint is published by General Healthcare Inc. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs 2.1 Japan (CC BY-NC-ND 2.1 JP) License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

FundingI am doing research without any funding.

Competing interestThe author has declared that there is no conflict of interests

PatentTwo patents filling under process.

Corresponding authorDr. Suresh Sagadevan
AddressDepartment of Physics, Sree Sastha Institute of Engineering and Technology, Chembarambakkam, Chennai, 600123, India
E-mailsureshsagadevan@yahoo.co.in

Introduction

Defect free bulk single crystals are needed for electronic industries because of their usage in the field of semiconductor, nonlinear optical (NLO), piezoelectric devices and so on. The research on organic crystals from the application point of view was started during 1980s. For practical applications, we need good optical transparency and also crystal should withstand high optical powers, and should have chemical stability. It is difficult to find a material that satisfies most of the above-said requirements, however, amino acid crystals are good candidates for NLO applications. Especially the complex of amino acid and strong inorganic acid plays a vital role in SHG applications. Nonlinear optics (NLO) is an innovative area of research and development which will play a key role in the field of optoelectronics and photonics 1. Nonlinear optical (NLO) materials find extensive optoelectronic applications such as optical frequency conversion, optical data storage and optical switches in the initially confined laser fusion systems. Incorporation of metals into organic ligand gives a new dimension of study. An important aspect of utilizing these materials for nonlinear optics is their unique charge transfer transitions either from metal to ligand or from ligand to metal. Due to this, in semiorganic materials, the metal – ligand bonding is expected to display large molecular hyperpolarizability, because of the transfer of electron density between the metal atom and the conjugated ligand system. The metal–organic coordination compounds as NLO materials have attracted much more attention for their considerable high NLO coefficients, stable physico-chemical properties and better mechanical intension.
The apparent development of semiorganic materials, where the organic ligand is ionically bonded with inorganic host refined the search of new materials with high optical nonlinearities which is an important area due to their optical applications such as optical communication, optical computing, Optical information processing, optical disk data storage, laser fusion reaction, laser remote sensing, colour display, medical diagnostics, etc 2. Nonlinear optical (NLO) materials play a major role in nonlinear optics and in particular they have a great impact on information technology and industrial applications. On account of the large flexibility for molecular design and higher nonlinear optical efficiency, there has been much progress in basic research on organic and semi-organic NLO materials. Semi organic crystals have large damage threshold, wide transparency range, less deliquescence, excellent nonlinear optical coefficient, low angular sensitivity and exceptional mechanical properties.
In the present work, a systematic investigation has been carried out on the growth of semi organic single crystals of potassium para-nitrophenolate dihydrate and the grown crystals have been subjected to single crystal X-ray diffraction analysis, optical, microhardness measurements, dielectric studies, and SHG, photoconductivity measurements and etching studies. The results of these studies have been discussed. Hence, the above properties of potassium para-nitrophenolate dihydrate were analyzed in the present work in order to substantiate the necessary requirements for the NLO behavior of the material potassium para-nitrophenolate dihydrate. In the present investigation, attention has been focused to confirm the dielectric behavior by the estimation of band gap energy using UV spectral analysis, dielectric studies. Mechanical properties were analysed to get an idea about the laser damage threshold. Photoconductivity studies have also been analysed to confirm the dielectric behavior which is responsible for the induced polarization in the medium. These studies were carried out to confirm dielectric behavior, non-centrosymmetric nature, polarization mechanism, mechanical stabilities of the material relevant to NLO behavior. The optical investigations and electrical conductivity studies are carried out first time for grown materials to find the suitability of the materials for device fabrications.

Materials and Methods

Experimental procedures

Semi organic single crystals of potassium para-nitrophenolate dihydrate were grown from paranitrophenol and potassium hydroxide, taken in equimolar ratio in an aqueous solution by the slow evaporation method. The solution was stirred continuously using a magnetic stirrer. The obtained saturated solution was further purified and allowed to evaporate at higher temperature which yields powder form of the synthesized potassium para-nitrophenolate dihydrate. Synthesized material was purified by repeated recrystallization process. Tiny seed crystals with good transparency were obtained due to spontaneous nucleation. Among them, a defect free seed crystal was selected and suspended in the mother solution, which was allowed to evaporate at room temperature. Large size single crystals were obtained due to the collection of monomers at the seed crystal sites from the mother solution, after the nucleation and growth processes were completed. In the modern world, the development of science in many areas has been achieved through the growth of single crystals. Large sized single crystals are essential for device fabrication and efforts are taken to grow large single crystals in short duration with less cost. The photograph of the grown potassium para-nitrophenolate dihydrate crystal is shown in Figure 1.

