Structural, optical, and mechanical properties of gamma beam-irradiated pure and CeCl3-doped potassium hydrogen phthalate (KHP) crystals for scintillating applications

Potassium hydrogen phthalate (KHP) is a high-energy scintillating material having superior properties. The solution crystal growth technique at room temperature is employed for growing undoped and CeCl₃-doped KHP crystals. The ⁶⁰Co γ-ray-irradiated crystals were analyzed through powder X-ray diffraction analysis to evaluate the cell parameters of the pure and doped KHP crystals. The functional groups and vibration modes were identified by Fourier transform infrared spectral analysis. The optical transparency of the grown crystals was analyzed by UV–vis spectroscopy which shows less absorption in the CeCl₃-doped KHP crystals when compared to pure KHP. The vibration properties were discussed with the use of Raman spectra of pure and doped crystals. The photoluminescence analysis reveals broad peaks from green to violet ranges for undoped and CeCl₃-doped KHP crystals. The microhardness test was carried out at different applied loads varying in the range 25–100 g. It is revealed that the Vicker's hardness values (Hv) increase with increasing applied load. The Meyer's index number (n) is determined and it confirms that the gamma-irradiated pure, CeCl₃-doped KHP crystals belong to the soft material category. The mechanical properties such as the elastic stiffness constant (C₁₁), yield strength (σ_v), brittleness index (Bi), and the fracture toughness (K_c) are measured for both pure and 1–4 mol% CeCl₃-doped KHP crystals.

The earlier investigations mainly focused on pseudo-organic NLO materials because of their qualitatively latent application in piezo and pyroelectric devices. The potassium hydrogen phthalate crystals, C8H5KO4 (KHP), crystallize in orthorhombic space group Pca21 and have four molecules for every each unit cell [[1]]. The high-energy gamma radiation causes impairment in the materials. The energy relocates to the atom in exceeding binding energy; the atom cannot collide from its lattice site to obtain the formation of flaw and glow centers [[3]]. Sodium, potassium, thallium, rubidium, urea, thiourea, cesium chloride, cerium chloride hydro-phthalate, and KHP crystals are the promising materials for subjective X-ray beam investigation of light components such as Si, Al, Mg, and Fe in all longwave spectral regions. Radioactive material such as cerium chloride-doped KHP emits radiation. Pure KHP crystals doped with Ce3+ ions have been divulged /described as potential radiation dose materials [[4]]. In this present study, to access the quality of the grown KHP crystals, the powder XRD, photoluminescence, and Raman spectroscopy analysis are performed. As one of the most significant properties, the scintillation light yield is evaluated from emission spectra acquired by pure and cerium chloride-doped KHP samples under the γ-ray excitation using Co60 source [[5]]. The higher energy gamma beam 60Co source relies upon the exactness of the radiation portion conveyed displacement damage in cerium chloride-doped materials. Due to the immense material properties of CeCl3-doped crystal, the present work is focused on the growth of unadulterated & CeCl3-doped potassium hydrogen phthalate (KHP) mono-crystals and the effect of γ-ray irradiation analysis on the as-grown crystal samples.

The present study materials such as potassium hydrogen phthalate (KHP) and cerium chloride (CeCl3) chemicals are of analytical reagent grade. The CeCl3-doped KHP crystals are grown by a slow evaporation solution growth technique. Distilled water is received from the deionizer water pre-filtration unit. The resistivity of the used distilled water is 18.4 MΩ cm. A clean container of 500 mL with double distilled water is taken and KHP salt added to it by stirring for 3 h with the help of a magnetic stirrer at ambient temperature. Potassium hydrogen phthalate salt is continuously added until the liquid gets saturated. Now, 1 mol%, 2 mol%, 3 mol%, and 4 mol% of CeCl3 are chosen as dopant materials and added to KHP in four separate beakers, and distilled water is to be added and to be stirred continuously for 3 h till it becomes saturated. Then the mixture is filtered using a filter paper. The solutions are kept in Petri-dish with the top closed with a perforated sheet. Small crystals will begin to appear in 7–10 days. The transparent good crystals are harvested after 3–4 weeks. The photos of the pure KHP crystals are displayed in Fig. 1 and CeCl3-doped KHP crystals are viewed in Fig. 2. The well-grown crystals are selected for gamma irradiation.

