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Distribution coefficients of Ca2+, Cd2+ and Pb2+ in KBr
Journal of Crystal Growth 71 (1985) 739-743 North-Holland, Amsterdam
739
DISTRIBUTION COEFFICIENTS OF Ca2+, Cd 2+ AND Pb 2+ IN KBr Paul R. COLLINS * and W.J. FREDERICKS
Cl~emistryDepartment,OregonState University,Corvallis,Oregon97331,USA Received 23 July 1984; manuscriptreceivedin final form 28 January 1985
Distribution coefficientsof Ca 2+, Cd2+ and Pb2+ in KBr were found to be 0.13, 0.0025 and 0.0135, respectively. These coefficients were obtained by fitting the impurity distribution along crystals pulled from melts which initially contained various impurity concentrations.The pulling rates and thermal balance were such that a flat crystal-melt interfacewas maintainedduring growth. Both the KBr and impuritieswere extensivelyprocessed to removeH20, OH- and organic impurities before crystal growth.
1. Introduction
from eq. (2), eq. (1) may be rewritten as
Few distribution coefficients of divalent impurities in alkali halides are known [1]. In this paper distribution coefficients for Pb 2+, Cd 2+ and Ca 2+ in KBr were estimated from the impurity gradient in Czochralski grown KBr crystals. When a distribution coefficient, D, is less than one, the dopant concentration will increase in the crystal as it grows. The concentration of dopant in the crystal is given by
Cc(x ) =DCm(x), where C¢(x) is the
(1)
Cm(x)=Cm(O)[
(2)
concentration of dopant in the crystal at a position x cm from the top, and Cm(X) is the concentration of dopant in the melt after x cm of crystal has grown. If D is much less than one, then Cm(X) is given by Mm(~)) ( x )~M~¢ m(0)],
where Mm(0) is the mass of the melt when x is zero, and Me(x) is the mass of the crystal after x cm of crystal has grown. If the cross-sectional area of the crystal, A, is nearly constant, then Me(x) = Apx, where p is the density of the crystal. Thus, * Present address: Union Carbide, Electronics Division, WashougalWashington98671, USA.
D = ~m(O) 1
Mm(O) .
(3)
Eq. (3) assumes the impurity concentration in the melt is only decreased by incorporation into the crystal. Some loss from evaporation of both KBr and the impurity occur during growth of the crystal. Solutions of divalent halides in molten alkali halides are non-ideal [2]. In general a negative deviation from Raoult's law has been observed. For KBr-CdBr 2 solutions Bredig [3] found the major impurity species to be CdBr 2-. Bloom and Hastie [4] have found KPbBr 3 to be the predominant complex in the KBr-PbBr2 system. The prevalent lead species in the vapor above an equimolar mixture was KPbBr3 [5]. Studies on CaBr2-KBr have not been reported. Schrier and Clark [6] studied MgC12-KC1 in detail. At KC1 mole fractions exceeding 0.6 the only vapor phase component containing Mg was KMgCI 3. Its partial pressure deviates negatively from Raoult's law as the KC1 concentration increases. The CaC12-KC1 system behaves in a similar way [7]. In the very dilute solutions fused salt solution studies show the expected order of partial pressures of the vapor species would be PKSr > P PKMBr3, where M is a divalent impurity. As evaporation occurs the melt becomes relatively richer in the
0022-0248/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
740
P.R. Collins, W.J. Fredericks / Distribution coefficients o f Ca 2 +, Cd 2 + and P b 2 + in K B r
divalent impurity. Thus CM(X ) was actually very slightly larger than its value estimated from (3).
