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Abstract
Ammonium dihydrogen phosphate (NH4H2PO4, ADP) is an excellent nonlinear optical crystal and has had wide application. It was the same type of potassium dihydrogen phosphate (KH2PO4, KDP) with a similar crystal structure. The difference in structure and properties of ADP crystal in the presence of defects, especially cluster defects, was one of the important issues of concern in the research. In this work, first-principles calculation, coupled with HSE06 functional and the van der Waals-Wannier function method, was applied to investigate the structural stability and electronic properties induced by oxygen vacancy cluster defects and FeP2-+VO2+ cluster defects. And some spectra experiments, such as Raman spectroscopy, the Fourier transform infrared spectroscopy and the ultraviolet absorption spectroscopy, were also applied to investigate the detailed influence for ADP crystal doped with different Fe3+ concentration, which was grown with the “point-seed” rapid growth method. Combined with the theoretical results and the spectra tests, it confirmed that the structural changes in ADP crystal caused by oxygen vacancy cluster defect and FeP2-+VO2+ cluster defect were smaller than that in KDP crystal, mainly due to the restriction of hydrogen bonds and NH4+ group. With the increase of defect concentration, the microstructure stress could also damage the crystal structure due to the microscopic stress induced by Fe3+. The defect states moved towards right from 1.1 eV to 6.6 eV with the concentration of oxygen vacancy increasing. Similarly, the defect state composed of Fe 3d and O 2p states induced by FeP2- defect also moved to the conduction band minimum. The absorption peaks around 220-350 nm induced by FeP2- defect and FeP2-+VO2+ cluster defect were along the xy plane. It provided a good suggestion based on the calculation that it was very important to minimize defects or control cluster defect concentration during crystal growth.
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1. Introduction
The ammonium dihydrogen phosphate (NH4H2PO4, ADP) was the analogue with potassium dihydrogen phosphate (KH2PO4, KDP) crystal, belonging to tetragonal system and space group for $D_{2{\rm{d}}}^{12} - I\mathop 4\limits^ - 2{\rm{d}}$ [1–3]. Therefore, the two crystals both possessed similar structures, the excellent nonlinear and electro-optical properties [4]. It could be applied for photoelectric modulation and frequency conversion devices, realizing second, third, fourth and even fifth harmonic generation [5–8]. Compared with the KDP crystal, ADP exhibited a higher laser damage threshold and its application had been attracted broad attention.
Firstly, several experimental researches have shown that metal impurity ions greatly affected the optical properties and destroyed the damage threshold of crystals [9–22]. For examples, Graces studied the optical absorption and charge transfer of Fe-doped KDP crystals with electron paramagnetic resonance method. The results showed that the absorption peak introduced by Fe3+ was near 270 nm caused by replacing K+, and the peak around 200-300 nm was mainly due to the replacement of P atom. At the same time, the effect of Fe3+ on the growth of KDP crystal was investigated by Wang et.al. It was found that a certain concentration of doping amount could improve the stability of the solution, while it had a great influence on the crystal prismatic sector. More importantly, there are a variety of trace impurity ions on the raw materials for crystal growth [23–28]. Fe3+ was the most common and representative metal impurity ion, and its influence on crystal properties could not be ignored. However, due to the limitations of experimental techniques, the effect mechanism of Fe doped defects on crystal properties and damage was still not clear. Thus, theoretical method was necessary in this case. As a similar crystal of KDP, the influence of metal ions on the crystal structure and properties of ADP cannot be ignored.
It could be seen from the reported research that the only part work was mainly focusing on nonlinear aspect for ADP crystals [29]. For example, A. A. Kaminskii et al. had pointed out the nonlinear laser effect of ADP crystal which could be applied for the dielectric material to modulate frequency conversion of Roman laser [30]. Besides, Ji et al focused on the high-efficiency frequency conversion into the deep ultraviolet in various deuterated ADP crystal. However, due to the influence of crystal quality, the crystal performance and properties would also be affected. For instance, Lian et al. studied the defect-induced damage behaviors of ADP crystal in 355 nm by fluorescence spectrum, positron annihilation spectrum and the online light scattering measurements [31]. They had found that various defects could influence the laser induced damage threshold of ADP crystal. While the mechanism of related defects in crystal was unclear, and the influence of other types of defects had not been clearly identified due to the limitation of experimental techniques, just like that in KDP crystal. In addition, the structure and electronic properties induced by the various intrinsic defects had been performed for KDP and ADP crystal in our previous work [32–34]. However, defects often existed in crystals with the form of cluster under intense laser irradiation in practice, which might also be factors causing crystal damage. The composition of defect cluster was complex, and it was only considered form the perspective of charge compensation to form FeP2-+VO2+ except for Fe3+ dopant defect. Therefore, the oxygen vacancy and Fe3+ dopant with its cluster defect FeP2-+VO2+ in ADP crystal would be considered.