Results and discussion

Single Crystal XRD

Single crystal X-ray diffraction is an analytical technique to determine the actual arrangement of atoms within a crystalline specimen. Single crystal X-ray diffraction (XRD) is a non-destructive tool to analyze crystal structure of compounds, which can be grown as single crystals. XRD is employed for finding unit cell parameters, space groups and three-dimensional co-ordinates of atoms in the unit cell. The single crystal X-ray diffraction analysis of the grown crystals was carried out to identify the cell parameters using an ENRAF NONIUS CAD4 automatic X-ray diffractometer. The lattice parameters are estimated to be a = 22.076 Å, b = 3.682 Å, c = 21.283 Å and β = 111.52°, and hence the crystal belongs to the monoclinic crystal system and which agree well with the available reported literature values 3. The structure of the grown crystal has been confirmed by single-crystal XRD which is very good agreement with Mahadevan et al 3. The ORTEP representation of the molecule with atom numbering scheme is shown in Figure 2. The asymmetric unit of the crystal shows two water molecules, one nitrophenolate moiety and another potassium atom and nitrophenol moiety. The water, nitrophenolate and nitrophenol are co-ordinated to the potassium atom through oxygen. It is nitro oxygen that is bonded to potassium rather than the phenoxide oxygen of the nitrophenolate. The water molecules are co-ordinated to potassium ions of the asymmetric unit and its b-translation 4.

Powder X-Ray Diffraction

The powder X-ray diffraction analysis was carried out to confirm the crystallinity and also to ascertain the purity of the grown potassium para-nitrophenolate dihydrate crystal. Powder XRD pattern was recorded by scanning the sample over the range 10–70° at a scan speed of 0.02°/min. The recorded XRD pattern of potassium para-nitrophenolate dihydrate is shown in Figure 3 and the planes were indexed. The appearance of sharp and strong peaks confirms the good quality of the grown semi organic potassium para-nitrophenolate dihydrate crystal. Using the obtained data, the lattice parameters were calculated as a = 22.076 Å, b = 3.682 Å, c = 21.283 Å and β = 111.52° and the results are found to be in good agreement with the results of single crystal XRD and the already reported values 5.

UV - Visible Spectroscopy

The UV-Visible spectrum was recorded using Perkin Elmer Lamda Instrument. The spectrum gives information about the structure of the molecule because the absorption of UV and visible light involves promotion of the electron in the σ and π orbital from the ground state to higher states. The optical absorption spectrum of potassium para-nitrophenolate dihydrate crystal shown in Figure 4 was recorded between 500 and 2000 nm. The lower cut-off wavelength of potassium para-nitrophenolate dihydrate was found to be at 470 nm. This transparent nature in the visible region is a desirous property for the material used for NLO applications.
The band gap of the crystal was estimated from the relation in equation (1)

The band gap value was found to be 2.65 eV. The absence of absorption bands in the visible region and the wide band gap of the grown crystal confirm to the suitability of the grown crystal for photonic and optical applications. The lower cut off wavelength is found to be 470 nm for potassium para-nitrophenolate dihydrate which is in fairly good agreement with reported values of 500 nm Mahadevan et al 3.

Determination of Optical Constants

The extinction coefficient (K) can be obtained from the following equation,

The transmittance (T) is given by

Reflectance (R) in terms of absorption coefficient can be obtained from the above equation. Hence,

Refractive index (n) can be determined from reflectance data using the following equation,

The refractive index (n) was found to be 1.672 at λ = 800 nm. From the optical constants, electric susceptibility (χ_c) can be calculated according to the following relation 6.

Hence,

where ɛ₀ is the permittivity of free space. The value of electric susceptibility χ_c is 0.153 at λ= 800 nm. The real part dielectric constant ɛ_r and imaginary part dielectric constant ɛ_i can be calculated from the following relations 7.

The value of real ɛ_r and ε_i imaginary dielectric constants at λ = 800 nm were estimated as 1.582 and 4.7502 x 10^-5, respectively. The moderate values of refractive index and optical band gap suggest that the material has the required transmission range for NLO application. The lower value of dielectric constant and the positive value of the material are capable of producing induced polarization due to intense incident light radiation. The optical investigations carried out in the present work are new findings which show higher values of both refractive index (n) and electric susceptibility (χ_c) indicating high transparency of the crystal to confirm its suitability for optical switch device fabrications. Since there is no previous report about the optical investigations of this material, comparative study is not possible.