Graph: Fig. 1 Pure potassium hydrogen phthalate (KHP)

Graph: Fig. 2 (a) 1 mol% CeCl3-doped KHP, (b) 2 mol% CeCl3-doped KHP, (c) 3 mol% CeCl3-doped KHP, (d) 4 mol% CeCl3-doped KHP

Such developed crystals are irradiated with γ-ray using 60Co irradiation source at the focal γ- container-5000 were using a protective and lightweight container is placed at CIF, Pondicherry University, India. The gamma beam-irradiated crystals were subjected to Powder XRD and were recorded in the range of 10°–60° in steps by using SEIFERT DIFFRACTOMETER with CuKα radiation with a wavelength of 1.54056 Å for the phase analysis. The γ-ray-irradiated pure and 1–4 mol% CeCl3-doped KHP crystals were subjected to optical absorption wavelength recorded from 250 to 400 nm using SHIMADZU UV–vis spectrometer. Photoluminescence is recorded with FLOUROLOG-3 fluorometer and a 450 W xenon lamp is used as the source and PMT as the detector. The double grating is used for excitation and emission spectrometer. FTIR analysis was done with Perkin Elmer BX Spectrometer by KBr pellet technique and the gamma-irradiated pure and 1–4 mol% CeCl3-doped KHP crystals. FTIR spectra were obtained from 400 to 4000 cm−1. A dispersive Raman spectrometer (Ranishow RM 2000) was used to the semiconductor laser diode (Argon-ion) of excitation wavelength of 514.5 nm. A combination of Vickers microhardness tester and optical microscope (Micro-Duromt 4000E) is used to find the microhardness of the grown crystals (Figs. 3 and 4).

Graph: Fig. 3 After irradiation, pure KHP and CeCl3-doped KHP crystals show 50–60% fading of absorbance in the first 24 h of exposure under environmental conditions

Graph: Fig. 4 A plot between gamma beam Co60 irradiation dose and change in optical absorbance dose for pure and CeCl3-doped KHP

Undoped KHP and 1–4 mol% CeCl3-doped KHP crystals are highly irradiated at various doses from 0.6 to 9 kGy with average γ-ray value of 1.25 × 106 eV. The self-supporting irradiators are specially designed for research and industrial, Nanotechnology, the Biodegradation process, NLO Applications such as pure KHP, CeCl3-doped KHP irradiation for precluding playback—sterilization devices [[7]]. A huge greater number of these are dry-storage irradiator chambers Co60 or Cs137 radioactive sources are suitable for radiation research [[8]]. These characteristics are performed by surrounding for undoped and 1–4 mol% CeCl3-doped KHP crystals with radiation sources excited in all directions. The sample is placed inside the chamber while pressing the control panel, the light beam is suddenly conveyed to the irradiation. At that point, the groundwork for illumination of an undoped and CeCl3-doped KHP test holder has been set in the stacking position. Depending on the dose rate of the day minimum 6 min, the clock on the control board is set to rely on the portion-grown crystals that were irradiated with various dosages 0.6 kGy, 0.7 kGy, 1 kGy, and 9 kGy, respectively. Then in 3 h, continually these were given to non-doped KHP and CeCl3-doped KHP crystals. This examination has a wide using Cobalt-60 irradiation optical dose radiation on the samples. It was observed that the non-doped and doped KHP crystals show a 50–60% blurring of absorbance in the 1 or 2 days of presentation under the room temperature. The effect of 60Co gamma-irradiated pure and 1–4 mol% CeCl3-doped KHP crystals show that there are no structural phase changes but changes in the lattice parameters only, because of gamma irradiation which was identified by the structural refinement performed on the irradiated KHP crystals. The color of crystals changes normally white to yellow because of the influence of Co60 using γ- ray highly irradiated for CeCl3-doped KHP crystal and a very minor change happened in the pure KHP crystals.

