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Morphology and structure studies of KDP and ADP crystallites in the water and ethanol solutions
Journal of Molecular Structure 740 (2005) 37–45 www.elsevier.com/locate/molstruc
Morphology and structure studies of KDP and ADP crystallites in the water and ethanol solutions Dongli Xua,b, Dongfeng Xuea,*, Henryk Ratajczakc,d a
State Key Laboratory of Fine Chemicals and School of Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, People’s Republic of China b College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, People’s Republic of China c Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland d Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wrocław 2, Poland Received 17 December 2004; accepted 6 January 2005
Abstract On the basis of calculations of the bond strength (of constituent chemical bonds that form during crystal growth process) and expanded capacity of low Miller index planes in KH2PO4 (KDP) and NH4H2PO4 (ADP) crystals, the ideal crystal morphologies and interactions between crystal surfaces and ethanol molecules are comprehensively studied. Our present results show that the crystallites morphology are mainly determined by the bond type, number, direction and strength, as well as the additive ingredient ethanol. Ethanol molecules may strongly affect ADP and KDP crystallites morphology by interacting with hydrogen bonds at crystal surfaces. q 2005 Elsevier B.V. All rights reserved. Keywords: Morphology; Ethanol–water mixed solution; Needle; SEM; X-ray diffraction
1. Introduction Potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP), which are well known for their good piezoelectric and nonlinear optical properties, have drawn many attentions for their significantly theoretical interests and commercial applications in the optical modulation, frequency conversion, electrooptics and so on [1,2]. In the past decades, many efforts have been made to promote the crystal quality and to increase the growth velocity with the aim to meet the requirements of inertial confinement fusion [3]. Yokotani et al. [4] have inspected KDP solution and developed a method to sterilize the microbes by ultraviolet light, which effectively improves the damage threshold 2–3 times (compared to those crystals grown by conventional technique). The quality dependence on impurities or additives such as trivalent cations, EDTA, borate etc. has been studied by Sasaki [5] and Yang [6]. * Corresponding author. Tel.: C86 41188993623; fax: C86 41188993623. E-mail address: [email protected] (D. Xue). 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.01.016
Using the high supersaturation (e.g., larger than 120%) and three-vessel method, KDP crystals of 62 mm in size have been achieved at a high-growth rate (excess of 50 mm/day) [5,7]. Although great advancements have been achieved, there are still many puzzles exist in the growth process of ADP and KDP crystal such as the morphology evolution with various additives [8–10], the form of growth units [11,12] and molecular interaction among growth units, solvents and crystal surfaces [13,14], which directly affect the crystal quality and growth velocity. In the present paper, the ideal crystal morphologies of ADP and KDP are deduced on the basis of calculation of the bond strength of all constituent chemical bonds (which are formed within basic growth units). On the basis of microscopic comparisons of the ideal and actual morphologies of crystallites grown in ethanol– water solvents, a possible interpretation of the evolution of crystal morphologies is proposed, which may provide us a useful way to comprehend the relation between chemical bonds and crystal figures, and to further find proper additives in controlling crystal morphologies.
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2. Crystal structures and the ideal morphology The crystallographic structures of KDP and ADP crystals have well been examined by X-ray analysis and neutron diffraction experiment [15–19]. There are four formula units per cell, both KDP and ADP isostructurally belong to noncentrosymmetric compounds at room temperature, which crystallize in the tetragonal system, space group I 42d, with ˚ , cZ7.546 A ˚ for ADP lattice parameters aZbZ7.502 A ˚ , cZ6.9717 A ˚ for KDP [19], [16] and aZbZ7.4528 A respectively. The very tiny distinction between them is their ˚ and different constituent cations (KC with the radius 1.33 A C ˚ NH4 with the radius 1.42 A [20]), which consequently arise the disparate interaction with the same anion H2 POK 4 and the final discrepancy between KDP and ADP crystallite morphologies. The unit cell structures are clearly shown in Fig. 1.
Fig. 1. Perspective view of the tetragonal unit cell of KDP and ADP crystals. All atoms except hydrogen atoms of H2 POK 4 are drawn in color solid spheres and labeled with element symbols, the color dotted lines represent various chemical bonds formed by body centered PO4 group with adjacent atoms. (For interpretation of references to color in this figure legend, the reader is referred to the web version of the article.)