2. Experimental The divalent cation dopants were added as their dibromide salts. CdBr 2 and CaBr 2 were made by dissolving Mallinckrodt SL grade CdCO 3 and CaCO 3 in aqueous HBr which had been made from gaseous HBr and type I deionized water. The slightly acidic solution was evaporated to dryness in a small Teflon dish inside a Teflon-coated vacuum oven. These salts were recrystallized from deionized water. After vacuum drying at 120°C the CaBr 2 retained 3.7 moles H 2 0 of hydration as measured by Ca atomic emission analysis. BDH ® 98% PbBr 2 was purified by three recrystallizations from - 0 . 0 1 M HBr. The PbBr 2 was then dried in the same way as the other salts. The KBr used for these crystals was Baker Analyzed ®. For these doped crystals, no special purification of the KBr was performed. The KBr crystals were pulled from the melt using the Czochraslki method in a growth apparatus described by Fredericks [8]. Only the features essential to this work need mention here. The KBr was contained in a quartz crucible 6½ cm in diameter and 8 cm in height which rested on a quartz stand inside a quartz tube 8 cm in diameter and 45 cm in height sealed at its bottom. A rhodium plated monel closure [9] provided the top seal for the large quartz tube. The quartz tube was sealed [9] into a graphite element vacuum furnace with 17 cm of the tube extending from the furnace case. A water-cooled rhodium-plated nickel tube extended through a three-stage seal in the center of the top seal. Kal-Rez ® O-rings lubricated with Krytox ® fluorinated ~rease seated in Teflon forruing rings sealed the nickel pulling rod in each of the three stages of the center seal. The top closure also contained 1 / 4 inch monel fittings; one to admit reactive gas and one to remove reactive gas products. Each fitting connected through Teflon tubing to separate greaseless manifolds. All crystals were grown in a similar way. The crucible was filled with KBr and dopants mixed with the KBr. The temperature was slowly in-
creased, and the growth tube was flushed with argon. When the temperature reached = 250°C, electronic-grade HBr was introduced to the growth tube. As the temperature was increased to the melting point of the KBr (734°C), the HBr was removed and replaced with fresh HBr a minimum of nine times. After the KBr had melted, at least three additional HBr exchanges were performed. Pure KBr crystals grown using similar reactive gas treatments had no detectable absorption in the" 214 nm O H - band when measured in 5 cm long crystals using a Perkin-Elmer Model 450 spectrophotometer. Based on Rolfe's [10] absorption coefficient for O H - in KBr and the spectrophotometer's detection limit the O H - concentration in these crystals was estimated to he less than 100 pb. The KBr : Ca crystals grown for these experiments exhibited similar spectra, but had a slightly greater baseline scattering. The scattering from precipitates in the annealed KBr : Cd crystals which have a broad weak 221 nm cadmium band and the strong 223 nm band in KBr : Pb prevented optical estimates of O H - concentration in these crystals. After the KBr was dried the HBr treatment sequence was interrupted at various times to add C12. Treatment sequences varied for each set of crystals and are given explicitly in table 1. Most alkali halides contain organic matter. An effective method of removing this is by oxidation of O H free salt by the corresponding halogen. In this process a limited amount of C12 was used to convert KBr to KCI and free bromine in the salt. Then the KC1 was reconverted to KBr by reaction with excess HBr. Other work [11] on purified KBr has shown this process effective. After the final HBr exchanges were made on the molten salt, the melt was placed under vacuum for - 45 min at -- 30°C above the melting point to remove any dissolved HBr. It was during this process that most of the evaporation occurred. then 3 / 4 atm of argon was introduced to the growth tube. From a partially melted seed the crystal was quickly expanded to 3 cm faces by lowering the melt temperature while pulling at approximately 1 c m / h with a 6 rpm rotation rate. During growth the 3 cm faces were maintained by minor temperature adjustments, the crystals used in this work