The powder XRD patterns of 60Co γ- ray-irradiated undoped and CeCl3 -doped KHP crystals are shown in Fig. 5. The crystallinity of pure and 1–4 mol% CeCl3-doped KHP crystal is quite clear from diffractograms using the presence of sharp peaks at specific Bragg's angles [[9]]. From the diffractograms, it's clear that the dopant of Ce3+ions in the altered formation of CeCl3-doped potassium hydrogen phthalate crystals leads to a shift in the positions of a peak shows that the doping has been brought about a change in the internal structure of crystals because of the change in bond length. The shifts appear from when the dopant is assimilated into the intermittent crystal lattice order PXRD diffraction analysis is performed to identify the crystallinity and crystal phase of grown crystals. The XRD data of pure was compared with the JCPDS data No.31-1855. The lattice constants a, b, and c and volume 'V' of the crystals were calculated for the orthorhombic crystal system using the following equations:

$$\lambda =2d_{\text{hkl}}sin{\theta }_{\text{hkl}}$$

Graph

$$\frac{1}{d^2}=\frac{h^2}{a^2}+\frac{k^2}{b^2}+\frac{l^2}{c^2}$$

Graph

where d is the lattice space, (h, k, l) is the miller indices, a, b, c are lattice parameters, λ is the wavelength, and 2θ is the diffraction angle. The Miller indices (h, k, l) of the alike planes were recorded. The value of lattice constants for undoped & doped crystals has been determined and the comparison of crystallographic data is given in Table 1 [[10]]. It is observed from the spectrum a change in relative intensity of the peak shift in the angular position of the peaks. Hence, the dopant has entered the crystalline matrix without much distortion and produced lattice strain. The lattice constant values for pure and CeCl3-doped KHP crystals measurable data show minor changes in the lattice parameter values and unit cell volume. The ionic radius of the dopant Ce3+ (115 pm) comparable to that of K (151 pm). It confirms that the dopant drop in the KHP crystalline matrix without causing so much distortion. The Braggs reflections were indexed for pure and CeCl3-doped KHP crystals using the cell parameters. The diffraction curves show a set of eminent peaks corresponding to (110), (111), (130), (140), (141), (151), (142), (113), (402), and (342) planes. In 1–4 mol% CeCl3-doped KHP, the peak intensities of reflections (140) and (142) increased with doping, and the peak intensities of reflections (111), (130), (151), (121), and (113) have been reduced with doping. The peaks (110), (402), and (342) are disappeared in the doping. Comparing the powder XRD patterns of undoped & CeCl3-doped KHP crystals, there is a small shift in the doped KHP crystals. This confirms the incorporation of cerium value in the pure KHP crystal lattice. It can be seen that there are no changes in the phase structure of gamma beam-irradiated crystals; however, there change in lattice parameters. The value of lattice constants for pure and doped crystals has been determined and the comparison of crystallographic data is given in Table 1.