Microscopically, chemical bonds affect and determine physicochemical properties of materials [21,22], which is also true for KDP and ADP crystals. The formed chemical bonds within growth units during the crystal growth, its strength and direction mainly determine the growth velocity of each crystallographic plane. On the other hand, those invariable bonds during the growth process can be regarded as a basic part of growth units, whose influences on the crystal growth velocity may be neglected. Cerreta et al. [11] show that the growth units of KDP and ADP crystals are K hydrated KC, NHC 4 and H2 PO4 groups, the bond length, strength, and direction, as well as chemical bonds formed between these growth units during growth process should be carefully considered. When investigating the above constituent chemical bonds in KDP and ADP crystals, one may find that anion– anion interactions are very similar. Two contiguous PO4 groups are interlinked through one hydrogen bond ˚ for KDP and 2.489 A ˚ (O–H/O) with a distant of 2.504 A for ADP, respectively. In fact, hydrogen atom in O–H/O bond is not properly on the line determined by these two oxygen atoms, it bonds to the nearer oxygen atom with a ˚ for these two crystals, and the short distance of 1.07 A ˚ for KDP separation to the farther oxygen atom is 1.434 A ˚ and 1.419 A for ADP. Therefore, these intervals and bond specialty for KDP and ADP are almost congruent. In both KDP and ADP crystals, each body-centered PO4 group connects to six adjacent cations by eight cationoxygen chemical bonds, that is, each cation closed to PO4 group along the c axis (there are two such cations) bonds to two oxygen atoms of PO4 group (which can form four cation-oxygen bonds), the other four cation-oxygen bonds are formed by the oxygen atoms of PO4 group with four separated cations surrounding it. However, the bond strength of KDP and ADP crystals is very different, chemical bonds of KDP are ionic while those of ADP are covalent, all calculated data of the bond strength are summarized in Table 1. As a result of replacing KC ion by NHC 4 group in ADP crystal, the dispersed positive charge density leads to a much weaker and more complex bonding between cation and anion. Compared to KC ion in KDP crystal, NHC 4 group may form much longer covalent bonds ˚ ) with anions. Due to the weaker contribution of (about 3 A longer chemical bonds, we thus neglect the plotting of weaker bonds in our figures. All constituent chemical bonds of KDP and ADP crystals may be categorized into three types by the direction and distribution in the lattice. The first kind of chemical bonds ˚ ), and that for ADP is for KDP is K–O ionic bonds (2.897 A ˚ ), which are practically H(N)–O covalent bonds (2.640 A parallel to c axis symmetrically and are drawn in blue dotted lines in Figs. 1, 2 and 5. Though H(N)–O bond length is a little shorter than K–O bond, the binding energy of ionic bonds is generally larger than that of covalent bonds, the bond strength along c axis are hence extraordinarily larger in KDP than ADP. The second kind of chemical bonds also
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Table 1 Crystallographic characteristics and chemical bond properties of KDP and ADP in one unit cell Bonds and Items KDP
ADP
Bond length ˚) (A
K–O 2.897 K–O 2.824 O/H 1.434 Total bond strength Number of identical planes ˚ 2) Area of plane (A Expanded capacity (!1000) Contribution of O/H bonds O–H(N) 2.640 O–H(N) 2.134 O/H(O) 1.419 Total bond strength Number of identical planes ˚ 2) Area of plane (A Expanded capacity (!1000) Contribution of O/H(O) bonds
Bond strength (v.u.) 0.1264938 0.1540824 0.2249472
0.013408 0.038097 0.234254
Crystal planes and contained bonds {100}
{001}
{101}
{110}
{112}
8 4 2 2.0782 2 51.959 19.998 4.329 8 4 2 0.7282 2 56.610 6.431 4.138
0 4 2 1.0662 4 55.544 4.799 2.025 0 4 2 0.6209 4 56.280 2.758 2.081
0 6 6 2.2742 2 76.058 14.950 8.873 0 6 6 1.6341 2 79.826 10.235 8.804
8 0 0 1.0119 4 73.481 3.443 0 8 0 0 0.1073 4 80.059 0.335 0
0 4 2 1.0662 3 66.596 5.337 2.252 0 4 2 0.6209 3 69.064 2.997 2.261
˚ ) in KDP and H(N)–O denote K–O ionic bonds (2.824 A ˚ covalent bonds (2.134 A) in ADP, its directions generally run parallel to (001) face and are illustrated in black dotted lines (Fig. 1), these ionic bond strengths are stronger than those of covalent bonds as well. The last kind of chemical bonds in KDP and ADP crystals particularly denote hydrogen bonds occurring between two adjacent PO4 groups via O–H/O bonds, which are perpendicular to c axis and represented by red dotted lines. The contributions of all chemical bonds determine the ideal morphology of crystals, and the fine distinctions between KDP and ADP structures may be the main reason for the variance of crystal outlines described below. According to Bravais rule, the most prominent faces of a crystal are those parallel to internal planes having the greatest density of lattice points (atoms, growth units), in some senses, it is in accordance with periodic bond chain theory. The planes having the greater density of lattice points may form more numbers of chemical bonds paralleling to them. Due to the atom interactions (chemical bonds) between growth units and crystal surfaces, the growth units prefer to aggregate along strong chemical bond directions, hence the tangential growth velocity of the plane is increased with the increasing number of chemical bonds at specific area, and the planes having the greatest density of atoms have more big area and emerge finally. Here we introduce a variable—expanded capacity—to reflect the proportion of every plane, the planes having bigger expanded capacity may emerge at the end of growth while the planes having smaller expanded capacity vanish finally. The expanded capacity are determined mainly by two factors: Tangential growth velocity, that is to say, is directly proportional to the number and strength of chemical bonds parallel to specific area, and the number of identical planes in one unit cell, determined by the symmetry of space group,
just defined as below. Expandedcapacity Z
ðbondnumberÞ!ðbondstrengthÞ ðnumberof theidenticalplaneinoneunitcellÞ!ðplaneareaÞ
Considering the symmetry of the tetragonal system, we may find that the low Miller index planes are made up by these three kinds of chemical bonds, which determine the ideal structure of these two isomorphs. The calculated results are listed in Table 1, the bond number and bond type are illustrated in Figs. 2–6, and the bond strength is quantitatively calculated by bond-valence model [23]. Our present calculations indicate that the expanded capacity of {100} and {101} planes are predominant within all planes, which thus lead to a morphology (or appearance) composed of a tetragonal prism ended with a tetragonal bipyramid. The schematic drawing of the ideal morphology of both crystals is shown in Fig. 7, which is in good agreement with Hartman’s analysis [24] and the following experimental results.
