Theoretical Studies On The Structure And Property Induced By Point Defects Of KDP And ADP Crystals

Potassium dihydrogen phosphate(KH2PO4;KDP)is the only nonlinear optical crystal material for Inertial Confinement Fusion(ICF)application due to the large electro-optical coefficient,low half-wave voltage,excellent transmittance and the ability of growing bulk crystal.However,optical elements are seriously damaged by ultraviolet laser irradiation in the current application,and the actual damage threshold is far lower than its theoretical value,severely limiting the output energy of the laser and reducing the service life of the components.Thus,it becomes a prominent problem restricting the development of ICF at present stage.Ammonium dihydrogen phosphate(NH4H2PO4;ADP)is the analog crystal with KDP crystal.The two crystals both possess similar structures and the excellent nonlinear and electro-optical properties.More importantly,ADP crystal has the lager effective nonlinear coefficient,higher damage threshold under the condition of short wavelength and the output characteristic of non-critical phase matching quadruple frequency at room temperature.Therefore,it has a broader potential than KDP crystal in application.Defects are considered as the main factors affecting the damage threshold.The sources and types of defects are numerous,and the composition is relatively complex.Point defects are the basis defects in crystal,and their effects on crystals are the key to research the problem of damage.However,the mechanism of specific defects on the properties and on the microscopic stress for KDP and ADP crystals are not very clear yet.Moreover,the different internal origins of the damage threshold between KDP and ADP crystals are also not well understood.Therefore,based on the"point-seed" rapid growth of KDP and ADP crystals,many experimental methods,such as X-ray energy dispersive spectroscopy were used to determine the intrinsic point defects in crystals.Combined with the theoretical research method,the optical absorption properties and the mechanism of microscopic stress induced by point defects were studied systematically.And the internal origins for the different damage threshold between KDP and ADP crystals were analyzed deeply.It could provide a reference and theoretical support for improving crystal quality and light damage threshold and promoting practical application for KDP and ADP crystals.The main contents of the research are as follows1.The KDP and ADP crystals were grown by the "point seed" rapid growth method.The main elements in these two crystals were obtained by X-ray energy dispersive spectrum.The results showed that the content of metal impurity ions in crystals was lower than the detection limit.The optical band gaps of these two crystals were measured by ultraviolet visible absorption spectra.It showed that there was no obvious absorption caused by impurity ions as well.The decrease of the optical band gap was due to the intrinsic point defects in crystals.The positron annihilation technique indicated that there were multiple intrinsic point defects in KDP and ADP crystals,such as oxygen vacancy,hydrogen vacancy and related defect clusters.2.First-principles method was applied to investigate the hydrogen vacancy and interstitial hydrogen defects in KDP and ADP crystals.Their influence on optical absorption properties was also studied.Considering the effect of electron exchange correlation potential on the accuracy of calculation,the results of lattice parameters and electronic properties obtained by PBE,HSE06,PBEO and HSE03 functionals were compared.It was found that GGA-PBE was more suitable for crystal structure calculations and HSE06 for electronic structure calculations.The van der Waals correction was added into the calculation to consider the influence of the weak interaction force in crystals.And it showed that ADP crystal was affected more seriously by van der Waals correction.Therefore,in the following calculations,van der Walls correction was added to all systems to ensure the accuracy of the results.The calculation method of defect formation energy was discussed according to the growth method of KDP and ADP crystals,and the traditional method was finally carried out to analysis the energy of the system.Theoretical calculation results showed that there was only one VH defect location in H2PO4" group for KDP crystal,while two locations in ADP crystal,namely VH1 defect in H2PO4" group and VH2 xii defect in NH4+group.In ADP crystal,VH2 defect was easier to be formed than VH1 defect,but the energy barrier was relatively small,which could be ignored in experiment.Thus,both defects were coexistent in ADP crystal.While Hi defect easily existed in both KDP and ADP crystals.The O-H bond showed great change of microscopic stress through the link of O-H…O bond,and the change magnitude caused by VH defect was VH2>VH>VH1.The influence on structure and microscopic stress caused by Hi defect was larger than that caused by VH defects,and it had much more influence on ADP crystal.Both VH+defect in KDP and VH1+defect in ADP could introduce defect states contributed by P-O bond and O-O peroxyl bridges,leading to the intrinsic absorption caused by the electron transition between valence band and defect state,while VH2+in ADP could not introduce any absorption.Differently,the changes induced by Hi defects