Graph: Fig. 5 XRD pattern of pure and CeCl3-doped KHP crystals

Table 1 Comparative unit cell parameters for pure and CeCl3-doped KHP crystals

Figure 6 shows the absorption transmission spectra corresponding to pure and CeCl3-doped KHP crystals. The optical absorption increases in gamma dose for 1–4 mol% CeCl3-doped KHP crystals. The effects of gamma irradiation are more on CeCl3-doped KHP crystals, but a very small change in the undoped KHP crystals. Excitation of a bound electron from higher occupied molecular orbital increases the spatial intensity of the electron diffusion. The absorption in the entire visible region of the Ce3+ ion-doped KHP crystals increased as that of pure KHP crystals. The absorption coefficient (α) obeys the following relation hυ = A(hυ − Eg)1/2, where 'A' is a coefficient constant, 'Eg' is the optical bandgap, 'h' is the Planck's value (h = 6.625 × 10–34 J/s), the absorption coefficient α = 2.303/t log (1/T) [[12]], and 't' is the sample thickness (3 mm). The value of the bandgap was found to be 4.09 eV and 5.42 eV, 4.92 eV, 4.95 eV, and 5.33 eV, respectively, for pure and CeCl3-doped KHP crystals. But as the doping 1–4 mol%, CeCl3-doped crystal doping level is increased and the transmittance curve decreases due to the interstitially occupied dopant produces a defect in the crystals, developing more grain boundaries. The lower cut-off wavelength for the pure KHP at around 325 nm and 1–4 mol% CeCl3-doped KHP crystals was found to be at around 325 nm, 319 nm, 319.86 nm, and 318.70 nm. The orbital P electron shift in cerium arises from the mesomeric effect. This P electron shift is bound for nonlinear optical response and absorption in the near UV region. As the doping level increased, the absorption curve decreases because interstitially dopant produces dislocations in the crystals developing more grain boundaries. The grown crystals have a good transmission in UV as well as the visible region.

Graph: Fig. 6 UV–vis absorbance spectra for pure & CeCl3-doped KHP crystals

Figure 7 shows the recorded photoluminescence spectrum of the pure KHP 1–4% mol CeCl3-doped KHP crystals that were scanned between 400 and 800 nm. For the PL spectrum of pure and 1–4 mol% CeCl3-doped KHP crystals when excited with the laser of wavelength 518.63 nm, 566.03 nm, 566.03 nm, 569.27 nm, 515.59 nm, respectively. The results indicate that grown crystals have a bright emission of the visible region and the broad peaks from green to violet emissions for pure and doped KHP crystals observed [[14]]. The broad peaks and greenish emission of high-intensity peaks are observed at 569.27 nm and 694.84 nm, respectively, for pure KHP and cerium chloride-doped KHP crystals which confirm that they emit green fluorescence to suggest that excellent for NLO applications and scintillators [[15]]. There is no significant change in peak but a shift from 415 to 518 nm which is observed with increased intensity. It shows the enrichment in fluorescence. A strong emission peak or the maximum intensity peak is observed at 694.84 nm which is assigned to a lattice related process. At a higher wavelength region, the photoluminescence intensity is slowly reduced.

Graph: Fig. 7 Photoluminescence spectra for pure and CeCl3-doped KHP

The FTIR spectra of undoped and CeCl3-doped KHP crystals are shown in Fig. 8. The IR radiation promotes transition in a molecule between spinning and reverberation energy levels of the ground electronic energy state. Here, the dopant Ce3+ ions enter into the interstitial sites of the pure KHP crystals without changing the carboxylate hydrogen ion. The characteristic FTIR spectrum exposes the presence of K–H bond, aldehyde bond, alkenes bond, and carboxylate bond. The FTIR spectra of irradiated pure and CeCl3-doped KHP samples are almost similar. A minor change in vibrational frequencies is revealed. The absorption peak [[17]] OH appears at 3470 cm−1, 2790 cm−1, 2620 cm−1 , and 2480 cm−1 corresponding to OH stretching in the pure KHP crystals. These frequencies are compared with doped cerium chloride compounds, and there was a shift in the range of 3470–3421 cm−1. The absorption peaks at 3650 cm−1& 4000 cm−1correspond to K–H symmetric stretching for pure and CeCl3-doped KHP crystals. When the frequencies are compared with the doped cerium chloride KHP compound, there would be a shift in the frequency range 3420–3470 cm−1. This shift towards higher energy is due to interstitial cerium in the doped KHP. The broadband lying in the range of 1750 cm−1, 1950 cm−1 corresponds to C=C extended. In addition to that, the carboxylate group C–O–O proportional stretching is present in the range of 2350 cm−1 to 2700 cm−1. The vibrational frequency at 3503 cm−1 and 3510 cm−1, 3496 cm−1, 3518 cm−1, and 3503 cm−1 is due to K–H stretching for pure and 1–4 mol% CeCl3-doped KHP crystals. The C–H stretches at 897 cm−1& C–O vibrate at 684 cm−1, 687 cm−1 to 776 cm−1. The FTIR spectra of gamma beam undoped KHP and CeCl3-doped KHP crystals are almost similar but little change in vibrational frequencies. It is identified that the chemical constituent of the crystals remains unchanged. Destruction of few absorption bands and new absorption peaks were noticed. It shows the enhancement of the amorphous nature of the gamma beam using cobalt-60-irradiated KHP crystals. This observed bands with vibrational frequency and their assignments are presented in Table 2.