741
P.R Collins, W.J. Fredericks / Distribution coefficients of Ca 2 +, Cd 2 + and Pb 2 + in KBr
Table 1 Crystal growth parameters ID number
Dopant
34-82 37-82 316-81
Ca z + Ca 2+ Cd 2"~
365-81
Cd 2+
116-82 123-82 228-81 127-82
Cd 2+ Cd 2+ Pb 2 + Pb 2+
Concentration in melt (mppm)
Reactive gas treatment
Cooling time (h)
1934
257-9HB4-3C12 - M - 3 H B r - A r 257-8HBr-4CI 2-M-4HBr-Ar 235-8HBr-3Cl2-M-Clz 3HBr-Ar 264-6HBr-4C12 - 3 H B r - M - 3 H B r - A r 2 5 4 - 8 H B r - 3 C I 2- M - 4 H B r - A r 260-8HBr-3Cl 2-M-4HBr-Ar 252-4HBr-3Cl 2-6HBr-M-2HBr-Ar 256-8HBr-3Cl 2-M-SHBr-Ar
18 7.5 18.5 19 19 20 17 17
. 893
316 342 1034 1604 5230 2711
were 6 to 9 cm long. these crystals were almost square, grown with flat (100) planes using a flat melt-solid interface. Growth was terminated by quickly pulling the flat crystal bottom from the melt. The finished crystals were annealed for the times given in table 1. Impurity concentration profiles are shown in figs. 1 through 3 for calcium, cadmium, and lead doped crystals. Lead and cadmium concentrations were determined by atomic absorption on PerkinElmer Model 403 spectrophotometer with a 4-inch air-acetylene burner. The calcium concentrations were determined by atomic emission using the same instrument. These analyses were performed by comparison of 105[ KBr sample solutions with two sets of reference standards prepared with KBr from pure crystals. The two sets of reference standards were
prepared with analyte concentrations spanning the impurity concentration range of the samples and with KBr concentrations of = 115[ and = 65[. By interpolating between the analyte concentrations determined from these two sets of standards, an accurate analyte concentration could be determined in the sample solutions. It is very important in this type of analysis that the solution used to prepare the standards be free of any analyte species. For the lead analysis, the absence of a characteristic ultraviolet absorption band in the crystalline KBr ensured a lead concentration well below the detection limit of atomic absorption. For the calcium analysis, the absence of a calcium emission line from the pure reference solutions ensured a negligible calcium concentration. At the wavelength of the cadmium absorption line the background absorption from KBr is
~g
Col~|umOop=dCryoEaIo*
25
/
/ /
C=d.ium Oop.d Cry.~..l.=
Eo- 0 M _
~_. ~ - 3
v
c ry . k =]
123-82 365-81
fix/
cry.~=l
ale-e!
f
• ...... o ...... o ......
Cry.E=I
,"
.
/
/
I /~
[
5 ,~ . 4 m -
.OL lg--
l"
t-
O t.J
2
±
0 I I
g.O
l.g
:
I I
2.8
PoeiEion in
..
I I
9..g Cr'yeEo!
:
5-
.
~
°
{
4. g
5. g
(om)
Fig. 1. Calcium concentration as a function of distance from top of crystal.
='
E
r 2
',
I 4
PooiEion in
,
I •
CrymEol
,
t e
, 11
(ore)
Fig. 2. C a d m i u m concentration as a function of distance from top of crystal.
742
P . R . Collins, W . J . F r e d e r i c k s / Distribution coefficients o f C a 2 +, C d 2 + a n d P b 2 + in K B r
4~0
LtadDop,~dCr-ye~.al.:
?
/
• --_---_-cr~.~.122o-Bl
a. 13- 311g-
/
/
/
g o ..~ 2811u" L .4J E W lell0 [ tJo
0
,
I
,
2
I
,
4
Poai~ion in
I
,
0
Crye~al
I
,
8
IB
(cm)
Fig. 3. Lead concentration as a function of distance from top of crystal.