only focused on the structure and microscopic stress with no obvious absorption both in KDP and ADP crystals.As a whole,the effect of hydrogen defects on ADP crystal was significantly smaller than that on KDP crystal,and hydrogen bonds were the main factor for the difference of the damage threshold between the two crystals.3.First-principles method was applied to investigate oxygen vacancy and interstitial oxygen in KDP and ADP crystals.Their influence mechanism on optical absorption properties was also studied.The theoretical results showed that the oxygen defects in KDP and ADP crystals could cause intramolecular dehydration leading to the structural collapse.In ADP crystal,the strong hydrogen bond network linked by P-O-H…O-P structure could interact with the O-O peroxyl bridge to prevent the formation of H2O.The influence induced by the oxygen defects was alleviated through the larger structure relaxation and the expansion of micro-stress in ADP crystal,showing relatively higher structural stability than KDP crystal.The defect formation energies of Vo and Oi defects in KDP crystal were about 1.5 eV lower than those in ADP crystal,indicating that the O atom was more likely to lose from its initial position to form vacancy or interstitial defects.Besides,Vo2+defect in KDP caused the obvious optical absorption mainly concentrating near the absorption edge with high strength,while it was weak absorption in ADP crystal.Therefore,ADP has stronger ability of resistance to damage and the higher damage threshold than KDP crystal,mainly due to the higher proportion of hydrogen bonds.4.Based on first-principles method,the intrinsic cation defects in KDP and ADP crystals,such as VK,VP,VN,and Ki,were investigated.The theoretical results showed that Ki defect had relatively lower defect formation energy,but the entry of K atom could cause great microscopic stress and lattice relaxation,which would destroy the stability of the crystal structure.Thus,it was not easy to form in the crystal.The defect formation energy of VP5-defect in KDP crystal was about half of that in ADP crystal.The absence of P atom would lead to the collapse of PO43-skeleton structure,causing great local microscopic stress as well as seriously damage of the structure.In terms of the energy and structure stability,it would need much more energy to lose P atom from ADP crystal with less disruptive than KDP crystal.Thus,ADP crystal possessed the stronger ability of resistance to damage than KDP crystal.Although VN3+in ADP and VP5-in KDP had similar defect formation energies,VN3+could only destroy NH4+ group and release H2,which would not affect the main PO43-skeleton structure and the connected hydrogen bond network.The damage degree to ADP was not as great as that of VP5-defect to KDP crystal.In addition,the VP5-defect in KDP caused larger and stronger optical absorption than that in ADP crystal,decreasing the performance of the crystal.Thus,the intrinsic cation defects would be one of the reasons of lower damage threshold of KDP than that of ADP crystal.5.First-principles method was applied to investigate the Fe ion and its influence on optical absorption properties of KDP and ADP crystals.The theoretical results showed that Fe atom entered the structure and replaced P atom,obtaining the lowest defect formation energy.Different magnetic conditions would have little effect on the structure of these two crystals,but it could significantly change the charged defect states in KDP.The influence of the microscopic stress caused by the Fe atom was less than that induced by the intrinsic point defects.It could cause large and broad optical absorption peaks composed of Fe 3d state.A wide range of optical absorption near the 200-300 nm was existent no matter how magnetic condition changed,impacting the performance of this two crystals and leading to the reduction of the damage threshold.Therefore,the Fe dopant would decrease the characteristic and properties of KDP and ADP crystal.6.The intrinsic point defects and Fe dopant point defects in KDP and ADP crystals affected the structural stability,caused local or wide-area microscopic stess and introduced defect states and additional optical absorption peaks.It would reduce the ability of resistance to damage and the practical damage threshold of the crystals.The effect on the mictroscopic stress induced by the intrinsic point defects was obviously greater than that induced by Fe dopant in KDP and ADP crystals.While the influence on the electronic and optical property caused by Fe dopant was far larger than that caused by the intrinsic defects.The research results showed that ADP had higher structural stability and stronger damage resistance than KDP crystal due to a large proportion of hydrogen bonds interacted with other chemical bonds.It might be the main factor that ADP has much higher damage threshold than KDP crystal.In summary,there exists vacancy,interstitial and metal ion defects in KDP and ADP crystals.These defects influence on the electronic structure and optical absorption properties through structure,energy and microscopic stress,decreasing the damage threshold of crystals.Thus,it is necessary to decrease the content and types of the intrinsic and dopant point defects as much as possible.And it is vital significant to improve the actual damage threshold and promote the application of KDP and ADP crystals.