Graph: Fig. 8 The FTIR Spectrum for pure & CeCl3-doped KHP

Table 2 FTIR frequency assignments of pure KHP and CeCl3-doped KHP crytals

The Laser Raman spectrum recorded for γ-ray-irradiated pure KHP and 4 mol% cerium-doped KHP crystal is in the region of 200–2000 cm−1. The peak positions of the viewed bands are given in Table 3. Laser Raman spectra for pure and CeCl3-doped KHP crystals are shown in Fig. 9. With this Raman system, it could detect and specify the different gyration modes of various insecure organic compounds [[19]]. The results showed the shift in frequencies range of ± 2 cm−1, which indicates reasonable accuracy, simplicity of the Raman system. Then, the surface enhancement technique of the Raman spectrum was employed in the present system of investigation. The aromatic ring C–H bending vibrations produce a peak below 1100 cm−1. The O–H bending vibration of the carboxyl group gives an intense peak around 1700 cm−1. The ring C–O–O extended vibration occurs at 1750–1857 cm−1. Laser Raman is due to asymmetric stretching of C–O–O. It can be observed Raman spectra show a strong conversion peak at 1043.53 cm−1 due to K–H stretching for pure and cerium-doped KHP crystals. Raman shift in relative wavenumbers concern the excitation radiation, absolute wave number υ◦ = 1/λ◦, where λ◦ is the wavelength of the excitation radiation (514.5 nm). Its use of lasers is necessary because of the low intensity of the scattered light. The absorption of energy is given by Einstein's equation E = hυ◦, where h is the Planck's value and υ◦. Absolute wave number. Raman shift [[20]] in wavelength ∆λR, J = λR, J − λ◦, where λR, J absolute wavenumber for a stokes Raman line (Table 4).