3. Experimental procedure KDP, ADP, ethanol of analytical grade and deionized water are used in our present experiments, which are carried out at ambient temperature 19 8C. The saturated KDP and ADP solutions of 17 8C are prepared, respectively, by the mass accurate up to 0.0001 g and allowed to achieve stabilization at 30 8C for 24 h. After filtration (with a porosity 60 mm), the temperature of the solutions, which are labeled as initial solutions, is decreased to ambient
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Fig. 2. {100} Planes (i.e., the prismatic faces) of KDP and ADP crystals (the upper is KDP and the below is ADP), viewed alone a axis. Three kinds of chemical bonds are shown with different colors and labeled with respective ˚ ). Blue dotted lines represent the first kind of chemical bond length (A bonds, black dotted lines depict the second kind of chemical bonds, and the third kind of chemical bonds is drawn in red dotted lines. (For interpretation of references to color in this figure legend, the reader is referred to the web version of the article.)
temperature. Since the amount of solute is measured at 17 8C, the initial solutions would be undersaturated at ambient temperature 19 8C. Initial KDP solution of 50 ml is transferred into a 100 ml glass beaker with agitating by magnetic stirrer and evaporates at ambient temperature for about half an hour, lots of KDP crystallites labeled as XA are grown spontaneously. XB is gained by pouring 15 ml KDP initial solution into 35 ml ethanol while XC is obtained by
Fig. 3. {001} Planes of KDP and ADP crystals, viewed along c axis. Red dotted lines represent hydrogen bonds between two PO4 groups and black dotted lines are the second kind of chemical bonds. (For interpretation of references to color in this figure legend, the reader is referred to the web version of the article.)
pouring 15 ml ethanol to 35 ml KDP initial solution in the process of agitating. XB and XC crystallites grow instantaneously when these two liquids are mixed. XD crystallites are prepared by the same process of XB except the KDP solution used in experiment is diluted to half of the initial concentration, this diluted KDP solution is used in preparing XE sample as well. The crystallites labeled as XE are relatively special one, no crystallite is observed immediately until adding 25 ml ethanol to the 35 ml diluted KDP solution and its growth period is thus a little longer in comparison with other samples. All of crystallites are filtrated immediately and dry at 40 8C for 24 h.
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Fig. 5. {110} Planes of KDP and ADP crystals, projected to (110) plane. There is only the first kind of chemical bonds in this plane. Fig. 4. {101} Planes (i.e., the pyramidal face) of KDP and ADP crystals, viewed along [101] direction. Green dotted lines represent chemical bonds outside the invested internal plane, and the meaning of other dotted lines is identical to that in Fig. 2. (For interpretation of references to color in this figure legend, the reader is referred to the web version of the article.)
The processes of growing ADP crystallites are identical with KDP, and ADP samples are labeled as ZA, ZB, ZC, ZD and ZE respectively, the detailed ratios of solution and ethanol are summarized in Table 2. To observe crystallites morphology directly, a field emission scanning electron microscopy (JSM-6700F) operated at 10 kV is employed here. The as-grown samples are characterized by an X-ray diffractometer (RINT 2200HF), and the position of diffraction peaks of every sample match well with the Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 35-0807 for KDP and No. 37-1479 for ADP.
4. Results and discussion Calculations of the low Miller index planes indicate that {100} and {101} planes have the most rapid expanded capacity under unconstrained condition, therefore, as the result of the growth competition, other planes disappear and the shapes of both crystals are composed of these two planes. It can be confirmed by SEM images (confer Fig. 8) of KDP and ADP crystallites grown in aqueous solutions by evaporation method, the growth conditions are approximate to the ideal unconstrained conditions, and the crystallites obtained are on the whole coincide with Fig. 7, though there are various magnitudes in volume and many defects. In more detail, because the expanded capacity of {100} planes is larger than that of {101} planes for KDP while the status for ADP is opposite, consequently KDP crystallites have much bigger {100} planes and relatively smaller {101} planes compared to ADP crystallites, that is to say, KDP crystallites
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D. Xu et al. / Journal of Molecular Structure 740 (2005) 37–45 Table 2 Contents of initial solution and ethanol for the preparation of all samples Samples
KDP solution (ml)
Ethanol (ml)
Samples
ADP solution (ml)
Ethanol (ml)
XA XB XC XD* XE*
50 15 35 15 35
0 35 15 35 25
ZA ZB ZC ZD* ZE*
50 15 35 15 35
0 35 15 35 25
Note: Samples marked with * are prepared by solutions diluted to half of the initial concentration.
Fig. 6. {112} Planes of KDP and ADP crystals, projected to (112) plane. The meaning of other dotted lines is identical to that in Fig. 2.
are a little slightness, and ADP crystallites may be more podgy, this phenomenon can well be observed in Fig. 8. Due to the different atom interactions among additives, growth units, solvent and chemical bonds at crystal surfaces,
Fig. 7. The scheme of the ideal morphology of both KDP and ADP crystals in an unconstrained system are drawn above.
the effect of ethanol on crystal morphology may be classified into two aspects: extracting solvent water from solution and adsorbing onto different crystal surfaces to suppress crystal growth. The former factor influences the volume of crystallites, while the later mainly affects the crystal morphology. When an amount of ethanol is poured into the undersaturated solution, ethanol molecules may form large number of hydrogen bonds with water molecules and emit heat, its effect is equivalent to reduce solvent water from the undersaturated solution, the solution then becomes instantaneously supersaturated and tremendous crystal nuclei engender and grow rapidly, all these samples except XA and ZA are gained by this method. Meanwhile, ethanol molecules may interact with hydrogen bonds of crystal surfaces through hydroxyl, the ethyl blocks the normal diffusion of growth units, which consequently affects the expanded capacity of every crystallographic plane, therefore, those crystal surfaces containing lots of hydrogen bonds are mainly influenced. To obtain KDP samples, a quantity of ethanol is mixed with KDP solution. For the crystal nuclei are huge and the total content of solute is limited, every nucleus thus cannot grow into large bulk but numerous crystallites, hence, the more amount of ethanol is used the little dimension of KDP crystallites we can harvest, this is in agreement with dimension of XB, XC and XD, XE samples (in Fig. 9). Compared with XC and XE the amounts of ethanol to prepare XB and XD are abundant, as a result, XB and XD are much smaller than XC and XE. For the solution to vegetate XD are diluted to half of the initial solution, mother solution concentration of XD is sparse than XB, hence XD are more little than XB for short of solute. The state seems should to be the same with XB and XD that XE should be little than XC, but in fact XE is the most big crystallites than other KDP samples, the reason is simple: Mother solution used in preparing XE are diluted and contain most amount of water and least amount of solute, until adding 25 ml ethanol into it can the mother solution become slightly supersaturated, during the adding of ethanol process, only a few crystal nuclei (comparing with XB, XD and XC) appear and grow for a relatively long time, hence, XE crystallites are much bigger than XB, XD and XC. The second effect of ethanol is adsorbed on particular sites of crystallites, which change the expanded capacity of
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Fig. 8. Images of KDP and ADP crystallites grown in aqueous solutions by the evaporation technique with agitating, photographed by scanning electron microscopy (JSM-6700F).
