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International Journal of Modern Physics B, Vol. 16, Nos. 1 & 2 (2002) 114-121 © World Scientific Publishing Company
THE IMPURITY EFFECTS ON GROWTH, STRUCTURE AND PHYSICAL PROPERTIES OF STRONTIUM NITRATE CRYSTALS
CHAORONG LI LIJUN WU WANCHUN CHEN
Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R.China. National Microgravity Laboratory, Chinese Academy of Sciences,Beijing 100080, P.R.China.
E-mail: [email protected]
Received (15 June 2001) Revised (15 August 2001)
The effects of Ba2+ and Pb2+ impurities on the crystal growth, structure and physical properties of Sr(N03)2 crystal have been investigated. The in situ crystal growth observations show that in pure or Pb2+ doped solutions the crystal growth rate on {100} surfaces is larger than that on {111} surfaces, that is Rioo>Rui, in the low supersaturation region, while Rioo<Riu in the higher supersaturation region. However, in the case of Ba2+ doped crystals, the scenario was reversed. Therefore, the doping of Ba2+ will influence the crystal growth morphology of the strontium nitrate crystals. Composition distribution show that the Ba2+ ions were enriched in the {100} growth sectors, while Pb2+ ions were enriched in {111} sectors. The impurities will deteriorate the crystalline quality from the high resolution x-ray diffraction results. Raman dominating peaks of the Ba2+ or Pb2+ doped Sr(N03)2 crystals will be shifted to lower frequencies and the shifting value depends on impurity composition..
1 Introduction
The impurities are a common problem in crystal growth, due to the purity limitation of the raw materials and deliberately doping for the purpose of properties. Usually, the impurities are inhomogeneously distributed in both natural and synthetic crystals. One of the typical appearances of composition inhomogeneity is sector zoning, which refers to a compositional difference between coeval growth sectors, or portions of growth sectors, in a crystal. Most workers have considered either rate effects or surface effects, or both, to explain the origin of the compositional zoning, and some quantitative models have been proposed.1"3
Impurity effects on the crystal growth kinetics have been intensively studied theoretically and experimentally.4"10 Impurities adsorbed on the crystal growth surface will block the movement of the advancing steps. The crystal growth may even completely stop when the driving force is lower than a critical value o-* = (A/J/KT)*'7
This is the well known "dead zone" phenomenon in crystal growth. The nitrate crystals have been widely used as a model crystal for crystal growth
mechanism investigations both on ground and under microgravity due to their simple cubic structures.11'12 Furthermore, the nitrate single crystals can be used in stimulated
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The Impurity Effects on Growth, Structure and Physical Properties 115
Raman scattering devices to shift the emission frequency of lasers to different spectral regions.13'14 Therefore, the understanding of the impurity effects on the growth, structure and property of nitrate crystals is also important for device applications. In this paper, the impurity of Me2+( Me2+=Ba2+or Pb2+) distribution in Sr(N03)2 crystals and its effects on crystal growth behavior, crystalline quality and the physical property are systemically studied.
2 Experimental
Sr(N03)2 crystals doped with Ba2+ or Pb2+ ions were grown on pure Sr(N03)2 seeds from aqueous solution by a temperature cooling method, details has been described elsewhere.15'16 The as grown crystals showed a polyhedral morphology, typically with a dodecahedron shape bounded with {100} and {111} faces, as shown in Fig. 1(a). The crystals were cut along (110) lattice plane, as indicated in Fig. 1(a), for composition, structure and physical properties measurements. The cross section with growth sector orientations is depicted in Fig. 1(b).
Fig.l. (a) A schematic drawing of the as grown Sr(N03)2 crystal and the cross sectioned orientation plane; (b) The cross section of the crystal along (110) and the growth sectors.
The mean concentrations of barium or lead and strontium in the initial growth solutions and final solid solutions were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The qualitative and quantitatively composition of the crystals were studied by back-scattered-electron (BSE) imaging in a JXA8800R scanning electron microscope after the crystals were cut, polished and carbon coated.
The crystal/solution interface structures were in situ studied using Mach-Zehnder interferometer with 488.0nm emission line of Ar+ laser. The average crystal growth rates of different crystalline faces were determined by measuring the moving distance of the measured surface at a time interval.
The crystalline quality was determined by high resolution x-ray diffraction rocking curve (HRXDRC) technique with Cu Kal radiation. Rocking curves with (440) symmetrical diffraction were recorded on the cross-sectioned samples. In order to measure the crystalline quality at different positions on the sample surface, the x-ray illuminating size was confined by a pin-hole slit made of lead with a diameter of about 0.5mm.