In this work, the oxygen vacancy cluster defect and the FeP2-+VO2+ cluster defect in ADP crystal was performed by density functional theory (DFT). And its influence on structure, electronic and optical properties were also calculated. Beyond this, the ADP crystals grown by “point-seed” rapid growth method doped with different concentration of Fe3+ was investigated by multiple spectrum experiment. It would be beneficial for understanding the difference of structural and property changes between KDP and ADP crystal induced by cluster defect. And the effect of defects on damage would be more systematic and clearer.
2. Computational details and experimental methods
2.1 Computational details
The Vienna Ab Initio Simulation Package (VASP) based on the density functional theory (DFT) developed by the University of Vienna was applied to this work [35,36]. The projector-augmented-wave (PAW) potential was used to describe the interaction between electrons and ions. To optimize the configurations, the electron exchange and correlation (XC) functional of the generalized gradient approximation (GGA) with Perdew, Burke, and Ernzerhof (PBE) functional was used. All the energetic, electronic and optical properties were employed within the Heyd, Suseria, and Ernzerhof (HSE06) hybrid functional [37–39]. The H 1s1, P 3s23p3, O 2s22p4, N 2s22p3 and Fe 3d64s2 states were treated as valence electrons. The screening length and mixing parameter were fixed at 10 Å and 0.25. Besides, the force convergence criterion for the structural relaxation was set to 0.01 eV/Å, and the electronic wave functions are expanded in plane waves using an energy cutoff of 400 eV. Monkhorst-Pack k-point meshes of 1 × 1 × 1 was used for cluster calculations [40]. To improve the accuracy of the calculation results, the van der Waals-Wannier function method (DFT/vdW-WF2) was used to describe the hydrogen bond interaction in this crystal [41–43]. All these parameters used in this work have been obtained by performing the convergence tests in our previous work.
The tetragonal supercells with 8 NH4H2PO4 units (384 atoms) were used to model the defect structures. The lattice vectors were refined as A = (ai+aj), B = (ai−aj), C = ck with the lattice constants of unit cell a = b = 7.50 Å, c = 7.55 Å [44,45]. The oxygen vacancy cluster defect model was constructed by removing oxygen atom in PO43- group one by one with the concentration of VO defect for 0.26%, 0.52%, 0.78% and 1.04%, respectively. Fe doped defect (FeP2-) model were constructed by replacing a phosphorus atom with Fe3+ in the pristine ADP supercell as KDP crystal. And its neighboring oxygen atom was removed to form FeP2-+VO2+ cluster defect. The related defect structures were also fully relaxed.
The defect formation energies Ef of defects with charge state q dependent on the Fermi level position was defined as [46–48]:
2.2 Experimental methods and characterization
The ADP crystals doped with different concentration of Fe3+ were grown with the “point-seed” rapid growth method, and the detailed growth procedure was reported in previous work. The bulk crystal was transparent without visible defect as shown in Fig. 1(a)–1(c). Samples for this work were cut from the two different positions from the as-grown bulk crystal, as shown in Fig. 2. The samples were 10 mm×10 mm×1 mm with type-II cut (θ = 59°), ultra-precision fly cut and surface polishing treatment. The A1-PR, A2-PY were used to define the prismatic and pyramidal sectors of ADP crystal doped with 10 ppm and 20 ppm, respectively.
Fig. 1. The rapid growth ADP crystals with different Fe3+ concentration for (a) 10 ppm, (b)20 ppm, and (c)30 ppm.
The Raman spectra was a lossless analytical technique based on the interaction of light and chemical bonds within a material. Raman spectra could provide detailed information on the chemical structure, phase and morphology, crystallinity and molecular interactions of a sample. In this work, it was used to observe the change of crystal microstructure with Fe3+ concentration. The experiment was carried out in the visible wavelength using a laser of 532 nm, and the spectrum scanning range was 200-1400 cm-1. The Fourier Transform Infrared spectroscopy was a mathematical processing of Fourie transform. It combined computer technology with infrared spectroscopy to identify the atomic group and chemical bonds in order to analysis the structure of materials. In this work, it could be used to observe the crystal structural stability with the change of Fe3+ concentration. The scope of this experiment was 400-4000 cm-1 with 32 scan times and resolution ratio of 4 cm-1. The ultraviolet absorption spectra could be used to analyze the composition of substances according to the degree of absorption of ultraviolet rays at different wavelengths. In this work, the influence of Fe3+ in ADP crystal was to be considered. LS5-Lambda 950 in the range of 190-800 nm at room temperature was applied to this experiment. The scanning wavelength was 1 nm and the sample test reference was air.