Table 2 Distribution coefficients Calcium in KBr
Cadmium in KBr
Lead in KBr
ID
D ID number
D number
ID
D number
37-82 34-82
<0.14 <0.12
123-82 365-81 316-81 116-82
<0.0029 <0.0020 < 0.0028 <0.0022
228-81 127-82
<0.0177 <0.0193
Average
<0.13
Average
<0.0025
Average
<0.0185
negligible. Thus, the absence of cadmium absorption in the pure reference solution could be determined directly. The data in figs. 1 through 3 has been fitted to eq. (3) using a least-squares procedure. From these plots, using values for Cm(0) from table 1, the distribution coefficients were calculated. These are listed in table 2.
3. Discussion
The variation of D for each impurity is relatively small for this type of measurement considering the large variations in Cm(0). This supports the premise that the impurity is lost from the melt as an equimolar complex and KBr is present in
such excess that the concentration Cm(0) remains relatively constant even though the evaporation occurs during reactive gas treatment. From the magnitude of D(Cd) and D(Ca) the solubility of Cd 2÷ is about 200 times less than that of Ca 2÷. This is in agreement with other studies on Cd 2÷ on Hg 2÷ [11,151 which show Group lib ions to be much less soluble than IIa ions even though their ionic radii are nearly the same and their charge identical. Ikeya et al. [1] found the distribution coefficient for Ca2+ in KBr to be 0.35 based on zone refining measurements. This substantially larger D(Ca) may have occurred because their purification method allowed the formation of a calcium-anionic impurity complex in the KBr. In any solution such complexes increase the solubility of slightly soluble ions [13]. Fritz et al. [14] have shown Ca2+ in KCI: O H forms a complex which either binds the charge carrying vacancy or compensates the excess Ca2÷ charge. A mole fraction of 2.1 X 10 -5 of OHinKC1 can increase the amount of dissolved Hg 2+ by a factor of about five by forming a very stable, immobile complex [14]. If the mercury-hydroxide complex is treated with C12 at high temperatures it is converted to an oxygen complex [16]. If an alkali halide contains only OH- effusion at high temperature is greater enhanced in a halogen atmosphere [17]. Ideda [18,19] has shown that in the enhancement process the halogen combines with OH- increasing the apparent effusion of OHby forming a hydrogen halide and 0 2 . Before zone refining Ikeya et al. vacuum dried the KBr-CaBr 2 mixture, then fused it before treating with Br 2. It was reported by Johnson, in 1935 [20], that fusion of nominally dry alkali halides caused hydrolysis to the hydroxide. Rolfe [21] reported that if KBr was heated in a vacuum for 88 h while slowly increasing the temperature, then growing a crystal in a dried, oxygen free nitrogen atmosphere he did not observe the 214 nm OHband. Later Fredericks, Schuerman and Lewis [22] found that treatment by hydrogen halides was required to completely remove OH- from alkali halides and halogens were effective in removing organic residues. Other investigators reported similar results [23,24].
P.R. Collins, I'E.J. Fredericks / Distribution coefficients of Ca 2 +,
Based o n this b e h a v i o r of alkali h a l i d e - d i v a l e n t h a l i d e m i x t u r e s it a p p e a r s that d u r i n g the i n i t i a l f u s i o n K B r : C a ( O H ) 2 : Ca2 ÷ was f o r m e d . D u r i n g the s u b s e q u e n t h a l o g e n a t i o n K B r : C a O : C a 2÷ was p r o d u c e d . T h e C a O w o u l d b e h a v e as a c a l c i u m c o m p l e x i n c r e a s i n g the a n a l y t i c a l c a l c i u m c o n centration above that in samples containing only C a 2÷, w h i c h c a u s e s D ( C a ) to b e larger t h a n its t r u e value.