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116 C. Li, L. Wu & W. Chen
Raman spectra were recorded with back scattering geometry by a Spex-1403 Raman spectrometer using 488.0nm emission line of Ar+ laser source at room temperature. The step and the recording time were respectively 0.1 cm"1 and 0.5 seconds.
3 Experimental Results
3.1 Impurity Distribution
Table 1 shows the concentration relationships of the initial aqueous solutions (YBa, aq and Ypb, aq = mole fraction of Ba2+ and Pb2+ in the aqueous solutions) and the mean compositions of the crystals (XBa,s and XPbtS= solid-phase Ba(N03)2 and Pb(N03)2 mole fractions), respectively. From table 1, one can see that a small fraction of Ba2+ or Pb2+ in the solution will result in a high concentration in the solid.
Table 1. Concentrations relationship of the solutions (YBa>aq and YPbi„q) and the mean compositions of the crystals (XBa,s and XPbtS) obtained by the inductively coupled plasma-atomic emission spectrometry
-t Ba.ag Ypb,at Xph
(a)
0.0 0.0059 0.0078 0.01 0.024 0.046 0.09 1.0
,- ' -
0.0 0.084 0.10 0.15 0.40 0.75 0.91 1.0
0.0 0.032 0.1
0.18 0.23 0.29 0.36 1.0
0.0 0.081 0.37 0.7
0.79 0.86 0.93 1.0
0.076
2 0.072
Ill-type sector Ill-type seel
(b)
Fig.2. (a) The qualitative composition measurement image and the locus for quantitative measurement of the Ba2+ doped sample; (b) the quantitative measurement result.
Usually, the segregation of Ba2+ or Pb2+ in the crystal is not homogeneous and shows compositional zoning. Fig.2(a) and Fig.3(a) are the BSE images of typical Ba2+
and Pb2+ doped crystals respectively, and clearly show compositional zoning of the impurities. The brightness of the image is determined by the composition, which allows the compositional heterogeneities to be distinguished in a qualitative way.17 In order to ensure this visual phenomenon, the spot analyses (the spot size was 10 urn) of
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The Impurity Effects on Growth, Structure and Physical Properties 117
quantitative electron microprobe analyses were applied to measure the distribution of the substituting ions. The straight lines on the photograph locate the measured spots. The quantitative results are illustrated in Fig.2(b) and Fig.3(b). The quantitative measured positions were indicated by straight lines in Fig.2(a) and labeled spots in Fig.3(a). As can be observed, for the Ba2+ doped sample, the BSE image is brighter in {100} sectors than in {111} sectors, indicating that Ba is enriched in {100} sectors since Ba has the larger atomic number than Sr. On the other hand, as for the Pb2+ doped sample, the back scattered electron image is brighter in {111} sectors than in {100} sectors, indicating that Pb is enriched in {111} sectors. The quantitative results confirm the qualitative results. The average composition in the seed, {100} sectors and {111} sectors are listed in Table 2 and 3 for Ba2+ and Pb2+ doped samples, respectively, in which X represents the mole fraction of Me2+ in the crystals. In the barium-doped crystal, the seed contains barium, caused by the small amount of Ba2+ impurity in the raw material, which is easy to segregate in the solid phase when crystallized, see Table 1.
0.069
0.066
0.063
0.060
0.057
0.054
-
-2
1 3 4 5 6
: I * I * i * i 7
100-type sector
8
llU 9 1 0 11
111 -type sector
(a) (b)
Fig.3. (a) The qualitative composition measurement image and the locus for quantitative measurement of Pb2+
doped sample; (b) the quantitative measurement result.
Table 2. The concentration of Ba2+, FWHM of HRXDRC , the peak position and FWHM of Sr(N03)2 crystal dominating Raman scattering peak at different sample areas.
Area
XBO.S
FWHM of HRXDRC (degree) Center of Raman peakCcm"1)
FWHM ofRamanpeakfcm'1)
Seed 0.014
0.0057 1057.0 4.5764
{111} _ 0.068
0.0284 1056.6 4.6233
(100) 0.078 0.0413 1056.4 4.6966
Table 3. The concentration of Pb2+, FWHM of HRXDRC , the position and FWHM of Sr(N03)2 crystal dominating Raman scattering peak at different sample areas.