3. Results and discussion
3.1 Stability and structure of the cluster defects
The stability of the cluster defects was necessary to be confirmed, because of that the point defects in crystals were easy to combine and form defect clusters under intense laser irradiation. Thus, the formation energies were calculated. Firstly, the concentration of the oxygen vacancy cluster, composed of different numbers of oxygen vacancies in the same tetrahedral PO43- group, have been considered. The related result was shown in Table 1. It could reflect that the formation energies increased with the defect concentration. When one oxygen atom in the PO43- group was lost to form oxygen vacancy with the concentration of 0.26%, the formation energy was 4.35 eV. It was much higher than that of the same situation for KDP crystal with the formation energy of -5.10 eV. Besides, the formation energy was 14.18 eV when the concentration reached to 0.78% in ADP crystal, which was 8.73 eV higher than that in KDP crystal. Thus, compared with KDP crystal, it was more difficult to form oxygen vacancy and its cluster defect in ADP crystal, mainly due to the hydrogen bond in ADP crystal. It could show much stronger structural stability in ADP compared with KDP crystal, which was consistent with our previous conclusions.
Table 1. The formation energy of ADP with different concentration of VO defect
Since the case of a single oxygen vacancy has been investigated in previous work, the stability of the cluster with different distance for the two oxygen vacancies whose concentration was 0.52% would be discussed. The two oxygen vacancies model was composed of different position of the oxygen atom in different PO43- group. One oxygen atom was in the central PO43- group in the whole ADP crystal, and another was far from it with the multiple of the length of r, which was equal to the P-O bond length, as shown in Fig. 3. The distance of r-6r was correspond to O2-O7. The related formation energy of the model was calculated, and the results was shown in Table 2. It has been seen that the two atoms closest to each other had the lowest energy to form cluster defect. Besides, there was little difference in formation energy between the atoms father apart from each other. This was obviously different from the defect formation in KDP crystal which increased with distance, mainly due to the hydrogen bond in ADP crystal. Thus, in this work, the model of the lowest formation energy was considered for the calculation of the cluster defect
Fig. 3. Schematic diagram of the position of oxygen vacancy cluster defect. r is the bond length of P-O 1.55 Å. The distance of r-6r was correspond to O2-O7. The red, green, white gray balls represent the H, N, O and P atom, respectively. Here is the top view.
Table 2. The formation energy of ADP with different location of VO defect
Structural distortion caused by defects was usually one of the main concerns in defect research, especially the cluster defects. Hence, the structural stability of cluster defects with low formation energy would be considered. Firstly, the structures of oxygen vacancy cluster defect have been calculated as shown in Fig. 4(a)–4(e). One oxygen atom in the PO43- group losing could cause the whole hydrogen atoms to shift somewhat. When there was another oxygen atom losing to form oxygen vacancy cluster defect, the torsion of the hydrogen atom increased on the original basis as shown in Fig. 4(c). Besides, the P atom shifted downward to form an angle with the N atom, which should appear in a straight line when viewed from above. As for three oxygen vacancy forming in the PO43- group, the position of the hydrogen atom was almost unchanged from before, while the P atom shifted symmetrically upward. And when it ended up with all oxygen atoms left in the PO43- group, the P atom went back to its original position. While the hydrogen atom originally attached to the oxygen atoms were shifted toward the P atom due to the lack of binding. The change of the structure for oxygen vacancy defect with different concentration in ADP crystal was similar to that in KDP crystal. That’s probably because of the similar structure of these two crystals.
Previous theoretical and experimental results showed that the formation energy of FeP2- defect in KDP crystal was the lowest when P atom was replaced by Fe impurity atom [49,17]. ADP and KDP crystal have similar crystal structure, and the formation energy of each intrinsic defect in ADP crystal was roughly the same as that in KDP crystal. Therefore, in this model, the formation of FeP2- defect in ADP crystal could be lower than that of other substitution points. Only the cluster model with such doping defect would be studied in our following research. Then the relative stability of FeP2-+VO2+ cluster defects have been considered due to the consideration of charge compensation, and the formation energy was shown in Table 3. When there was only FeP2- defect in ADP crystal, the formation energy was -1.94 eV, which was much lower than that in KDP crystal with 4.63 eV. It indicated that Fe3+ doped substituent defect was more easily formed in ADP crystal. When VO2+ defect adjacent to the FeP2- defect occurred, the formation energy rose to -0.53 eV, increasing about 1.41 eV. Besides, with the distance of VO2+ defect increasing, the formation energy increased gradually. Although the distance had little effect on defect formation energy. It was obviously different from the phenomenon in KDP crystal that the distance of the defect had great influence on the formation energy. This was mainly because of the regulating effect of the hydrogen bond and NH4+ group in ADP crystal to maintain the stability of the lattice. Correspondingly, FeP2-+VO12+ cluster defects were more able to form with the lowest formation energy of -0.53 eV. Therefore, the property of FeP2- and its adjacent VO2+ defect would be focused in the following calculation. It also explained that defects showed obvious clustering effect.