References [1] M. Ikeya, N. Itoh and T. Suita, Japan. J. Appl. Phys. 7 (1968) 837. [2] J.W. Hastie, Thermodynamic Studies, by Mass Spectrometry, of Molten Mixed Halide Systems, in: Advances in Molten Salt Chemistry, Vol. 1, Eds. J. Brannstein, G. Mamantov and G.P. Smith (Plenum, New York, 1971) p. 225. [3] M.A. Bredig, J. Chem. Phys. 37 (1962) 451. [4] H. Bloom and J.W. Hastie, Australian J. Chem. 21 (1968) 583. [5] H. Bloom and R.G. Anthony, Australian J. Chem. 24 (1971) 2001. [6] E.E. Schrier and H.M. Clark, J. Phys. Chem. 6 (1963) 1259. [7] G.I. Novikov and F.G. Gavryuchenkov, Russ. Chem. Rev. 36 (1967) 156. [8] W.J. Fredericks, Purification of Potassium Bromide and
Cd 2 +
and Pb 2 + in KBr
743
Alkaline Earth Fluorides for Laser Components, in: Laser Induced Damage in Optical Materials, Eds. A.V. Glass and A.H. Guenther, NBS Special Publication 509 (National Bureau of Standards, Washington, DC, 1977). [9] W.J. Fredericks, J. Sci. Instr. 44 (1967) 561. [10] J. Rolfe, Can. J. Phys. 41 (1963) 1525. [11] L.W. Barr and A.B. Lidiard, Defects in Ionic Crystals, in: Physical Chemistry: An Advanced Treatise, Vol. X (Academic Press, New York, 1970) p. 188. [12] W.J. Fredericks, P.R. Collins and D.F. Edwards, J. Phys. Chem. Solids 45 (1974) 471. [13] J.N. Butler, Ionic Equilibrium (Addison-Wesley, Reading, MA, 1964) p. 214. [14] B. Fritz, F. Luty and J. Anger, Z. Physik 174 (1963) 240. [15] C.A. Allen and W.J. Fredericks, Phys. Status Solidi (b) 55 (1973) 615. [16] C.A. Allen and W.J. Fredericks, Phys. Status Solidi (a)3 (1970) 143. [17] C.A. Allen and W.J. Fre,dericks, J. Solid State Chem. 1 (1970) 205. [18] T. Ikeda, Japan. J. Appi. Phys. 12 (1973) 1810. [19] T. Ikeda, J. Phys. SOc. Japan 41 (1976) 1968. [20] C.R. Johnson, J. Phys. Chem. 39 (1935) 791. [21] J. Rolfe, Phys. Rev. Letters 1 (1958) 56. [22] W.J. Fredericks, L.W. Schuerman and L.C. Lewis, An Investigation of Crystal Growth Processes, Final Report, AF-AFORS-217-63 (1966) p. 23. [23] J.M. Peach, D.A. Bower and P.O. Pohl, J. Appl. Phys. 38 (1967) 2166. [24] R. CapeUetti, V. Fano and M. Scalvini, Science Fische 38 (1968) 886.
739
DISTRIBUTION COEFFICIENTS OF Ca2+, Cd 2+ AND Pb 2+ IN KBr Paul R. COLLINS * and W.J. FREDERICKS
Cl~emistryDepartment,OregonState University,Corvallis,Oregon97331,USA Received 23 July 1984; manuscriptreceivedin final form 28 January 1985
Distribution coefficientsof Ca 2+, Cd2+ and Pb2+ in KBr were found to be 0.13, 0.0025 and 0.0135, respectively. These coefficients were obtained by fitting the impurity distribution along crystals pulled from melts which initially contained various impurity concentrations.The pulling rates and thermal balance were such that a flat crystal-melt interfacewas maintainedduring growth. Both the KBr and impuritieswere extensivelyprocessed to removeH20, OH- and organic impurities before crystal growth.