Area Seed {111} {100}
Xpb,s
FWHM of HRXDRC (degree) Center of Raman peak(cm_1)
0.0 0.0077 1056.4
0.067 0.0201 1056.1
0.055 0.0148 1056.3
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118 C. Li, L. Wu & W. Chen
3.2 Impurity Effects on Crystal Growth Rate
The starting solutions were set to be saturated at 30°C for the crystal growth rates measurements. The Ba2+ concentration YBa>aq in the Ba2+ doped solution was about 0.00482 and the Pb2+ concentration YPb>aq in the Pb2+ doped solution was about 0.0149. The supersaturation was obtained and varied by reducing the temperature. Since the solubility changes only slightly for Sr(N03)2 with temperature variation, a relative small supersaturation range has been tested. Fig.4 shows the dependence of averaged normal crystal growth rates of {111} and {100} surface for Sr(N03)2 crystal growth from the pure, Ba2+ and Pb2+ doped Sr(N03)2 solutions. Results are summarized in Table 4.
1 o
a:
\-l
—•—(111) ••-•-•- (100)
A Pb(H i ) v Pb(100) ••: Ba(111)
m Ba(100)
l [ ,•••--
s* /
. - < ^ ^ 4
. 1 i 1 i L
•
/ / --/-» """
y . i . i i _
+
•
/
n
A :-;
I . I .
A
<?
I I I i
0.2 0.3 0.4 0.5 0.6 0.7 0.8
supersaturation <J(%)
Fig.4. The averaged normal crystal growth rates of the {111} and {100} surfaces for the crystals grown from pure, Ba2+ and Pb2+ doped solutions.
For the crystal growth from pure Sr(N03)2 solution, the normal crystal growth rate of
{100} surface #100 is larger than that of {111} surface Rm (Rl0o>Rm) in the low supersaturation range. However in the higher supersaturation range, the normal growth
rate of {100} surface will be smaller than that of {111} surface (#ioo <#m). The transition of that growth behavior happened at the supersaturation <jt around 0.34 from Fig.4. Therefore the crystal morphology of Sr(N03)2 crystal grown from pure Sr(N03)2
solution will be different when grown at different supersaturation. The typical impurity effects on reducing the crystal growth rate can be seen in Fig.4
and Table 4. The normal growth rates of either {111} or {100} surface of the crystal grown from doped solutions were smaller than that grown from pure solution at the same supersaturation. For the crystal grown from Pb2+ doped solution, as shown in Fig.4 the
impurity Pb2+ obviously reduced both the normal growth rate ^ n i and ^i0 0 , but the
variation of #,n and ^i00 with a is similar as the crystal grown from pure solution, that
is tfioo > #iu in the low supersaturation region and #i00 < Rm in the higher a region. However, the transition point a, has moved to a higher value, 0.622<<jt<0.829.
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The Impurity Effects on Growth, Structure and Physical Properties 119
Pure
Pb2+ doped
Ba2+ doped
0.261 0.521 0.622 0.829 0.408 0.651
1.816 6.659 1.87 3.24 0.877 1.985
3.012 3.27 2.07 2.34 0.716 2.521
From Fig.4 and Table 4, different scenario was occurred for the crystal grown from
Ba2+ doped solution. The Ba2+ hinders the normal growth rate Rm more effectively than
Rlu in the low supersaturation region. One interesting thing is that the relative growth
rate of Rm and Rm will also be changed with the supersaturation. In the high
supersaturation region, the relative normal growth rates will be changed to Rm > Rm. Again although we have not given the exact transition point at here, we still can say that the transition did not happen either at the same <jt of crystal grown from pure solution or from Pb2+ doped solution (see Fig.4). The transition happened at the 0.408<<r{<0.651 at the current impurity concentration.
Table 4 The average normal crystal growth rates of (111) and (100) surfaces in pure, Ba2+ and Pb2+ doped Sr(N03)2 solutions under different supersaturation
Solution Supersaturation (%) Rm Rioo Comparison
.fiioo > .fini Rioo < Rm Rioo > Rm Rioo < Rm Rioo < Rm Rioo > Rm
The Mach-Zehnder interferometer can reveal the concentration distribution at both the crystal/solution interfaces of (111) and (100) at the same time. The experiments show that the interference fringes distorted more obviously at the faster growing interface, hence the concentration gradient of the solute is larger at the relative fast growing interface. Therefore, the relative growth rate of (111) and (100) surface can also be qualitatively determined from the distortion of the interference fringes. The explanation for those phenomena can be based on the impurity effects on thermodynamic and surface kinetics. In the low supersaturation region, the driving force is not high enough to overcome the 2D nucleation energy barrier, so that the dominating effect is on the surface kinetics. The impurities will reduce the growth rate by reducing the step advancing rate, which is impurity density related. Therefore, the growth rate will be smaller in the higher impurity concentration sectors as shown in Fig.2 and Table 4. On the other hand, in the higher a region, the impurities will promote the heterogeneous 2D nucleation by reducing the nucleation energy barrier in the high supersaturation region18. Therefore, the dominating effect of the impurities on crystal growth is on thermodynamics, hence the growth rate is relatively larger in the higher impurity
concentration sector, as shown in Fig.2 and Table 4, Rwo > Rm for Ba2+ doped crystal
and Rioo <Rm for Pb2+ doped crystal.