Table 3. The formation energy of ADP with FeP2-+ VO2+ defect
And the structure of the defect formed by Fe3+ substituting P atom and its cluster defects have also been performed, as shown in Fig. 5(a)–5(c). As for the form of FeP2- defect, the adjacent four oxygen atom in the original PO43- group had a little torsion. The two opposite oxygen atoms were close to and away from the FeP2- defect, leading to the structural aberration in the defect area. When the adjacent oxygen atom lost to form VO2+ defect, the FeP2- defect moved close to the VO2+ defect, mainly due to the effect of the charge compensation and Coulombic force of attraction. It promoted the clustering of defect to form FeP2-+VO2+ cluster defect. Correspondingly, the hydrogen atom attached to the oxygen atom had a large degree of displacement, compared with the situation for only FeP2- defect. At the same time, the NH4+ group also had a slight deviation. However, the degree of structural change for the FeP2-+VO2+ cluster defect was relatively smaller than that for oxygen vacancy cluster defect. It probably because that the charge compensation made up for the structural distortion caused by electric charge. Thus, the efforts of the electronic structure and its optical absorption would be major concern of these cluster defects. In addition, the structure change caused by the FeP2-+VO2+ cluster defect in ADP crystal was relatively large compared with that in KDP crystal, mainly due to the NH4+ group and hydrogen bond. And the large dopant atomic radius would also contribute to the result. This was consistent with the results of intrinsic defect in our previous research.
3.2 Electronic structure and optical absorption of cluster defects
The partial density of states (PDOS) of cluster defects, including oxygen vacancy cluster defect with various concentration and FeP2-+VO2+ cluster defect, have been investigated to define the efforts of the electronic structure and its optical absorption. The results were shown in Fig. 6 and Fig. 7. It could be seen that the VO defect introduced the empty defect states composed of P 3p and O 2p states at about 1.1 eV. Unlike the previous results, the defect at 1.1 eV was caused by the interaction of P-O bond rather than the action of the N-H bond, which was the same with that in KDP crystal. When the concentration of VO defect increased to 0.52% (2VO), the position of the defect state was almost unchanged. While the role of the H atom began to become apparent. This was consistent with the torsion of hydrogen bonds in the H2PO4- group in the structure with Fig. 4(c). As the defect concentration continued to increase to 3VO, another defect state contributed by P-O bond was added at about 2.8 eV on the basis of the original defect. It indicated that the increase of defect concentration intensified the structure change of P-O bond adjacent to the position of defect cluster. Moreover, the defect states moved right at 6.2 and 6.6 eV when all the oxygen atom lost in ADP crystal. In addition to the original P-O bond, the contribution of hydrogen bond was particularly obvious and dominant. It was the same as the performance in structure for hydrogen bond. Overall, the electronic properties of oxygen cluster defects in ADP crystal were distinctly different from those in KDP crystal. It mainly because the hydrogen bond in ADP crystal could regulate the changes of crystal structure and electronic structure. And it was consistent with the conclusion of the intrinsic point defects in our previous work for KDP and ADP crystal.
Fig. 6. The partial density of states (PDOS) for the oxygen vacancy cluster defects with various concentration.
Fig. 7. The partial density of states (PDOS) for FeP2- defect and FeP2-+VO2+ cluster defect for ADP crystal.
The electronic structure and physical properties of Fe dopant and its cluster defect in ADP crystal was also one of the concerns. Then the partial density of states for FeP2- defect and FeP2-+VO2+ cluster defect was calculated, as shown in Fig. 7. From the results, it could be seen that the FeP2- defect in ADP crystal introduced four defect states at 0.5, 1.5, 3.8 and 4.8 eV, respectively, composed of Fe 3d and O 2p states. The optical absorption peak in Fig. 8. was more inclined to right along the direction of y in ADP crystal. The change of optical absorption caused by impurities was not significant, perhaps due to the low content of FeP2- defect. When there was VO2+ defect appearing, the defect states were almost unchanged, expect for the position moving to the right. Correspondingly, the hydrogen bond had no contribution to the defect states compared with that of oxygen vacancy cluster defects, mainly due to the effect of the charge compensation and Coulombic force of attraction. It was the same with the electronic structure in KDP crystal for Fe dopant defect and FeP2-+VO2+ cluster defect. And the electronic charge differences in Fig. 9(a)–9(b) could also reflect the conversion of electric charges to FeP2- defect to its around O atom. In addition, the optical absorption peak was along the xy plane in ADP crystal, which was also not obviously. The performance in optical absorption in ADP crystal was the same as that in KDP crystal, mainly due to the similar crystal structure.
Fig. 8. Imaginary part of the dielectric function ε2(ω) and absorption coefficient α(ω) for pristine ADP, FeP2- defect and FeP2-+VO2+ cluster defect in ADP crystal.
Fig. 9. Electronic charge differences of (a) and FeP2- defect and (b) FeP2-+VO2+ cluster defect for ADP crystal. Blue and yellow regions represent electron depletion and accumulation, respectively. The yellow region in (a) is covered by the blue area and is around FeP2- defect.