every plane. Generally speaking, only hydrogen bonds are disturbed by hydroxyl of ethanol molecules and the ionic bonds remain its initial property. For KDP crystallites, the hydrogen bonds, which propagate along h100i directions, are suppressed by adsorbing of ethanol molecules. Hence the KDP crystallites prefer to grow along strong ionic bonds directions h001i, the crystallites morphology transform from tetragonal prism to sharp needles, which are confirmed by
the SEM images of KDP specimen. In company with the weight ratio of ethanol in solution, the KDP crystallites become more little and sharp. However, the magnitudes of ADP crystallites are almost equal when mixing various amount of ethanol with ADP solution (confer Fig. 10). The reason is simple: because all of the chemical bonds in ADP crystal are covalent bonds that are more infirm than ionic bonds, hence the nucleation
Fig. 9. SEM images of KDP crystallites, which are grown in solutions with different contents of ethanol and water, photographed by scanning electron microscopy (JSM-6700F).
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Fig. 10. Images of ADP crystallites, which are grown in solutions with different contents of ethanol and water, photographed by scanning electron microscopy (JSM-6700F).
rate and crystal growth velocity of ADP are more laggard than those of KDP crystal, as a result, ADP has a smaller (relatively) quantity of nucleus and longer growth period, thus it may crystallize in a big volume. Meantime, all chemical bonds of ADP crystal are partly composed of hydrogen atoms, which may interact with hydroxyl of ethanol molecules, as a result, every bond direction of ADP has been suppressed by ethyl in some degree, which brings on the leveling effect of the growth velocity of every direction and plane, the more content of ethanol and the clearer leveling effect in ADP. Hence, both ZB and ZD samples cannot preserve a regular shape under agitating for the major contents of ethanol, only when ethanol concentration becomes minor such as ZC and ZE samples, can the crystallites present a regular figure. When a few amount of ethanol is mixed with ADP solution, the ethanol molecules prefer to attract H–O bonds of ADP crystal along h100i direction for the resemble of hydroxyl, which lead to the preferential growth alone c axis, hence, the crystal morphology of ZC and ZE transfer from the tetragonal prism to needle in common with KDP. In short, the more ethanol concentration, the faster growth along c axis, when the ethanol concentration exceeds special contents or agitating override a special rate, the crystallites cannot maintain a regular figure.
The powder diffraction angles of KDP crystallites coincide with the standard JCPDS Card No. 35-0807 very well except the intensity of diffraction peaks, which indicate that the interactions between the crystal surface and ethanol are weak than normal chemical bonds, and ethanol molecules may not enter crystallites to produce new phase under agitating. Since the powder diffraction experiments are conducted using crystallites directly instead of grinded
Fig. 11. Powder X-ray diffraction patterns of KDP specimens.
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are composed by a tetragonal prism ended with a tetragonal bipyramid. After comparing it with the actual crystallite shapes obtained in water–ethanol mixed solution, we propose the possible mechanism of crystal morphology evolution. That is, ethanol molecules interact with hydrogen bonds of KDP and ADP crystal through hydroxyl and the ethyl thus suspend the normal diffusion process of growth units, which restrain the crystal development along hydrogen bonds direction, hence the morphologies of crystal are changed.
Acknowledgements Fig. 12. Powder X-ray diffraction patterns of ADP specimens.
powder, the peak intensity indeed reflect the proportions of every plane and orientations of crystal growth. According to our present calculations, {100} planes have the largest area for X-ray irradiations, which give rise to the strongest peak intensity of {200} peak. Other diffraction peaks (i.e., intensities) mainly depend on the plane area and probability to be irradiated, the high Miller index planes of small crystallites have more probability irradiated by X-ray than big ones, the smaller crystallites and the stronger peak intensity, which can be detected in Fig. 11. Crystallites labeled as XB, XC and XD have the small dimension comparison to XA and XE, which have stronger diffraction peaks of high Miller index planes than XA and XE as well, the morphology and size of XA and XE are similar, their X-ray patterns are thus similar. Analogy with KDP, the powder diffraction angles of ADP crystallites (Fig. 12) are in agreement with standard JCPDS Card No. 37-1479, there are no other phases emerge besides the tetragonal system. The peaks intensity of ZB and ZD are similar to the standard JCPDS card, since these crystallites are irregular, which provides every plane an opportunity to be irradiated, the peaks of every plane have fairish intensity in X-ray pattern. However, for ZA, ZC and ZE, the orientation and uniform figure of crystallites strongly affect the diffraction peak intensity, since these three crystallites have the biggest {100} plane compared to other planes, which provide a biggest probability to meet the condition of Bragg diffraction, hence {100} diffraction peak becomes the strongest signal instead of {101} as usual.
5. Conclusions Based on the calculations of constituent chemical bonds forming during growth process in both KDP and ADP crystals, the ideal morphologies of them are deduced, which
A Foundation for the Author of National Excellent Doctoral Dissertation of People’s Republic of China (FANEDD #200322) is thanked for the financial support.