Table 3 Laser Raman spectra analysis assignments for pure & CeCl3-doped KHP

Graph: Fig. 9 Laser Raman spectra for pure and CeCl3-doped KHP

Table 4 Microhardness parameter values for pure and CeCl3-doped KHP crystals

Table 4 Microhardness parameter values for pure and CeCl3-doped KHP crystals

In this present investigation, the microhardness measurements were carried out on the undoped & 1–4 mol% do CeCl3-doped KHP crystals using a microhardness tester. In the present study, material hardness value and Mayer's index number (n) that is based on the structural crystalline solid [[22]] have been found out. Further, the number of bonds per unit volume of the Co60-irradiated KHP crystal elastic coefficient (C11), fracture toughness value (Kc), the brittleness indices(Bi), and yield strength(σv) were estimated [[23]]. This test was done at different applied loads of indentation range 25–100 g with a uniform indentation time interval 25 s each load. Vicker's hardness value has been determined using this relation Hv = 1.854 kg $\frac{\mathrm{P}}{{\mathrm{d}}^2}$ /mm2. Here, P is the applied load in kg, and d is the mean Stroke length in mm. A graph plotted between hardness numbers Hv with the applied load P is shown in Fig. 10. It is observed from the graph that the value of hardness increases and attains almost saturation with the increase of an applied load due to the influence of γ-ray irradiated KHP. In Fig. 11, a plot obtained with log P and log d gives a straight line, which is derived from the relation connecting the applied load P = adn, where n is the Meyer index [[25]] work hardening coefficient and 'a' is the constant for a given substance. The work hardening coefficient has been calculated from the slope of the straight line. Here, the hard materials 'n' lies between 1.397 and 2.0 and it is having more than 1.7 for soft materials. The value of the work hardening coefficient of pure KHP and cerium-doped crystal was found to be 1.698 and 1.727, which shows that the material is soft. For the pure KHP and cerium-doped KHP materials showing the increase in Hv, the Meyer index n is between 1.3 and 2. The elastic stiffness C11 for various loads has been determined using the relation C11 = Hv7/4 [[27]]. Figure 12 shows the plot in between the load (P) and elastic stiffness (C11). It is observed that an increase in Stiffness constant with an increase in load is observed which explains the bonding nature of the irradiated pure KHP and cerium ions-doped KHP. The yield strength is an important property for NLO fabrication which was calculated by relation (Fig. 13) (σv) = $\frac{{\mathrm{H}}_{\mathrm{V}}}{3}$ (0.1)n−2. The fracture toughness could be determined using this relation of (Kc) = $\frac{\mathrm{P}}{\mathrm{\beta }}$ C3/2 [[29]] where 'C' is the crack length and β' is the geometrical constant. For Vicker's indenter β = 7, the fracture toughness explains about the ability of the KHP crystals and has been calculated for each load by the following relation, (Bi) = $\frac{{\mathrm{H}}_{\mathrm{V}}}{{\mathrm{K}}_{\mathrm{C}}}$ [[30]]. Figure 14 is the plot drawn in between the load P and Brittleness index (Bi), which shows decreases in the Brittleness index with the increases in the load(P) due to the influence of Co60 gamma irradiation ions for the pure and doping materials. This microhardness test reveals that the CeCl3-doped KHP crystals have more elastic stability, well suitable yield strength, and low brittleness to control the fracture by the mechanical properties.

Graph: Fig. 10 Plot of Load P with hardness values (Hv) for pure and CeCl3-doped KHP

Graph: Fig. 11 Plot of Log P with log d for pure and CeCl3-doped KHP

Graph: Fig. 12 Plot of Load P with elastic stiffness constant (C11) for pure, CeCl3-doped KHP

Graph: Fig. 13 Plot of Load P with yield strength (σv) for pure, CeCl3-doped KHP

Graph: Fig. 14 Plot of load P with brittleness index (Bi) for pure, CeCl3-doped KHP

We have successfully grown pure and CeCl3-doped KHP crystals by slow evaporation solution growth technique. Gamma beam cobalt-60-irradiated KHP crystals appear such as slight color changes because of gamma-induced effect. The powder XRD of irradiated pure KHP and 1–4 mol% CeCl3-doped KHP samples confirms no structural changes but there are lattice parameter changes only due to lattice strain developed. Further gamma beam-irradiated KHP crystals have the functional groups COO–, OH, C=C, and CHO. The functional groups confirm the carboxyl groups present in the CeCl3-doped KHP crystals identified by FTIR studies. The powder XRD confirms its structure and crystalline perfection. The lattice parameters for undoped KHP crystals are found to be a = 9.625 Å, b = 13.319 Å, c = 6.460 Å; for 4 M % doped KHP, a = 9.670 Å, b = 13.401 Å, c = 6.530 Å. UV–vis studies confirm that absorption increases, with decreases in radiation dose due to the formation of defects and gamma irradiation effect. Raman laser spectrum confirms the carboxyl groups present in the CeCl3-doped KHP crystals. Photoluminescence emission spectra confirm radioactive recombination indicates that the crystals have green fluorescence emission and an increase in the intensity of CeCl3-doped KHP crystals. A low percentage of dopants are superior when compared to a pure and high percentage of dopants. This proves KHP doped with a low percentage of dopants could be broadly utilized for optical applications. Microhardness studies revealed the soft material category and also mechanical properties such as Vickers microhardness number, elastic stiffness, yield strength, fracture toughness, brittleness, and work hardening coefficient of undoped and doped KHP were determined by microhardness analysis.

The authors thank Karunya Institute of Technology and Sciences (KITS), Coimbatore, for providing the required facilities to carryout this research work. Also, the authors express their heart felt gratitude to KITS, Coimbatore, for extending the necessary characterization facilities that we have utilized in this research paper.

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