3.3 Spectrum measurements
The Raman spectra of the ADP crystal with different Fe3+ concentration was shown in Fig. 10(a)–10(b). As seen in the figure, the peak at 924 cm-1 was the characteristic peak of ADP crystal, which represented the stretching vibration of PO43- group and O-H-O bond [50,51]. And the peak at 339 cm-1 and 475 cm-1 were the asymmetric bending vibration of PO43- and the bending vibration of O-H-O bond. The symmetric bending vibration of PO43- group, the bending vibration of O-H-O and the twisting of NH4+ group were located at 550 cm-1. In addition, the antisymmetric bending vibration of the NH4+ group was at 1437 cm-1, while the peak at 1659 cm-1 was the symmetric bending vibration of NH4+ group and the bending vibration of O-H-O. The peak at 3125 cm-1 was the antisymmetric stretching vibration of NH4+ group and the stretching vibration of O-H-O. It could be seen that the peak at 3125 cm-1 shifted and the intensity of most peaks in prismatic sector of ADP crystal decreased with the increase of the concentration of Fe3+, mainly due to the influence of Fe3+.The skeleton structure of PO43- was broken when the Fe3+ doped into crystal. And the charge distribution of atoms and groups around Fe3+ have changed, leading to the deformation of O-H-O bond and the NH4+ group, which was also confirmed by the theoretical calculation result shown in Fig. 9. It would affect the stability of ADP crystal. The situation of the pyramidical sector was the same, except for the incomplete peak for sample of A3-PY. It might be due to the partial deliquescence with relatively high Fe3+ concentration.
Fig. 10. The Raman spectra of ADP samples with different Fe3+ concentration for the (a) prismatic and (b) pyramidal sectors.
The result of the Fourier Transform Infrared spectroscopy was shown in Fig. 11(a)–11(b). The peak at 543, 1397 and 1260 cm-1 represented the bending vibration of PO43- group, NH4+ group and the asymmetric bending vibration of PO43- group, respectively. And the peak at 1039, 1683, 2383, 2866 cm-1 were the stretching of P-O-H bond, P = O bond, P-OH bond and (O=) PO-H bond. In addition, 3218 cm-1 was the vibration of N-H in NH4+ group, stretching of O-H bond and P-O-H bond. As it could be seen from the figure, with the increase of Fe3+ concentration, the peak position had a moderate shift, especially the absorption peak at 1039 cm-1 moving to a higher wave. It was because the P = O and P-OH bonds in ADP crystals were affected by Fe3+, which destroyed the connection between skeleton structure. It would lead to a relatively large deformation of O-H bond and the instability of the crystal structure, which also had been confirmed with calculation results in Fig. 5. With the increase of defect concentration, the microstructure stress could also damage the crystal structure due to the microscopic stress induced by Fe3+.
Fig. 11. The Fourier Transform Infrared spectroscopy of ADP samples with different Fe3+ concentration for the (a) pyramidal and (b) prismatic sectors.
Because the Fe3+ preferred to enrich in the prismatic sectors, the ultraviolet absorption of this sector had attracted more attention. Therefore, the ultraviolet absorption spectra of prismatic sector were shown in Fig. 12. It could be seen that there was a wide absorption peak in the ADP crystal doped with Fe3+ at the range of 220-350 nm. Combined with the results of calculation, it might be caused by the FeP2- defect and FeP2-+VO2+ cluster defect. The ultraviolet absorption caused by Fe3+ was also consistent in KDP crystal, which has been investigate by the reported work [52]. It also demonstrated that the Fe3+ would affect the optical property in crystal.
4. Conclusion
Overall, the stability, structure, electronic and optical absorption induced by oxygen vacancy cluster defect and FeP2-+VO2+ cluster defect in ADP crystal was investigated by the DFT method with hybrid HSE06 functionals coupled with van der Waals-Wannier function method. And the structural and properties changes caused by cluster defects in ADP crystal were compared with those in KDP crystal during the research. In addition, spectral experiment with different concentrations of Fe3+ were also used to investigate the effect of Fe3+ on ADP crystals. Combined the theoretical calculation and spectral experiment result showed that, similar to KDP crystal, the larger the defect concentration in crystal, its stability was lower, and the defects showed aggregation effect. However, the existence of cluster defects, including oxygen vacancy cluster defects and FeP2-+VO2+ cluster defect only caused slightly structural changes in ADP crystal. The degree of it was not great, mainly due to the contribution of hydrogen bond and NH4+ group. In addition, with the increase of the concentration of oxygen vacancy defect, the defect level in ADP crystal moved towards right from 1.1 eV to 6.6 eV. It was different with that in KDP crystal. Due to the effect of the charge compensation and Coulombic force of attraction, the defect states composed of Fe 3d and O 2p induced by FeP2-+VO2+ cluster defect moved right compared with the situation with only FeP2- defect. However, the optical absorption peak along xy plane was not obviously due to the low concentration of defects. In spite of this, it was necessary to reduce the existence or concentration of cluster defects in order to improve crystal quality and crystal properties.