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Morphology and structure studies of KDP and ADP crystallites in the water and ethanol solutions Dongli Xua,b, Dongfeng Xuea,*, Henryk Ratajczakc,d a
State Key Laboratory of Fine Chemicals and School of Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, People’s Republic of China b College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, People’s Republic of China c Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland d Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wrocław 2, Poland Received 17 December 2004; accepted 6 January 2005
Abstract On the basis of calculations of the bond strength (of constituent chemical bonds that form during crystal growth process) and expanded capacity of low Miller index planes in KH2PO4 (KDP) and NH4H2PO4 (ADP) crystals, the ideal crystal morphologies and interactions between crystal surfaces and ethanol molecules are comprehensively studied. Our present results show that the crystallites morphology are mainly determined by the bond type, number, direction and strength, as well as the additive ingredient ethanol. Ethanol molecules may strongly affect ADP and KDP crystallites morphology by interacting with hydrogen bonds at crystal surfaces. q 2005 Elsevier B.V. All rights reserved. Keywords: Morphology; Ethanol–water mixed solution; Needle; SEM; X-ray diffraction
1. Introduction Potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP), which are well known for their good piezoelectric and nonlinear optical properties, have drawn many attentions for their significantly theoretical interests and commercial applications in the optical modulation, frequency conversion, electrooptics and so on [1,2]. In the past decades, many efforts have been made to promote the crystal quality and to increase the growth velocity with the aim to meet the requirements of inertial confinement fusion [3]. Yokotani et al. [4] have inspected KDP solution and developed a method to sterilize the microbes by ultraviolet light, which effectively improves the damage threshold 2–3 times (compared to those crystals grown by conventional technique). The quality dependence on impurities or additives such as trivalent cations, EDTA, borate etc. has been studied by Sasaki [5] and Yang [6]. * Corresponding author. Tel.: C86 41188993623; fax: C86 41188993623. E-mail address: [email protected] (D. Xue). 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.01.016
Using the high supersaturation (e.g., larger than 120%) and three-vessel method, KDP crystals of 62 mm in size have been achieved at a high-growth rate (excess of 50 mm/day) [5,7]. Although great advancements have been achieved, there are still many puzzles exist in the growth process of ADP and KDP crystal such as the morphology evolution with various additives [8–10], the form of growth units [11,12] and molecular interaction among growth units, solvents and crystal surfaces [13,14], which directly affect the crystal quality and growth velocity. In the present paper, the ideal crystal morphologies of ADP and KDP are deduced on the basis of calculation of the bond strength of all constituent chemical bonds (which are formed within basic growth units). On the basis of microscopic comparisons of the ideal and actual morphologies of crystallites grown in ethanol– water solvents, a possible interpretation of the evolution of crystal morphologies is proposed, which may provide us a useful way to comprehend the relation between chemical bonds and crystal figures, and to further find proper additives in controlling crystal morphologies.
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2. Crystal structures and the ideal morphology The crystallographic structures of KDP and ADP crystals have well been examined by X-ray analysis and neutron diffraction experiment [15–19]. There are four formula units per cell, both KDP and ADP isostructurally belong to noncentrosymmetric compounds at room temperature, which crystallize in the tetragonal system, space group I 42d, with ˚ , cZ7.546 A ˚ for ADP lattice parameters aZbZ7.502 A ˚ , cZ6.9717 A ˚ for KDP [19], [16] and aZbZ7.4528 A respectively. The very tiny distinction between them is their ˚ and different constituent cations (KC with the radius 1.33 A C ˚ NH4 with the radius 1.42 A [20]), which consequently arise the disparate interaction with the same anion H2 POK 4 and the final discrepancy between KDP and ADP crystallite morphologies. The unit cell structures are clearly shown in Fig. 1.
Fig. 1. Perspective view of the tetragonal unit cell of KDP and ADP crystals. All atoms except hydrogen atoms of H2 POK 4 are drawn in color solid spheres and labeled with element symbols, the color dotted lines represent various chemical bonds formed by body centered PO4 group with adjacent atoms. (For interpretation of references to color in this figure legend, the reader is referred to the web version of the article.)