3.3 Impurity Effects on Crystal Structure Perfection
The impurity effects on crystalline quality and its homogeneity were evaluated from the values of full width at half maximum (FWHM) of HRXDRC. The samples were cut along (110) plan as shown in Fig. 1(b). HRXDRCs at different areas were recorded with (440) symmetric diffraction. The typical FWHM results of Ba2+ and Pb2+ doped samples
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120 C. Li, L. Wu & W. Chen
in the seed, {100} and {111} sector areas are listed in Table 2 and 3, respectively. From Table 2 and 3, one can see that the FWHM value of seed-grown crystal is larger than that of the seed itself for the Ba2+ or Pb2+ doped samples. Therefore, crystalline quality was deteriorated by the impurities. For the Ba2+ doped sample from table 2, the crystal quality in {111} sectors is relative higher than that in {100} sectors, due to the smaller FWHM value in {111} sectors, which corresponds well with that the composition of Ba2+ is higher in {100} sectors. On the other hand, for the Pb2+ doped sample, the crystalline quality of {100} sectors is better than that of {111} sectors, corresponding to the lower Pb2+ composition in {100} sectors. Those Results coincide well with the results of the dependence of the crystalline quality on the composition of SrxBai.x(NO2)2 and SrxPb].x(N02)2 solid solution.15
3.4 Impurity Effects on Raman Scattering
The Raman spectra of the Ba2+ and Pb2+ doped samples were recorded in the area of seed, {111} and {100} sectors. Positions and full widths at half maximum (FWHM) of the Raman peak at different areas were listed in Table 2 and 3 for the Ba2+ and Pb2+
doped sample respectively. The distributions of impurity concentration were given in Fig.2 and Fig.3 and the mean concentrations were also listed in Table 2 and 3, for the Ba2+ and Pb2+ doped sample respectively. The samples were re-polished for the Raman experiments after the composition measurements.
Comparing the Raman peak positions of the seed, {111} and {100} sector areas, peak positions of the seed-grown crystal moved to the lower wavenumber side. Therefore, barium and lead will shift the Raman dominating peaks to the lower frequency. Further more, the shifting value is composition dependent. The Ba2+ concentration is higher in (100) sector, so that the Raman peak of this area shifts more. On the contrary, Pb2+
enriched in (111) zone, so the Raman peak of (111) shifts more. The reason for the Raman dominating peak of Ba2+ and Pb2+ doping crystal shifting to the low frequency direction can be understood by the Raman dominating peak positions of pure nitrate materials. For instance, in the Raman spectra of the single crystal of Ba(N03)2, the dominating vibronic mode is the nitrate moiety "breathing" mode vi(A1' )=1047 cm'\ while for Sr(N03)2 single crystal has a Raman active "breathing" mode at 1055.6 cm"1 14. Therefore, the Ba2+ in Sr(N03)2 single crystal will adjust its Raman dominating vibronic mode and shift the Raman peak to the frequency of pure Ba(N03)2 single crystal dominating vibronic mode, that is to the lower frequency direction.
From Table 2 and 3, FWHM of the Raman peak in the Me2+ doped areas is broadened and the amount of broadening is related to the concentration of Me2+. Higher the concentration is, wider the Raman peak will be. Those results suggest that barium or lead will degrade the quality of the crystalline phase and that coincides quite well with the crystal structure properties outcome by high resolution x-ray diffraction technique.
4 Summary and Conclusion
The compositional zoning of impurities of Ba2+ and Pb2+ in Sr(N03)2 crystal, and its effects on the crystal growth, structure and physical properties of the crystals have been
„2+ systemically studied. The compositional distribution of Ba is different from that of Pb2+ in the Sr(N03)2 crystal. The Ba2+ ions are enriched in the {100} growth sectors, while Pb2+ ions are enriched in {111} sectors. The compositional zoning of impurities
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The Impurity Effects on Growth, Structure and Physical Properties 121
will influence the growth behavior on different sectors due to their effects on thermodynamics and surface kinetics. These zoning effect will also influence the structural and physical property. Impurities will degrade the crystalline quality and Raman scattering property, due to the lattice stress induced by the replacement of Sr2+
with the impurity ions of Ba2+ or Pb2+. The Raman scattering dominating peaks of Sr(N03)2 will be shifted to lower frequency by the impurities and the shifting value depends on impurity concentration.
Acknowlegements
This work was supported by the Claim Project of National Commission of Science and Technology under Grant Nos. 95-Yu-34 and National Microgravity Laboratory.
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