Funding
University of Science and Technology Beijing; Shandong University.
Acknowledgements
This work was supported by the Department of Physics, School of Mathematics and Physics, University of Science and Technology Beijing. And thanks to the support of Institute of Crystal Materials, Shandong University.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. L. N. Rashkovich, KDP-family Single Crystals (Published under the Adam Hilger imprint by IOP Publishing Ltd, 1991).
2. D. N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey, (Springer, 2001).
3. S. L. Wang, X. Sun, and X. T. Tao, “Growth and characterization of KDP and its analogs,” Springer Handbook of Crystal Growth (Springer, 2010).
4. J. J. Barrett and A. Weber, “Temperature Dependence of Optical Harmonic Generation in KDP and ADP Crystals,” Phys. Rev. 131(4), 1469–1472 (1963). [CrossRef]
5. S. H. Ji, F. Wang, M. X. Xu, L. L. Zhu, Z. P. Wang, and X. Sun, “Room temperature, high-efficiency, noncritical phase-matching fourth harmonic generation in partially deuterated ADP crystal,” Opt. Lett. 38(10), 1679 (2013). [CrossRef]
6. S. H. Ji, F. Wang, L. L. Zhu, X. G. Xu, Z. P. Wang, and X. Sun, “Non-critical phase-matching fourth harmonic generation of a 1053-nm laser in an ADP crystal,” Sci. Rep. 3(1), 1605 (2013). [CrossRef]
7. H. Gao, B. Teng, Z. Q. Wang, D. G. Zhong, S. H. Wang, J. Tang, N. Ullah, and S. H. Ji, “Room temperature noncritical phase matching fourth harmonic generation properties of ADP, DADP, and DKDP crystals,” Opt. Mater. Express 7(11), 4050–4057 (2017). [CrossRef]
8. F. Wang, X. W. Deng, L. Q. Wang, F. Q. Li, W. Han, W. Zhou, X. X. Huang, H. W. Guo, and Q. H. Zhu, “Efficient fifth harmonic generation of Nd: glass laser in ADP crystals,” Chinese Opt. Lett. 17(12), 57–62 (2019).
9. N. Zaitseva, J. Atherton, and R. Rozsa, “Design and benefits of continuous filtration in rapid growth of large KDP and DKDP crystals,” J. Cryst. Growth 197(4), 911–920 (1999). [CrossRef]
10. L. N. Rashkovich and V. N. Kronsky, “Influence of Fe3+ and Al3+ ions on the kinetics of steps on the {1 0 0} faces of KDP,” J. Cryst. Growth 182(3-4), 434–441 (1997). [CrossRef]
11. T. Sasaki and A. Yokotani, “Growth of large KDP crystals for user fusion experiments,” J. Cryst. Growth 99(1-4), 820–826 (1990).
12. T. A. Eremina, V. A. Kuznetsov, N. N. Eremin, T. M. Okhrimenko, N. G. Furmanova, E. P. Efremova, and M. Rak, “On the mechanism of impurity influence on growth kinetics and surface morphology of KDP crystals—II: experimental study of influence of bivalent and trivalent impurity ions on growth kinetics and surface morphology of KDP crystals,” J. Cryst. Growth 273(3-4), 586–593 (2005). [CrossRef]
13. I. M. Pritula and Y. N. Velikhoy, “Some aspects of UV-absorption of NLO KDP crystals,” SPIE's International Symposium on Optical Science, Engineering, and Instrumentation, International Society for Optics and Photonics (1999), pp. 202–208.
14. V. I. Salo, M. I. Kolybayeva, V. M. Puzikov, I. Pritula, and V. G. Vasil’Chuk, “The effect of impurities on the value of bulk laser damage threshold of KDP single crystals,” International Conference on Optical Diagnostics of Materials and Devices for Opto-, Micro-, and Quantum Electronics (1998), pp. 549–552.
15. V. I. Salo, L. V. Atroschenko, S. Garnov, and N. V. Khodeyeva, “Structure, impurity, composition and laser damage threshold of the subsurface layers in KDP and KD*P single crystals,” Laser-Induced Damage in Optical Materials, 1995, International Society for Optics and Photonics, Boulder, (1996), pp. 197–201.