Microscopically, chemical bonds affect and determine physicochemical properties of materials [21,22], which is also true for KDP and ADP crystals. The formed chemical bonds within growth units during the crystal growth, its strength and direction mainly determine the growth velocity of each crystallographic plane. On the other hand, those invariable bonds during the growth process can be regarded as a basic part of growth units, whose influences on the crystal growth velocity may be neglected. Cerreta et al. [11] show that the growth units of KDP and ADP crystals are K hydrated KC, NHC 4 and H2 PO4 groups, the bond length, strength, and direction, as well as chemical bonds formed between these growth units during growth process should be carefully considered. When investigating the above constituent chemical bonds in KDP and ADP crystals, one may find that anion– anion interactions are very similar. Two contiguous PO4 groups are interlinked through one hydrogen bond ˚ for KDP and 2.489 A ˚ (O–H/O) with a distant of 2.504 A for ADP, respectively. In fact, hydrogen atom in O–H/O bond is not properly on the line determined by these two oxygen atoms, it bonds to the nearer oxygen atom with a ˚ for these two crystals, and the short distance of 1.07 A ˚ for KDP separation to the farther oxygen atom is 1.434 A ˚ and 1.419 A for ADP. Therefore, these intervals and bond specialty for KDP and ADP are almost congruent. In both KDP and ADP crystals, each body-centered PO4 group connects to six adjacent cations by eight cationoxygen chemical bonds, that is, each cation closed to PO4 group along the c axis (there are two such cations) bonds to two oxygen atoms of PO4 group (which can form four cation-oxygen bonds), the other four cation-oxygen bonds are formed by the oxygen atoms of PO4 group with four separated cations surrounding it. However, the bond strength of KDP and ADP crystals is very different, chemical bonds of KDP are ionic while those of ADP are covalent, all calculated data of the bond strength are summarized in Table 1. As a result of replacing KC ion by NHC 4 group in ADP crystal, the dispersed positive charge density leads to a much weaker and more complex bonding between cation and anion. Compared to KC ion in KDP crystal, NHC 4 group may form much longer covalent bonds ˚ ) with anions. Due to the weaker contribution of (about 3 A longer chemical bonds, we thus neglect the plotting of weaker bonds in our figures. All constituent chemical bonds of KDP and ADP crystals may be categorized into three types by the direction and distribution in the lattice. The first kind of chemical bonds ˚ ), and that for ADP is for KDP is K–O ionic bonds (2.897 A ˚ ), which are practically H(N)–O covalent bonds (2.640 A parallel to c axis symmetrically and are drawn in blue dotted lines in Figs. 1, 2 and 5. Though H(N)–O bond length is a little shorter than K–O bond, the binding energy of ionic bonds is generally larger than that of covalent bonds, the bond strength along c axis are hence extraordinarily larger in KDP than ADP. The second kind of chemical bonds also
D. Xu et al. / Journal of Molecular Structure 740 (2005) 37–45
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Table 1 Crystallographic characteristics and chemical bond properties of KDP and ADP in one unit cell Bonds and Items KDP
ADP
Bond length ˚) (A
K–O 2.897 K–O 2.824 O/H 1.434 Total bond strength Number of identical planes ˚ 2) Area of plane (A Expanded capacity (!1000) Contribution of O/H bonds O–H(N) 2.640 O–H(N) 2.134 O/H(O) 1.419 Total bond strength Number of identical planes ˚ 2) Area of plane (A Expanded capacity (!1000) Contribution of O/H(O) bonds
Bond strength (v.u.) 0.1264938 0.1540824 0.2249472
0.013408 0.038097 0.234254
Crystal planes and contained bonds {100}
{001}
{101}
{110}
{112}
8 4 2 2.0782 2 51.959 19.998 4.329 8 4 2 0.7282 2 56.610 6.431 4.138
0 4 2 1.0662 4 55.544 4.799 2.025 0 4 2 0.6209 4 56.280 2.758 2.081
0 6 6 2.2742 2 76.058 14.950 8.873 0 6 6 1.6341 2 79.826 10.235 8.804
8 0 0 1.0119 4 73.481 3.443 0 8 0 0 0.1073 4 80.059 0.335 0
0 4 2 1.0662 3 66.596 5.337 2.252 0 4 2 0.6209 3 69.064 2.997 2.261
˚ ) in KDP and H(N)–O denote K–O ionic bonds (2.824 A ˚ covalent bonds (2.134 A) in ADP, its directions generally run parallel to (001) face and are illustrated in black dotted lines (Fig. 1), these ionic bond strengths are stronger than those of covalent bonds as well. The last kind of chemical bonds in KDP and ADP crystals particularly denote hydrogen bonds occurring between two adjacent PO4 groups via O–H/O bonds, which are perpendicular to c axis and represented by red dotted lines. The contributions of all chemical bonds determine the ideal morphology of crystals, and the fine distinctions between KDP and ADP structures may be the main reason for the variance of crystal outlines described below. According to Bravais rule, the most prominent faces of a crystal are those parallel to internal planes having the greatest density of lattice points (atoms, growth units), in some senses, it is in accordance with periodic bond chain theory. The planes having the greater density of lattice points may form more numbers of chemical bonds paralleling to them. Due to the atom interactions (chemical bonds) between growth units and crystal surfaces, the growth units prefer to aggregate along strong chemical bond directions, hence the tangential growth velocity of the plane is increased with the increasing number of chemical bonds at specific area, and the planes having the greatest density of atoms have more big area and emerge finally. Here we introduce a variable—expanded capacity—to reflect the proportion of every plane, the planes having bigger expanded capacity may emerge at the end of growth while the planes having smaller expanded capacity vanish finally. The expanded capacity are determined mainly by two factors: Tangential growth velocity, that is to say, is directly proportional to the number and strength of chemical bonds parallel to specific area, and the number of identical planes in one unit cell, determined by the symmetry of space group,
just defined as below. Expandedcapacity Z
ðbondnumberÞ!ðbondstrengthÞ ðnumberof theidenticalplaneinoneunitcellÞ!ðplaneareaÞ
Considering the symmetry of the tetragonal system, we may find that the low Miller index planes are made up by these three kinds of chemical bonds, which determine the ideal structure of these two isomorphs. The calculated results are listed in Table 1, the bond number and bond type are illustrated in Figs. 2–6, and the bond strength is quantitatively calculated by bond-valence model [23]. Our present calculations indicate that the expanded capacity of {100} and {101} planes are predominant within all planes, which thus lead to a morphology (or appearance) composed of a tetragonal prism ended with a tetragonal bipyramid. The schematic drawing of the ideal morphology of both crystals is shown in Fig. 7, which is in good agreement with Hartman’s analysis [24] and the following experimental results.
