Crystallographic Analysis of Tapering of ADP Crystallites

JOURNAL OF RARE EARTHSVol.24 , Spec. Issue , Mar.2006 ,p. 228Crystallographic Analysis of Tapering of ADP Crystallites"Xu Dongli( 许东利) , Xue Dongfeng( 薛冬峰)*( State Key Laboratory of Fine Chemicals , Department of Materials Science and Chemical Engineering , School ofChemical Enginering , Dalian University of Technology , Dalian 116012 , China )Abstract : On the basis of crystallographic characteristics of ADP ( ammonium dihydrogen phosphate ) crystals and the se-lected growth conditions , the growth habit of ADP crystals was studied. In comparison with pyramidal planes , the growthrate of prismatic faces is slower and more sensitive to the additives and impurities for ADP crystals. W hen the supersatura-tion is low , the advance of growth steps on prismatic face can be blocked by ethanol or impurities , the crystal morphologyis changed from the tetragonal prism to shuttle( i. e. ,the tapered shape ). The tapering formation of ADP crystallites wasstructurally studied in a novel view.Key words : crystal structure ; chemical bond ; growth habit ; ADP ; hydrogen bond ; taperingCLC number :0782*.1Document code :AArticle ID : 1002 - 0721( 2006 )- 0228 -05The crystal morphology ,as a synergetic result ofchemical properties of ADP crystals are clearlythe crystal structure and specific growth conditions ,changed. Therefore ，the investigation of the growthgenerally reflects the detailed growth history andhabit and its dependence on the selected growth condi-strongly affects many macroscopic properties of the ob-tions is helpful for us to understand the crystal growthtained crystals. Therefore , comprehensive understand-process and select growth conditions.ing and controlling of the crystal morphology is veryIn the present paper ,ADP erystallites with a reg-important and necessary , the crystallographic analysisular or tapered shape were prepared under water-etha-should be carefully performed when the optimal growthnol mixed solution. The dependence of the crystalconditions are to be determined.morphology on the growth time and supersaturetion wasADP ( ammonium dihydrogen phosphate ) crystalsinvestigated on the basis of crystallographic analysis ofare well known for their excellent piezoelectric , elec-ADP crystals.trooptic , and nonlinear optical properties , which areStructural Characteristics Analysisextensively used in the electrooptic and electroacousticADP crystallizes in the tetragonal system withfields and as a monochromator for X-ray analysis 1~4].space group I 42d at room temperature8,91.As shownDue to their rapid growth rate and relatively simplein Fig. 1 , the hydrogen bonds connecting the adjacentgrowth condition , ADP crystals with large size anddihydrogen phosphate anions are represented byhigh optical quality have been successfully grown indashed lines , while the hydrogen bonds between amthe aqueous solution by cooling or slow evaporationmonium ions and dihydrogen phosphates are distin-method15. Moreover , for the purpose of further in-guished by both dotted and chain dotted lines. Othercreasing the growth rate and enhancing the physico-stronger chemical bonds are depicted as solid lines ,chemical properties of ADP crystals , various kinds ofwhich are almost invariable during the whole crstalli-additives and dopant , such as EDTA , urea , KMnO4zationprocess.and phosphoric acid , are added into ADP aqueous so-Even there are various kinds of chemical bondslution to adjust the metastable zone width and theirexisting in ADP crystals ，only the weaker chemicalspecific properties 5-77. However , with including thebonds formed during the crystallization process havadditives and dopant , the growth habit ( or morpholo-the dominant influence on the crystal growth'31 . Nu-gy ), the growth rate and the corresponding physico-中国煤化工MYHCNM HGFoundation item : Project supprted by the National Natural Science Foundation of China ( 20471012 ) , the Foundation for the Author of NationalExcellent Doctoral Disertation of China ( 200322 ), the Research Fund for the Doctoral Program of Higher Education( 20040141004 ) and the Scienifice Research Foundaion for the Returned Overseas Chinese Scholars , State Education MinistryBiography iXye Dongli( 1975 - ) , Male , Doctoral candidate ; Working on crystal growth* Correspondinguthor ( E-mail :dfxue@ chem. dlut. edu. en)XuDLetal.Crystallographic Analysis of Tapering of ADP Crytalies229H ee=::0on these hydrogen bonds between growth units , while :the selected growth conditions influence the final mor-phology via such hydrogen bonds.The surface atomic structure of KDP ( potassiumdihydrogen phosphate , the isomorph of ADP ) crystalsNkhas ever been examined13] , which indicates that theH -.269.:0pyramidal planes are terminated with K + ions ratherthan( H2PO4 )~ groups in the aqueous solution. ThisHis in accordance with the common sense that the|H -........ramidal planes should be covered by positive ions withFig. 