16. V. V. Azarov, L. V. Atroshchenko, Y. K. Danileiko, M. I. Kolybaeva, Y. P. Minaev, V. N. Nikolaev, A. V. Sidorin, and B. I. Zakharkin, “Influence of structure defects on the internal optical strength of KDP single crystals,” Quantum Electron. 99(1), 820–826 (1990). [CrossRef]
17. N. Y. Garces, K. T. Stevens, L. E. Halliburton, M. Yan, N. P. Zaitseva, and J. J. DeYoreo, “Optical absorption and electron paramagnetic resonance of Fe ions in KDP crystals,” J. Cryst. Growth 225(2-4), 435–439 (2001). [CrossRef]
18. T. A. Eremina, V. A. Kuznetsov, T. M. Okhrimenko, and M. Rak, “Structures of impurity defects in KDP and their influence on quality of crystals,” Proc. SPIE 5136, 153–158 (2003). [CrossRef]
19. R. Malekfar and N. Naghavi, “Raman backscattering spectroscopy of KDP crystal doped by Fe3+ at low temperatures in the lower hydrogen bonds region,” Am. J. Phy. 935(126), 127–131 (2007). [CrossRef]
20. G. Endert and M. L. Martin, “The effect of chromium impurities on the laser damage threshold of KDP crystals,” Cryst. Res. Technol. 16(5), K65–K66 (1981). [CrossRef]
21. B. Wang, S. L. Wang, C. S. Fang, X. Sun, Q. T. Gu, Y. P. Li, K. P. Wang, and Y. N. Li, “Effects of Fe3+ ion on the growth habit of KDP crystal,” J. Synth. Crys. 34(02), 16–19 (2005). [CrossRef]
22. Y. Q. Liu, X. C. Li, J. Wu, B. A. Liu, D. S. Geng, and Y. N. Zhang, “First-principles studies on the electronic and optical properties of Fe-doped potassium dihydrogen phosphate crystal,” Comput. Mater. Sci. 143, 398–402 (2018). [CrossRef]
23. I. M. Pritula, M. I. Kolybayeva, V. I. Salo, and V. M. Puzikov, “Defects of large-size KDP single crystals and their influence on degradation of the optical properties,” Opt. Mater. 30(1), 98–100 (2007). [CrossRef]
24. H. Yin, M. Li, C. Zhou, and J. Song, “KDP single crystal growth via three-dimensional motion growth method,” Cryst. Res. Technol. 51(8), 491–497 (2016). [CrossRef]
25. M. Nakatsuka, K. Fujioka, T. Kanabe, and H. Fujita, “Rapid growth over 50 mm/day of water-soluble KDP crystal,” J. Cryst. Growth 171(3-4), 531–537 (1997). [CrossRef]
26. Y. C. Liang, W. Q. Chen, Q. S. Bai, Y. Z. Sun, G. D. Chen, Q. Zhang, and Y. Sun, “Design and dynamic optimization of an ultraprecision diamond flycutting machine tool for large KDP crystal machining,” Int. J. Adv. Manuf. Tech. 69(1-4), 237–244 (2013). [CrossRef]
27. Q. Xu, J. Wang, W. Li, X. Zeng, and S. Y. Jing, “Defects of KDP crystal fabricated by single-point diamond turning,” Proc. SPIE 3862, 236 (1999). [CrossRef]
28. L. G. Liu, D. L. Wang, Z. X. Cui, C. Y. Shen, M. R. Xu, F. Meng, S. K. Yao, and S. L. Wang, “Influence of agitation intensity on solution stability for rapidly grown KDP crystal through theoretical and experimental research,” Mater. Today Commun. 24, 101007 (2020). [CrossRef]
29. Y. F. Lian, J. B. Wen, F. Wang, T. T. Sui, J. Huang, X. Sun, and X. D. Jiang, “Nonlinear optical characteristics of an ADP crystal grown in a defined direction,” Opt. Mater. Express 8(9), 2614 (2018). [CrossRef]
30. A. A. Kaminskii, V. V. Dolbinina, H. Rhee, H. J. Eichler, K. Ueda, K. Takaichi, A. Shirakawa, M. Tokurakawa, J. Dong, and D. Jaque, “Nonlinear-laser effects in NH4H2PO4(ADP) and ND4D2PO4(DADP) single crystals: almost two-octave multi-wavelength Stokes and anti-Stokes combs, cascaded lasing in UV and visible ranges with the involving of the second and third harmonic generation,” Laser Phys. Lett. 5(7), 532–542 (2008). [CrossRef]
31. Y. F. Lian, D. T. Cai, T. T. Sui, M. X. Xu, Y. A. Zhao, X. Sun, and J. D. Shao, “Study on defect-induced damage behaviors of ADP crystals by 355 nm pulsed laser,” Opt. Express 28(13), 18814 (2020). [CrossRef]
32. T. T. Sui, Y. F. Lian, M. X. Xu, L. S. Zhang, Y. L. Li, X. Zhao, X. G. Xu, and X. Sun, “Comparison of hydrogen vacancies in KDP and ADP crystals: a combination of density functional theory calculations and experiment,” Phys. Chem. Chem. Phys. 21(11), 6186–6197 (2019). [CrossRef]
