Influence Of MO₄^2-(M=Cr,Mo,W) Anions On Growth And Optical Properties Of KDP Crystal And High Temperature Behavior Of KDP Crystal

KDP (Potassium dihydrogen phosphate) crystal has excellent piezoelectric, electro-optical properties. At the earlier period, KDP crystal was widely applied as piezoelectric transducer and sonar devices. In addition, with the application of high power laser system on the controlled nuclear fusion, the investigation and application of KDP crystal has come into a new stage. The acquisition of KDP crystal is mainly through growing from solution. When grown KDP crystal from solution, the influence of impurities on both growth and optical properties of KDP crystal should be taken into consideration. Nowadays, the research about anions effect among the investigations of impurities effect on both growth and optical properties of KDP crystal is relative few. Accordingly, we chose CrO,*2', MOO42" and WO42" as additives to study their influence on growth habits and optical properties of KDP crystal because of their structural similarities and dissimilarities to PO43", which is the primary anionic growth unit for KDP crystal. Moreover, we also explored the issue about "high temperature phase transition" in KDP crystal. The primary coverage is as follows:1. The dependence of growth rate of (100) face of KDP crystal on supersaturation was measured by using laser polarization interference technique. The morphologies of the growth steps on (100) and (101) faces were observed through AFM technique at a series of supersaturations. Moreover, the relaxations with the depth of half of the unit cell along [100] and [101] directions on (100) and (101) faces of KDP crystal was calculated by using DmolJ module in Material Studio software. As for (100) face, the results showed that at the supersaturation of 1.8%, elementary steps were in priority on (100) face, whose height was about 0.366 ran, equivalent to half of the unit cell along [100] direction. When the supersaturation was increased, elementary steps began bunching together. At the initial bunching stage, the step height and step width were increased. With a rise of supersaturation, more elementary steps participated in the bunching process, and the velocity of the macrosteps increased as well. At the same time, the slopes of the macrosteps reached stable ultimately. At lower supersaturation, there were lots of two-dimensional nucleuses on (101) face and the topmost of the nucleus were quite flat. The growth steps, which were in small dimension, were derived from the nucleus. The height of these growth steps was in the range from 0.47 to 0.53 nm, equivalent to half of the unit cell along [101] direction. When the supersaturation was relative high, the height of the growth steps on (101) face was about 0.51 nm, suggesting that no step bunching occured. The relaxation calculation results revealed that P-O bonds in PO43" tetrahedron had changed, and the O-H-O bonds had changed as well. The results also suggested that alternations occurred for the electron structure. The width of the forbidden band was narrowed, which was caused by p and s orbitals.2. By altering the concentrations of the additives, a series of KDP crystals were grown by conventional and rapid growth methods, respectively. The growth habits of the as-grown KDP crystals suggested that CrO42- made a little contribution to the growth habits and made KDP crystals colered of yellow-green. CrO42-didn't cause KDP crystals tapering or expanding. Simultaneously, CrO42-didn't introduce any liquid inclusions or simultaneous crystals. MoO42- made KDP crystals tapering for conventional temperature cooling method. When grown KDP by rapid growth method, MoO42- could induce liquid inclusions or simultaneous crystals. With lower concentration in growth solution, WO42- could made KDP crystal tapering for conventional growth method and higher concentration of WO42- could induce liquid inclusions and simultaneous crystals for rapid growth method. The influence of the additives on growth kinetics revealed that CrO42- played a little role on both growth rate and morphology of the growth steps. MoO42- poisoned the growth steps, embodying in postponing the step bunching and making the step edge curving and knaggy to reduce the edge free energy ratio y/kT. Ultimately, the width of the dead zone was increased and the growth rate of(100) face was decreased. WO42- could decrease the growth rate and increase the growth dead zone. AFM investigation suggested that WO42- could impede step bunching. High concentration of WO42- could kink the terrace of the macro-steps and create another series of macro-steps. The motion direction of the new creative steps was vertical to that of the original steps. When the motions of the vertical steps were both inhibited by WO42-, the steps would become tapering and (100) face would twist, which ultimately led to the exposure of (512) facets. The interaction between the additives and the (100) face of KDP crystal were analyzed according to the experiment and theory as below. The configurations of CrO42- in saturated KDP solution were confirmed to be Cr2O72- and HCrO4-. Through electrostatic attractive force, Cr2O72- and HCrO4- were selectively absorbed onto (101) and (100) faces, respectively. The absorbed Cr2O72- on(101) face and HCrO4- on (100) face exhibited a little effect on growth of KDP crystal due to the structural similarities of CrO42- to PO43-. Additionally, the influence of CrO42-on growth of KDP crystal was similar to that of Cr2O72-. The configurations of MoO42- in saturated KDP solution were confirmed to be HMoO4- and H2MoO4. The O-H terminals of HMoO4- and H2MoO4 make them local positive characteristic, which can be absorbed onto (100) face through charge assised-hydrogen bonds. The absorbed HMoO4- and H2MoO4, owing to their