1. Introduction
from eq. (2), eq. (1) may be rewritten as
Few distribution coefficients of divalent impurities in alkali halides are known [1]. In this paper distribution coefficients for Pb 2+, Cd 2+ and Ca 2+ in KBr were estimated from the impurity gradient in Czochralski grown KBr crystals. When a distribution coefficient, D, is less than one, the dopant concentration will increase in the crystal as it grows. The concentration of dopant in the crystal is given by
Cc(x ) =DCm(x), where C¢(x) is the
(1)
Cm(x)=Cm(O)[
(2)
concentration of dopant in the crystal at a position x cm from the top, and Cm(X) is the concentration of dopant in the melt after x cm of crystal has grown. If D is much less than one, then Cm(X) is given by Mm(~)) ( x )~M~¢ m(0)],
where Mm(0) is the mass of the melt when x is zero, and Me(x) is the mass of the crystal after x cm of crystal has grown. If the cross-sectional area of the crystal, A, is nearly constant, then Me(x) = Apx, where p is the density of the crystal. Thus, * Present address: Union Carbide, Electronics Division, WashougalWashington98671, USA.
D = ~m(O) 1
Mm(O) .
(3)
Eq. (3) assumes the impurity concentration in the melt is only decreased by incorporation into the crystal. Some loss from evaporation of both KBr and the impurity occur during growth of the crystal. Solutions of divalent halides in molten alkali halides are non-ideal [2]. In general a negative deviation from Raoult's law has been observed. For KBr-CdBr 2 solutions Bredig [3] found the major impurity species to be CdBr 2-. Bloom and Hastie [4] have found KPbBr 3 to be the predominant complex in the KBr-PbBr2 system. The prevalent lead species in the vapor above an equimolar mixture was KPbBr3 [5]. Studies on CaBr2-KBr have not been reported. Schrier and Clark [6] studied MgC12-KC1 in detail. At KC1 mole fractions exceeding 0.6 the only vapor phase component containing Mg was KMgCI 3. Its partial pressure deviates negatively from Raoult's law as the KC1 concentration increases. The CaC12-KC1 system behaves in a similar way [7]. In the very dilute solutions fused salt solution studies show the expected order of partial pressures of the vapor species would be PKSr > P
0022-0248/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
740
P.R. Collins, W.J. Fredericks / Distribution coefficients o f Ca 2 +, Cd 2 + and P b 2 + in K B r
divalent impurity. Thus CM(X ) was actually very slightly larger than its value estimated from (3).
2. Experimental The divalent cation dopants were added as their dibromide salts. CdBr 2 and CaBr 2 were made by dissolving Mallinckrodt SL grade CdCO 3 and CaCO 3 in aqueous HBr which had been made from gaseous HBr and type I deionized water. The slightly acidic solution was evaporated to dryness in a small Teflon dish inside a Teflon-coated vacuum oven. These salts were recrystallized from deionized water. After vacuum drying at 120°C the CaBr 2 retained 3.7 moles H 2 0 of hydration as measured by Ca atomic emission analysis. BDH ® 98% PbBr 2 was purified by three recrystallizations from - 0 . 0 1 M HBr. The PbBr 2 was then dried in the same way as the other salts. The KBr used for these crystals was Baker Analyzed ®. For these doped crystals, no special purification of the KBr was performed. The KBr crystals were pulled from the melt using the Czochraslki method in a growth apparatus described by Fredericks [8]. Only the features essential to this work need mention here. The KBr was contained in a quartz crucible 6½ cm in diameter and 8 cm in height which rested on a quartz stand inside a quartz tube 8 cm in diameter and 45 cm in height sealed at its bottom. A rhodium plated monel closure [9] provided the top seal for the large quartz tube. The quartz tube was sealed [9] into a graphite element vacuum furnace with 17 cm of the tube extending from the furnace case. A water-cooled rhodium-plated nickel tube extended through a three-stage seal in the center of the top seal. Kal-Rez ® O-rings lubricated with Krytox ® fluorinated ~rease seated in Teflon forruing rings sealed the nickel pulling rod in each of the three stages of the center seal. The top closure also contained 1 / 4 inch monel fittings; one to admit reactive gas and one to remove reactive gas products. Each fitting connected through Teflon tubing to separate greaseless manifolds. All crystals were grown in a similar way. The crucible was filled with KBr and dopants mixed with the KBr. The temperature was slowly in-