Refractive Index Measurements

The refractive index of the crystals can be determined by Brewster’s angle method using He-Ne laser of wavelength 632.8 nm. A polished flattened single crystal of is mounted on a rotating mount at an angle varying from 0 to 90 degrees. The angular readings on the rotary stage was observed, when the crystal is perfectly perpendicular to the intracavity beam. The crystal is rotated until the laser oscillates and the angle has been set for maximum power output. Brewster’s angle (θp) for the crystal is measured. The refractive index is calculated using the equation,

where θp is the polarizing angle. In the present work, finely polished crystals of the as grown potassium para-nitrophenolate dihydrate were used for refractive index measurements. These crystals were cleaved and are placed on a rotating mount at an angle varying from 0 to 90 degrees. He-Ne laser of wavelength 632.8 nm was used as the source. Brewster’s angle (θp) for potassium para-nitrophenolate dihydrate was measured to be 59.08 degrees. The refractive index has been calculated using the equation n = tan θp, where θp is the polarizing angle and it is found to be 1.67.

NLO Studies

Recent interest is focused on to find the materials which have suitable nonlinear optical properties for use as the active media in efficient second harmonic generators, tunable parametric oscillators and broadband electro-optic modulators. Kurtz and Perry proposed a powder SHG method for comprehensive analysis of the second order nonlinearity. In order to confirm the NLO behaviour in the title compound, powdered samples were subjected to the Kurtz and Perry powder technique. A Q-switched Nd: YAG laser beam of wavelength 1064 nm and 10 ns pulse width with an input rate of 10 Hz, was used to test the NLO property of the sample. The output of the grown crystal was measured as 6 mV while the KDP gave an SHG signal of 15 mV for the input beam energy of 4.7 mJ/Pulse. The second harmonic signal generated in the crystalline sample was confirmed by the emission of the green radiation from the crystal. Even though the SHG efficiency of potassium para-nitrophenolate dihydrate it can be used for applications in photonic and optoelectronic devices. The SHG efficiency is decreased due to the lower polarizing ability of the material. The nonlinear optical study confirms the SHG property of the material equal to 1.5 times of the standard KDP and this prediction is comparable with the reported value 1.52 by Mahadevan et al 3.

Mechanical Property

Hardness is an important factor in the choice of ceramics for abrasives, bearings, tool bits, wear resistance coatings etc. Hardness is a measure of resistance against lattice destruction or the resistance offered to permanent deformation or damage. Measurement of hardness is a destructive testing method to determine the mechanical behaviour of the materials. As pointed out by Shaw 8, the term hardness is having different meanings to different people depending upon their areas of interest. For example, it is the resistance to penetration to a metallurgist, the resistance to cutting to a machinist, the resistance to wear and tear to a lubrication engineer and a measure of flow of stress to a design engineer. All these actions are related to the plastic stress of the material. For hard and brittle materials, the hardness test has proved to be a valuable technique in the general study of plastic deformation 9. The hardness depends not only on the properties of the materials under test but also largely on the conditions of measurement. Microhardness tests have been applied to fine components of clock and instrument mechanisms, thin metal strip, foils, wires, metallic fibers, thin galvanic coatings, artificial oxide films, etc., as well as the thin surface layers of metals which change their properties as a result of mechanical treatments such as machining, rolling, friction and other effects. The microhardness method is widely used for studying the individual structural constituent elements of metallic alloys, minerals, glasses, enamels and artificial abrasives.
Vickers test is said to be a more reliable method of hardness measurement. In order to get similar geometrical impression under varying loads, Smith and Sand land 10 suggested that a pyramid at tip substituted for a ball. The mechanical strength of a material plays a key role in device fabrications. It is a measure of the resistance, the lattice offers to local deformation 11. A smooth and flat surface of the grown crystal was subjected to a hardness study at room temperature, with the load range of 5–50 g using the Vickers hardness tester fitted with a diamond pyramidal indenter and attached to an incident light microscope. The indentation time was kept as 5 s for all the loads. The Vickers hardness number was calculated using the relation,

where P is the applied load in kg and d is the diagonal length of the indentation impression in mm. The variation of hardness value, H_v with the load P is shown in Figure 5. From the profile, it is observed that the hardness increase with an increase in the load satisfies the reverse indentation size effect (ISE). The well-known Meyer’s law gives the relationship between the load and size of the indentation as,

where k₁ and n are constants for a particular material. By plotting log P against log d, (figure 6) the values of work hardening coefficient was calculated as 3.45, which is greater than 2, establishing that the hardness increases with increase of load. The large value of n indicates large effect of dislocations. According to Onitisch 12, if n is greater than 2, the microhardness will increase with the increase of load. From the hardness study, the grown potassium para-nitrophenolate dihydrate crystal is found to be relatively soft material.