1 Bond graph of ADP crystala small size , the negative ions are often difficult tobond to the crystal surface due to their large size andmerous studies concerning the solution structure indi-specific bond orientation. Since the size and valencecate that the constituent atoms enter crystals in a formstate of K+ and( NH4 )* are similar , and the constitu-of fundamental growth units ，especially for ani-ent anions are also same for both KDP and ADP cry;ons 10-121. The fundamental growth units are oftentals ,it is convenient and reasonable to presume thatconstructed by the strong chemical bonds ( betweenthe pyramidal planes of ADP are also ended with theconstituent atoms ) , which participate in the crystalli-positive ions ,i.e. ,( NH4 )* ions( Confer Fig.2 , thezation as a whole cluster , the bond strength and num-dashed lines connecting nitrogen atoms ). Thereforeber within these growth units are practically invariablewhen growing ADP crystals in the aqueous solutionduring the whole crystallization proprocess.Therefore ,the negative ions are frequently attracted onto the py-the chemical bonds inside these growth units are oftenramidal planes ，while impurties with the positivestronger but have a small effect on the crystal growth.charge are often repulsed by the positively chargedOn the contrary , the weak chemical bonds( e.g. ,hy-surface ,that is to say , the growth of pyramidal planesdrogen bonds for ADP ) , which interlink the neighbor-is mainly affected by the negative ions rather than pos-ing growth units during the crystallization process , areitive ions.the determinative factors controlling the crystal growthOn the other hand , the prismatic planes of ADPrate and final morphology. Furthermore , for theseare covered by( NH, )* and( H2PO4 ) groups alter-weak chemical bonds and the configuration of growthnatively ,i. e. , the prismatic planes of ADP are elecunits are sensitive to the selected growth conditions.troneutral , both positive and negative ions havetherefore , the investigation associated with the config-practically equal possibility to be attracted by theseuration of growth units and the breaking and reformingprismat-ic planes. Considering the size of( NH4 )*of the weak chemical bonds is very important for thecrystallization , the actual crystal morphology can betuned by alternating the growth conditions consequent-Raman spectroscopy studies and theoretical cal-culations indicate that the fundamental growth units ofRpqpADP crystals in the aqueous solution are hydrated( NH4 )~ cation ,( H,PO4 )~ anions and the diners of( H,PO4 ) anionsS "11. All chemical bonds connect-●Ning these fundamental growth units are weak hydrogenbonds( O - H...0 and N-H... 0 ), which can beclassified into three types according to the bond va-中国煤化工,●Plence and orientation. The ideal morphology of ADPCNM H G\crystals ( Enclosed by the prismatic and pyramidal→bplanes ) has been successfully derived on the basis ofthe crystallographic analysis in our recent works 30.Fig. 2 Crystallographic structure of ADP crystals , viewed alongThe growth海熬瑪ADP cerystals is strongly dependentthe a axis ,hydrogen atoms are omitted for carify230JOURNAL OF RARE EARTHS , Vol. 24 , Spee. Issue , Mar.2006using pipette. The first sample A is taken out immedi-and( H,PO ) groups ，the anions are easily exposedately when the crystallites can be seen in the mixed so-to the aqueous solution due to their larger size , whenlution. The other samples ,such as B ,C ,D and Ethe ions diffuse from the mother solution to boundaryare piped out separately at5 , 15 ,30 and 60 min afterlayer , they are attracted firstly by the anions , in otherthe first sample. All samples are dried by filter paperswords , the cations are attracted by the prismatic planeand then observed using microscope ( XTB-A ). Topreferentially , which is in accordance with the previ-measure the sample size , the mixed samples of A to Eous experiments' I4].are photographed by a field emission scanning electronIn Fig.2 ,a zigzag anion chain is constructed bymicroscopy ( JSM -6700F ).