33. T. T. Sui, L. N. Wei, X. Z. Cao, M. X. Xu, L. S. Zhang, X. Zhao, Z. X. Chen, Y. L. Li, X. G. Xu, and X. Sun, “Comparison of oxygen vacancy and interstitial oxygen in KDP and ADP crystals from density functional theory calculations,” Comp. Mater. Sci. 182, 109783 (2020). [CrossRef]
34. T. T. Sui, L. N. Wei, Y. F. Lian, M. X. Xu, L. S. Zhang, Y. L. Li, X. Zhao, X. G. Xu, and X. Sun, “Structural stress and extra optical absorption induced by the intrinsic cation defects in KDP and ADP crystals: a theoretical study,” CrystEngComm 22(11), 1962–1969 (2020). [CrossRef]
35. G. Kresse and J. Furthmüller, “Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set,” Comput. Mater. Sci. 6(1), 15–50 (1996). [CrossRef]
36. G. Kresse and J. Furthmüller, “Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set,” Phys. Rev. B 54(16), 11169–11186 (1996). [CrossRef]
37. J. P. Perdew, K. Burke, and M. Ernzerhof, “Generalized gradient approximation made simple,” Phys. Rev. Lett. 77(18), 3865–3868 (1996). [CrossRef]
38. J. Heyd, G. E. Scuseria, and M. Ernzerhof, “Hybrid functionals based on a screened Coulomb potential,” J. Chem. Phys. 118(18), 8207–8215 (2003). [CrossRef]
39. A. V. Krukau, O. A. Vydrov, A. F. Izmaylov, and G. E. Scuseria, “Influence of the exchange screening parameter on the performance of screened hybrid functional,” J. Chem. Phys. 125(22), 224106 (2006). [CrossRef]
40. H. J. Monkhorst and J. D. Pack, “Special points for Brillouin-zone integrations,” Phys. Rev. B 13(12), 5188–5192 (1976). [CrossRef]
41. P. L. Silvestrelli, “van der Waals interactions in DFT made easy by Wannier functions,” Phys. Rev. Lett. 100(5), 053002 (2008). [CrossRef]
42. P. L. Silvestrelli, “van der Waals interactions in density functional theory using Wannier functions,” J. Phys. Chem. A 113(17), 5224–5234 (2009). [CrossRef]
43. L. Andrinopoulos, N. D. M. Hine, and A. A. Mostofi, “Calculating dispersion interactions using maximally localized Wannier functions,” J. Chem. Phys. 135(15), 154105 (2011). [CrossRef]
44. X. E. Ren, D. L. Xu, and D. F. Xue, “Crystal growth of KDP, ADP, and KADP,” J. Appl. Phys. (Melville, NY, U. S.) 310(7-9), 2005–2009 (2008). [CrossRef]
45. C. S. Liu, C. J. Hou, N. Kioussis, S. G. Demos, and H. B. Radousky, “Electronic structure calculations of an oxygen vacancy in KH2PO4,” Phys. Rev. B 72(13), 134110 (2005). [CrossRef]
46. C. G. Van de Walle and J. Neugebauer, “First-principles calculations for defects and impurities: Applications to III-nitrides,” J. App. Phys. 95(8), 3851–3879 (2004). [CrossRef]
47. A. F. Kohan, G. Ceder, D. Morgan, and C. G. Van de Walle, “First-principles study of native point defects in ZnO,” Phys. Rev. B 61(22), 15019–15027 (2000). [CrossRef]
48. J. He, R. K. Behera, M. W. Finnis, X. Li, E. C. Dickey, S. R. Phillpot, and S. B. Sinnott, “Prediction of high-temperature point defect formation in TiO2 from combined ab initio and thermodynamic calculations,” Acta Mater. 55(13), 4325–4337 (2007). [CrossRef]
49. T. T. Sui, C. B. Wan, M. X. Xu, X. Sun, X. G. Xu, and X. Ju, “Hybrid density functional theory for the stability and electronic properties of Fe-doped cluster defects in KDP crystal,” CrystEngComm 23(44), 7839–7845 (2021). [CrossRef]
50. X. C. Li, B. A. Liu, C. Y. Yan, J. Ren, C. Liu, and X. Ju, “Defects of laser-irradiated KDP crystals with different fluences studied by photoluminescence spectroscopy,” Matter and Radiat. at Extremes 5(3), 035402 (2020). [CrossRef]
51. D. Kanimozhi, S. Nandhini, and R. Indirajith, “Effects of dyes in the growth, optical, mechanical and dielectric properties of KDP crystals,” J. Mater. Sci. 30(11), 10244–10255 (2019). [CrossRef]
52. D. C. Guo, X. D. Jiang, J. Huang, F. R. Wang, H. J. Liu, X. Xiang, G. X. Yang, W. G. Zheng, and X. T. Zu, “Effects of γ-ray irradiation on optical absorption and laser damage performance of KDP crystals containing arsenic impurities,” Opt. Express 22(23), 29020–29030 (2014). [CrossRef]