The elastic stiffness constant (C₁₁) was calculated using Wooster’s empirical relation as 13.

As indentation initiates plastic deformation in a crystal, which is highly directional in nature the hardness measurement may be a function of the orientation of the indented crystal. The variation of Vickers hardness number (H_v) as a function of applied load on (012) plane shows that H_v increases with increase of load. The calculated stiffness constant for different load was tabulated (Table 1). Since there is no previous report about the stiffness constant of this material, comparative study is not possible.

Dielectric Study

The useful method of characterization of electrical response is the dielectric studies. A study on the dielectric properties of solids gives an electric field distribution within solid. The frequency dependence of these properties gives great insight into the materials applications. The range of measurement depends on the properties and the materials of interest. From the study of dielectric constant as a function of frequency, temperature etc., the different polarization mechanisms in solids such as atomic polarization of the dipoles, space-charge polarization etc., can be understood. One of the most important parameters widely used is the dielectric constant or relative permittivity. The dielectric constant of a material may be defined as the ratio of the field strength in vacuum to that in the material for the same distribution of charge. The dielectric constant of a substance is a property of the constituent ions. The dielectric characteristics of the material are important to study the lattice dynamics in the crystal. A sample of having silver coating on the opposite faces was placed between the two copper electrodes and a parallel plate capacitor was thus formed. The cut and polished single crystals of potassium para-nitrophenolate dihydrate were used for dielectric studies. The surface of the sample was electroded with silver paste for good ohmic contact. The dielectric studies of the grown crystal were carried out by using the instrument, HIOCKI 3532-50 LCR HITESTER. The capacitance of the sample was measured by varying the frequency from 50 Hz to 5 MHz and the graph is plotted between dielectric constant Vs logarithmic frequency is (Figure 7). Figure 7 shows the variation of the dielectric constant with log frequency at different temperatures. From the plot, it is observed that the dielectric constant is relatively higher in the region of 50 Hz–5 MHz and decreases further with an increase in the frequency, and this trend continues up to 5 MHz. After this, the dielectric constant remains almost constant at all other higher frequencies. The high value of the dielectric constant at low frequency is due to the presence of electronic, ionic, dipolar and space charge polarizations 14. In accordance with the Miller rule, the lower value of the dielectric constant at higher frequencies is a suitable parameter for the enhancement of the SHG coefficient 15. The variation of the dielectric loss with frequency is shown in Figure 8. The characteristic of a low dielectric loss with high frequency for the sample suggests that the crystal possesses enhanced optical quality with lesser defects and this parameter plays a vital role in the fabrication of nonlinear optical devices 16.
In the proposed relation only one parameter viz., the high frequency dielectric constant, is required as the input, to evaluate the electronic properties like valence electron plasma energy, average energy gap or Penn gap, Fermi energy and electronic polarizability of the potassium para-nitrophenolate dihydrate crystals. Theoretical calculations shows that the high frequency dielectric constant is explicitly dependent on the valence electron Plasmon energy, an average energy gap referred to as the Penn gap and the Fermi energy. The Penn gap is determined by fitting the dielectric constant with the Plasmon energy 17, 18. The electrical properties are necessary to analyze second harmonic generation efficiency of the title compound. The valence electron plasma energy, ћω_p, is calculated using the relation 19.

According to the Penn model 20, the average energy gap for potassium para-nitrophenolate dihydrate is given by

where ћω_p is the valence electron Plasmon energy and the Fermi energy 17 given by

Then we obtained electronic polarizability α using a relation 20, 21.

where S₀ is a constant given by

The value of α obtained from equation (16) closely matches with that obtained using Clausius-Mossotti relation,

Considering that the polarizability is highly sensitive to the band gap 22, the following empirical relationship is also used to calculate,

where E_g is the band gap value determined through the UV transmission spectrum. The high frequency dielectric constant of materials is a very important parameter for calculating the physical or electronic properties of materials. These values of potassium para-nitrophenolate dihydrate are compared with the standard material KDP and listed in Table 2.

From the table, it is observed that electrical properties are found to be higher than those of KDP. In particular, the polarizability of potassium para-nitrophenolate dihydrate is found to be 1.5 times higher than that of KDP. As the SHG efficiency depends upon the polarizability, the SHG efficiency is also found to be 1.5 times more than that of KDP. The polarizability calculated from Penn gap analysis agrees very well with the results obtained using standard Claussius –Mossotti relation. Hence, this new findings are very much useful to reveal the dielectric behavior of the crystal.