3. Experimental procedure KDP, ADP, ethanol of analytical grade and deionized water are used in our present experiments, which are carried out at ambient temperature 19 8C. The saturated KDP and ADP solutions of 17 8C are prepared, respectively, by the mass accurate up to 0.0001 g and allowed to achieve stabilization at 30 8C for 24 h. After filtration (with a porosity 60 mm), the temperature of the solutions, which are labeled as initial solutions, is decreased to ambient
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Fig. 2. {100} Planes (i.e., the prismatic faces) of KDP and ADP crystals (the upper is KDP and the below is ADP), viewed alone a axis. Three kinds of chemical bonds are shown with different colors and labeled with respective ˚ ). Blue dotted lines represent the first kind of chemical bond length (A bonds, black dotted lines depict the second kind of chemical bonds, and the third kind of chemical bonds is drawn in red dotted lines. (For interpretation of references to color in this figure legend, the reader is referred to the web version of the article.)
temperature. Since the amount of solute is measured at 17 8C, the initial solutions would be undersaturated at ambient temperature 19 8C. Initial KDP solution of 50 ml is transferred into a 100 ml glass beaker with agitating by magnetic stirrer and evaporates at ambient temperature for about half an hour, lots of KDP crystallites labeled as XA are grown spontaneously. XB is gained by pouring 15 ml KDP initial solution into 35 ml ethanol while XC is obtained by
Fig. 3. {001} Planes of KDP and ADP crystals, viewed along c axis. Red dotted lines represent hydrogen bonds between two PO4 groups and black dotted lines are the second kind of chemical bonds. (For interpretation of references to color in this figure legend, the reader is referred to the web version of the article.)
pouring 15 ml ethanol to 35 ml KDP initial solution in the process of agitating. XB and XC crystallites grow instantaneously when these two liquids are mixed. XD crystallites are prepared by the same process of XB except the KDP solution used in experiment is diluted to half of the initial concentration, this diluted KDP solution is used in preparing XE sample as well. The crystallites labeled as XE are relatively special one, no crystallite is observed immediately until adding 25 ml ethanol to the 35 ml diluted KDP solution and its growth period is thus a little longer in comparison with other samples. All of crystallites are filtrated immediately and dry at 40 8C for 24 h.
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Fig. 5. {110} Planes of KDP and ADP crystals, projected to (110) plane. There is only the first kind of chemical bonds in this plane. Fig. 4. {101} Planes (i.e., the pyramidal face) of KDP and ADP crystals, viewed along [101] direction. Green dotted lines represent chemical bonds outside the invested internal plane, and the meaning of other dotted lines is identical to that in Fig. 2. (For interpretation of references to color in this figure legend, the reader is referred to the web version of the article.)
The processes of growing ADP crystallites are identical with KDP, and ADP samples are labeled as ZA, ZB, ZC, ZD and ZE respectively, the detailed ratios of solution and ethanol are summarized in Table 2. To observe crystallites morphology directly, a field emission scanning electron microscopy (JSM-6700F) operated at 10 kV is employed here. The as-grown samples are characterized by an X-ray diffractometer (RINT 2200HF), and the position of diffraction peaks of every sample match well with the Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 35-0807 for KDP and No. 37-1479 for ADP.
4. Results and discussion Calculations of the low Miller index planes indicate that {100} and {101} planes have the most rapid expanded capacity under unconstrained condition, therefore, as the result of the growth competition, other planes disappear and the shapes of both crystals are composed of these two planes. It can be confirmed by SEM images (confer Fig. 8) of KDP and ADP crystallites grown in aqueous solutions by evaporation method, the growth conditions are approximate to the ideal unconstrained conditions, and the crystallites obtained are on the whole coincide with Fig. 7, though there are various magnitudes in volume and many defects. In more detail, because the expanded capacity of {100} planes is larger than that of {101} planes for KDP while the status for ADP is opposite, consequently KDP crystallites have much bigger {100} planes and relatively smaller {101} planes compared to ADP crystallites, that is to say, KDP crystallites
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D. Xu et al. / Journal of Molecular Structure 740 (2005) 37–45 Table 2 Contents of initial solution and ethanol for the preparation of all samples Samples
KDP solution (ml)
Ethanol (ml)
Samples
ADP solution (ml)
Ethanol (ml)
XA XB XC XD* XE*
50 15 35 15 35
0 35 15 35 25
ZA ZB ZC ZD* ZE*
50 15 35 15 35
0 35 15 35 25
Note: Samples marked with * are prepared by solutions diluted to half of the initial concentration.
Fig. 6. {112} Planes of KDP and ADP crystals, projected to (112) plane. The meaning of other dotted lines is identical to that in Fig. 2.
are a little slightness, and ADP crystallites may be more podgy, this phenomenon can well be observed in Fig. 8. Due to the different atom interactions among additives, growth units, solvent and chemical bonds at crystal surfaces,
Fig. 7. The scheme of the ideal morphology of both KDP and ADP crystals in an unconstrained system are drawn above.
the effect of ethanol on crystal morphology may be classified into two aspects: extracting solvent water from solution and adsorbing onto different crystal surfaces to suppress crystal growth. The former factor influences the volume of crystallites, while the later mainly affects the crystal morphology. When an amount of ethanol is poured into the undersaturated solution, ethanol molecules may form large number of hydrogen bonds with water molecules and emit heat, its effect is equivalent to reduce solvent water from the undersaturated solution, the solution then becomes instantaneously supersaturated and tremendous crystal nuclei engender and grow rapidly, all these samples except XA and ZA are gained by this method. Meanwhile, ethanol molecules may interact with hydrogen bonds of crystal surfaces through hydroxyl, the ethyl blocks the normal diffusion of growth units, which consequently affects the expanded capacity of every crystallographic plane, therefore, those crystal surfaces containing lots of hydrogen bonds are mainly influenced. To obtain KDP samples, a quantity of ethanol is mixed with KDP solution. For the crystal nuclei are huge and the total content of solute is limited, every nucleus thus cannot grow into large bulk but numerous crystallites, hence, the more amount of ethanol is used the little dimension of KDP crystallites we can harvest, this is in agreement with dimension of XB, XC and XD, XE samples (in Fig. 9). Compared with XC and XE the amounts of ethanol to prepare XB and XD are abundant, as a result, XB and XD are much smaller than XC and XE. For the solution to vegetate XD are diluted to half of the initial solution, mother solution concentration of XD is sparse than XB, hence XD are more little than XB for short of solute. The state seems should to be the same with XB and XD that XE should be little than XC, but in fact XE is the most big crystallites than other KDP samples, the reason is simple: Mother solution used in preparing XE are diluted and contain most amount of water and least amount of solute, until adding 25 ml ethanol into it can the mother solution become slightly supersaturated, during the adding of ethanol process, only a few crystal nuclei (comparing with XB, XD and XC) appear and grow for a relatively long time, hence, XE crystallites are much bigger than XB, XD and XC. The second effect of ethanol is adsorbed on particular sites of crystallites, which change the expanded capacity of
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Fig. 8. Images of KDP and ADP crystallites grown in aqueous solutions by the evaporation technique with agitating, photographed by scanning electron microscopy (JSM-6700F).