( H2PO4 ) groups via hydrogen bonds. To grow ADF3 Results and Discussioncrystals continuously , the whole zigzag chain must bWhen absolute ethanol is poured into the saturat-completed firstly. Since the chemical bonds connect-ed solution , some amount of solvent water is extracteding constituent anions are weaker hydrogen bonds , andby forming hydrogen bonds with ethanol moleculesthe size of these anions is big , the construction processthe supersaturation of the mixed solution becomes big-is often distorted or suspended by the positivelygest at this moment. Therefore , after a few minutescharged impurities absorbed on these anionic chains( Induction period ) ,numerous ADP crystallites appearconsequently the advance of the elementary step paral-in the mixed solution and grow rapidly. Since the su-lel to these zigzag chains is frequently blocked by thepersaturation of the mixed solution is biggest at the .impurities with positive charges. With the growth offirst stage , the growth rate of crystallites is also fastestcrystallites and the block of the elementary step , vari-at this moment，the increment of crystallites is alsoous tapered ADP crystals may be formed ( Confer Fig.bigger , which is in a good agreement with our experi-2 , the dotted lines ).mental results. Along with the increase of growthExperimental Proceduretime , the crstallites become bigger and bigger whileTo validate the above crystallographic analysis ofthe supersaturation of the mixed solution becomes low-ADP crystals ，a time-dependent experiment is deer and lower ,that is to say , the growth rate of crystal-signed in this work to investigate the basic relation be-lites becomes slower and slower. Finally , the mixedtween the growth conditions and crystal morphology.solution regresses to the saturated solution again , theThe saturated ADP solution is prepared at roomdimension of crystallites preserves the final size , whichtemperature by deionized water with AR grade ADPcan be clearly observed by comparing samples D andalt. After the filtration , the saturated solution is over-E ,the size and morphology of D and E are almostheated at 50 C for 24 h and then slowly cooled tosame.room temperature. Some absolute ethanol is added intoAnother rule may be observed clearly by compa-the saturated solution to extract the solvent water. Aring the morphology of the obtained crystallites( Fig.ter stirring the mixed solution for about 10 min( In-3 ). The crysallites obtained at the early stage canduction period ) ,a great deal of ADP crystallites ap-preserve their ideal morphology , though their size ispear in the mixed solution. All samples are collected200 umCD中国煤化工MHCNMHGFig.3 Photographs and SEM image of ADP crytallites ( Sample A is taken out immediately when the rytallites can be seen in themixed号彩留, the other samples ,B ,C,D and E ,are piped out separately alt5 ,15 ,30 and 60 min after Sample A)XuDLetal.Crystallographic Analysis of Tapering of ADP Crystallites23 1small and there are some defects existed. However ,the tapered ADP crystallites are formed in this stagewith the increase of growth time ，the tapered ADP( tapered crystallites I ). Finally , the supersaturationcrystallites emerge ，the tapering of crystallites be-of the mixed solution is very low , the growth of pris-comes more and more intense along with the growth ofmatic planes is practically ceased while the growth ofcrystallites and the decrease of supersaturation.pyramidal planes is still maintained , therefore , seriousOwing to the currently selected growth condi-tapered ADP crystallites are formed in this stage ( ta-tions , the tapered crystallites are the inevitable resultpered crystallites II ).of the crystallographic characteristics of ADP crystals.4 ConclusionSince the growth of prismatic planes is strongly deter-From the above analysis of ADP crystal growthmined by the hydrogen bonding between ( H2PO4 )with respect to the crystallographic structure and se-anioos , ethanol molecules added into the mixed solu-lected growth conditions , one may find that a stabletion also interact with the oxygen atoms or hydroxyls ofsupersaturation and purified mother solution are the es-( H2PO4 ) groups , therefore , the hydrogen bondingsential elements for the rapid growth of ADP crystalsbetween( H2PO4 ) anions can be suspended by etha-with a high optical quality. The higher supersaturationnol molecules. The growth rate of prism planes iscan decrease the harmful influencesof impurities onslowed down while that of pyramidal planes is almostthe crystal growth. The impurities ( Especially theinvariable ( for their positively charged surface ) , themental ions with high valence states and molecules at-crystallites thus become slender with respect to growthtracted by zigzag anion chain ) can block the normalime. Furthermore , the whole zigzag chains formed bygrowth of prism planes , on the other hand , the anions( H,P04 )~ anions via hydrogen bonds can be blockedwith the similar configuration with( H,PO4 ) anionsby few impurities absorbed on( H2PO4 ) anions , thatmay influence the growth of pyramidal planes inten-is to say , the elementary step parallel to < 100 >sively.crystallographic directions on the prismatic planes canReferences :be blocked by few impurities absorbed on( H,PO4 )-anions.[1] Bhat H L, Roberts K J , Shervood J N. 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