Photoconductivity

Photoconductivity measurements were carried out on a cut and polished sample of the grown single crystal by fixing it onto a microscope slide. The sample was connected in series with a DC power supply and KEITHLEY 485 Picoammeter. The sample was then exposed to light radiation and the photocurrent was recorded for the same values of the applied voltage. The field dependence of dark and photo currents of grown crystal is shown in Figure 9. The photocurrent is found to be less than the dark current at every applied electric field. This phenomenon is known as negative photoconductivity. Generally, this may be attributed to the loss of water molecules in the crystal 23. However, the negative photoconductivity in this case may be due to the reduction in the number of charge carriers or their lifetime in the presence of radiation 24. The decrease in lifetime with illumination could be due to the trapping process and increase in carrier velocity according to the relation,

where v is the thermal velocity of the carriers, s is the capture cross-section of the recombination centers and N is the carrier concentration. As intense light falls on the sample, the lifetime of the characterization decreases. In the Stockmann model, a two level scheme is proposed to explain negative photoconductivity 25. As a result, the recombination of electrons and holes takes place resulting in decrease in the number of mobile charge carriers, giving rise to negative photoconductivity.
Figure 9 shows the dependence of the dark current and photocurrent, with respect to the applied field at room temperature. The dark current and photocurrent increases linearly with respect to the applied field. At every instant, the dark current is greater than the photocurrent, which is due to the negative photoconductivity. This may be attributed to the decrease in either the number of free charge carriers or their lifetime when subjected to radiation. According to the Stockmann model, the forbidden gap in the material contains two energy levels in which one is situated between the Fermi level and the conduction band, while the other is located close to the valence band. The second state has high capture cross-sections for electrons and holes.
As it captures electrons from the conduction band and holes from the valence band, the number of charge carriers in the conduction bands gets reduced, and the current decreases in the presence of radiation. Thus the crystal is said to exhibit a negative photoconducting effect. The negative photoconductivity of the sample may be due to the reduction in the number of charge carriers to reveal the dielectric nature of the material. Hence, this new findings are very much useful to reveal the dielectric behavior of the crystal.

Etching Studies

Etching is a technique which is used to reveal the defects in crystals like dislocations, growth bands, twin boundaries, point defects etc. Normally when the crystal is dissolved in the solvent, well defined etch pits are formed. The formation of the etch pits is assumed to be the reverse of growth process. Etchants employed to reveal dislocations are taken in homologous series of water. Etching of the surfaces was carried out by dipping the plates in etchants for few seconds to few minutes at room temperature and then wiping them with dry filter paper. Etch patterns were observed and photographed under an optical (Carl-Zesis Jenavert) microscope in the reflected light. Elongated circular etch pits were ob-served when potassium para-nitrophenolate dihydrate single crystal was etched with water for five seconds as shown in Figure 10.

Conclusions

Single crystal of semi organic potassium para-nitrophenolate dihydrate has been grown by the slow evaporation technique. The single crystal XRD analysis confirms that the crystal belongs to the monoclinic system. The transparency nature of the crystal in the visible and infrared region from the absorption spectrum confirms the NLO property of the crystal.The optical constants like optical band gap (E_g), refractive index (n), electric susceptibility (χ_c) and dielectric constants were calculated to study the transparent and dielectric nature of the material. From the microhardness test, it is observed that the hardness number H_v increases with the increase in the load and the value of the Meyer index number or the work hardening coefficient n has been calculated as 3.45, which is greater than 2 confirming the increasing trend of hardness with the load. The high value of the work hardening coefficient shows higher dislocation in the grown crystal since the dislocations present in the crystal cause a work hardening coefficient. The calculation of the stiffness constant (C₁₁) reveals that the binding force between the ions is quite strong. The nonlinear optical study confirms the SHG property of the material. The dielectric constant and dielectric loss have been studied as a function of frequency at different temperatures. The electrical properties such as plasma energy, Penn gap, Fermi energy and polarizability were calculated. The higher value of polarizability indicates that the second harmonic generation efficiency is more than that of standard material KDP. The theoretical prediction of SHG efficiency was confirmed by Kurtz and Perry powder technique. However, this material can be used in photonic and optoelectronic devices which more stability. The photoconductivity studies confirm that the crystal possesses negative photoconductivity. From the etching studies well defined etch pits were observed on the surface of the specimen. It may due to vacancy of atoms on the surface of the specimen.

Acknowledgements

The author thanks the management and principal of Sree Sastha Institute of Engineering and Technology, Chembarambakkam, Chennai-600123 for their encouragements throughout this work.

Author Contributions

This work was carried out only by the author Suresh Sagadevan.

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