every plane. Generally speaking, only hydrogen bonds are disturbed by hydroxyl of ethanol molecules and the ionic bonds remain its initial property. For KDP crystallites, the hydrogen bonds, which propagate along h100i directions, are suppressed by adsorbing of ethanol molecules. Hence the KDP crystallites prefer to grow along strong ionic bonds directions h001i, the crystallites morphology transform from tetragonal prism to sharp needles, which are confirmed by
the SEM images of KDP specimen. In company with the weight ratio of ethanol in solution, the KDP crystallites become more little and sharp. However, the magnitudes of ADP crystallites are almost equal when mixing various amount of ethanol with ADP solution (confer Fig. 10). The reason is simple: because all of the chemical bonds in ADP crystal are covalent bonds that are more infirm than ionic bonds, hence the nucleation
Fig. 9. SEM images of KDP crystallites, which are grown in solutions with different contents of ethanol and water, photographed by scanning electron microscopy (JSM-6700F).
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Fig. 10. Images of ADP crystallites, which are grown in solutions with different contents of ethanol and water, photographed by scanning electron microscopy (JSM-6700F).
rate and crystal growth velocity of ADP are more laggard than those of KDP crystal, as a result, ADP has a smaller (relatively) quantity of nucleus and longer growth period, thus it may crystallize in a big volume. Meantime, all chemical bonds of ADP crystal are partly composed of hydrogen atoms, which may interact with hydroxyl of ethanol molecules, as a result, every bond direction of ADP has been suppressed by ethyl in some degree, which brings on the leveling effect of the growth velocity of every direction and plane, the more content of ethanol and the clearer leveling effect in ADP. Hence, both ZB and ZD samples cannot preserve a regular shape under agitating for the major contents of ethanol, only when ethanol concentration becomes minor such as ZC and ZE samples, can the crystallites present a regular figure. When a few amount of ethanol is mixed with ADP solution, the ethanol molecules prefer to attract H–O bonds of ADP crystal along h100i direction for the resemble of hydroxyl, which lead to the preferential growth alone c axis, hence, the crystal morphology of ZC and ZE transfer from the tetragonal prism to needle in common with KDP. In short, the more ethanol concentration, the faster growth along c axis, when the ethanol concentration exceeds special contents or agitating override a special rate, the crystallites cannot maintain a regular figure.
The powder diffraction angles of KDP crystallites coincide with the standard JCPDS Card No. 35-0807 very well except the intensity of diffraction peaks, which indicate that the interactions between the crystal surface and ethanol are weak than normal chemical bonds, and ethanol molecules may not enter crystallites to produce new phase under agitating. Since the powder diffraction experiments are conducted using crystallites directly instead of grinded
Fig. 11. Powder X-ray diffraction patterns of KDP specimens.
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are composed by a tetragonal prism ended with a tetragonal bipyramid. After comparing it with the actual crystallite shapes obtained in water–ethanol mixed solution, we propose the possible mechanism of crystal morphology evolution. That is, ethanol molecules interact with hydrogen bonds of KDP and ADP crystal through hydroxyl and the ethyl thus suspend the normal diffusion process of growth units, which restrain the crystal development along hydrogen bonds direction, hence the morphologies of crystal are changed.
Acknowledgements Fig. 12. Powder X-ray diffraction patterns of ADP specimens.
powder, the peak intensity indeed reflect the proportions of every plane and orientations of crystal growth. According to our present calculations, {100} planes have the largest area for X-ray irradiations, which give rise to the strongest peak intensity of {200} peak. Other diffraction peaks (i.e., intensities) mainly depend on the plane area and probability to be irradiated, the high Miller index planes of small crystallites have more probability irradiated by X-ray than big ones, the smaller crystallites and the stronger peak intensity, which can be detected in Fig. 11. Crystallites labeled as XB, XC and XD have the small dimension comparison to XA and XE, which have stronger diffraction peaks of high Miller index planes than XA and XE as well, the morphology and size of XA and XE are similar, their X-ray patterns are thus similar. Analogy with KDP, the powder diffraction angles of ADP crystallites (Fig. 12) are in agreement with standard JCPDS Card No. 37-1479, there are no other phases emerge besides the tetragonal system. The peaks intensity of ZB and ZD are similar to the standard JCPDS card, since these crystallites are irregular, which provides every plane an opportunity to be irradiated, the peaks of every plane have fairish intensity in X-ray pattern. However, for ZA, ZC and ZE, the orientation and uniform figure of crystallites strongly affect the diffraction peak intensity, since these three crystallites have the biggest {100} plane compared to other planes, which provide a biggest probability to meet the condition of Bragg diffraction, hence {100} diffraction peak becomes the strongest signal instead of {101} as usual.
5. Conclusions Based on the calculations of constituent chemical bonds forming during growth process in both KDP and ADP crystals, the ideal morphologies of them are deduced, which
A Foundation for the Author of National Excellent Doctoral Dissertation of People’s Republic of China (FANEDD #200322) is thanked for the financial support.
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