larger volume on (100) face, ultimately impeded the growth of KDP crystal. The configurations of WO42* in saturated KDP solution were confirmed to be H2W12O4210" and H2W12O406", whose polymeric degree was greater than CrC>42-and MOO42". H2W12O4210" and H2W12O406" could be also absorbed onto (100) face and their larger volumes caused inclusions and inhibitd the growth of KDP crystal with high additive concentrations. By comparing the inhibit capabilities of the three additives, we found that CrO42", M0O42" and WO42" could construct polymeric anionic groups with various polymeric degrees and hydrogen bonds in saturated KDP solution. Through electrostatic attractive force and charge-assisted hydrogen bond, these anionic groups could interact with (100) face and enter into KDP crystal lattice, and moreover, all of them could impede the growth of KDP crystal. The inhibit capability on growth of KDP crystal was depended on the ionic volumes of the additives. The ionic volumes of CrC>42-, MOO42" and WO42" increased one by one. The inhibit effect of CrC>42" whose ionic volume was much similar to that of PO43" was proved to be the minimal, and WO42" with the largest ionic volumes showed the greatest inhibit effect. Both the color of the KDP crystals and element analysis indicated that the anionic additives could enter into KDP crystal lattice, and the concentration of the corresponding element in KDP crystals increased with a rise of the additives in growth solution.3. The structural integrity was characterized by using high resolution X-ray diffraction (rocking curves), and the optical properties were accordingly mcarsured. The rocking curves revealed that the structural integrity was destroyed after CrC>42", M0O42" and WO42" entering into crystal lattice, embedding in the full width at half maximum was broadened. Additionally, satellite peaks appeared for rapid grown KDP crystals when the additive was CrO42". The static extinction ratio results showed that the static extinction ratio was decreased with a rise of CrO4 ", MOO4 " and WO4 " concentration, which suggested that CrC>42-, MOO42" and WO42" brought more residual lattice stress. We believed that the origin of the residual lattice stress was somehow different. CrC>42-could casuse residual lattice stress mainly through entering of CrC>42-into the crystal lattice. MOO42" introduced larger residual stress through two approaches. One was that MOO42" entered into the crystal lattice; another was MOO42" modified the growth steps and brought more mesoscopic defects. WO42" brought more mesoscopic defects principally through modifying the growth steps to introduce residual lattice stress. The influence of the additives on transmittance of the as-grown KDP crystals indicated that CrO42" brought two absorption peaks centered at 280 and 360 nm, and enhanced the absorption at 220 run, which were at the same band positions compared with the CrO42" or HCrCV transmittance spectra. MOO42" enhanced the optical absorption at ultraviolet band for pyramidal sector, and decreased the optical absorption in the range of 200-400 nm for prismatic sector, which was attributed to that MOO42" was prone to be absorbed onto (100) face, consequently, the incorporation of metallic cations with (100) face was blocked and the entrance probability of metallic cations into KDP crystal. WO42" enhanced the optical absorption at ultraviolet band for pyramidal sector; however WO42" had little influence for prismatic sector. Moreover, the investigation on optical homogeneity suggested that lower CrO42" concentration made a little role on optical homogeneity and higher CrC>42-concentration could decrease the optical homogeneity. The conoscopic interference indicated the optical homogeneity of KDP crystal was spoiled by MOO4\Additionally, the conoscopic interference images suggested WO4 " played a little role on optical homogeneity. The three additives could increase the density of light scatters and the size of single light scatter. The influence of the additives on laser induced damage threshold was not completely the same. The laser damage threshold was increased a rise of CrC^2" concentration, while the laser damage threshold was increase firstly and decreased with a rise of MOO42- and WO42" concentrations.4. The high temperature behavior of tetrahedral phase of KDP crystal was depicted by using thermo gravimetry-differential thermal analysis (TG-DTA), specific heat and in-situ infrared reflective spectra over the temperature range from 50 to 260℃, respectively. The results showed that no structural transition or dehydration occurred near 183℃, and the dehydration began at 207-210℃. The dehydration underwent three alterations as temperature increasing. In the first alteration, new peaks in representation of P2O72" appeared which meant dehydration occurred in the direction to K4P2O7. The second alteration was the continuous decomposition of the intermediates. The last alteration was the continuous decomposition of the intermediates yielded in the second stage and translated to KPO3. Based on the Kissinger method, the thermal dehydration kinetics of KDP crystal was calculated in terms of the TG data and the active energy were acquired for 101.7 Jmol-1 and 112.4 Jmol-1,respectively. The effect of pressure on thermal stability and decomposition of tetrahedral KDP phase was investigated by using in-situ infrared reflective spectra. The results suggested that the onset temperature of decomposition under pressure of 1 MPa was improved from 210℃to 213℃, suggesting the thermal stability of KDP was enhanced. Whereas, under pressure of 2 MPa, the thermal stability was deteriorated and KDP began decompose at 183℃. The effect of pressure on dehydration of KDP crystal was that pressure made KDP crystal translated to KPO3 directly without any other polymeric intermediates.