creased, and the growth tube was flushed with argon. When the temperature reached = 250°C, electronic-grade HBr was introduced to the growth tube. As the temperature was increased to the melting point of the KBr (734°C), the HBr was removed and replaced with fresh HBr a minimum of nine times. After the KBr had melted, at least three additional HBr exchanges were performed. Pure KBr crystals grown using similar reactive gas treatments had no detectable absorption in the" 214 nm O H - band when measured in 5 cm long crystals using a Perkin-Elmer Model 450 spectrophotometer. Based on Rolfe's [10] absorption coefficient for O H - in KBr and the spectrophotometer's detection limit the O H - concentration in these crystals was estimated to he less than 100 pb. The KBr : Ca crystals grown for these experiments exhibited similar spectra, but had a slightly greater baseline scattering. The scattering from precipitates in the annealed KBr : Cd crystals which have a broad weak 221 nm cadmium band and the strong 223 nm band in KBr : Pb prevented optical estimates of O H - concentration in these crystals. After the KBr was dried the HBr treatment sequence was interrupted at various times to add C12. Treatment sequences varied for each set of crystals and are given explicitly in table 1. Most alkali halides contain organic matter. An effective method of removing this is by oxidation of O H free salt by the corresponding halogen. In this process a limited amount of C12 was used to convert KBr to KCI and free bromine in the salt. Then the KC1 was reconverted to KBr by reaction with excess HBr. Other work [11] on purified KBr has shown this process effective. After the final HBr exchanges were made on the molten salt, the melt was placed under vacuum for - 45 min at -- 30°C above the melting point to remove any dissolved HBr. It was during this process that most of the evaporation occurred. then 3 / 4 atm of argon was introduced to the growth tube. From a partially melted seed the crystal was quickly expanded to 3 cm faces by lowering the melt temperature while pulling at approximately 1 c m / h with a 6 rpm rotation rate. During growth the 3 cm faces were maintained by minor temperature adjustments, the crystals used in this work
741
P.R Collins, W.J. Fredericks / Distribution coefficients of Ca 2 +, Cd 2 + and Pb 2 + in KBr
Table 1 Crystal growth parameters ID number
Dopant
34-82 37-82 316-81
Ca z + Ca 2+ Cd 2"~
365-81
Cd 2+
116-82 123-82 228-81 127-82
Cd 2+ Cd 2+ Pb 2 + Pb 2+
Concentration in melt (mppm)
Reactive gas treatment
Cooling time (h)
1934
257-9HB4-3C12 - M - 3 H B r - A r 257-8HBr-4CI 2-M-4HBr-Ar 235-8HBr-3Cl2-M-Clz 3HBr-Ar 264-6HBr-4C12 - 3 H B r - M - 3 H B r - A r 2 5 4 - 8 H B r - 3 C I 2- M - 4 H B r - A r 260-8HBr-3Cl 2-M-4HBr-Ar 252-4HBr-3Cl 2-6HBr-M-2HBr-Ar 256-8HBr-3Cl 2-M-SHBr-Ar
18 7.5 18.5 19 19 20 17 17
. 893
316 342 1034 1604 5230 2711
were 6 to 9 cm long. these crystals were almost square, grown with flat (100) planes using a flat melt-solid interface. Growth was terminated by quickly pulling the flat crystal bottom from the melt. The finished crystals were annealed for the times given in table 1. Impurity concentration profiles are shown in figs. 1 through 3 for calcium, cadmium, and lead doped crystals. Lead and cadmium concentrations were determined by atomic absorption on PerkinElmer Model 403 spectrophotometer with a 4-inch air-acetylene burner. The calcium concentrations were determined by atomic emission using the same instrument. These analyses were performed by comparison of 105[ KBr sample solutions with two sets of reference standards prepared with KBr from pure crystals. The two sets of reference standards were
prepared with analyte concentrations spanning the impurity concentration range of the samples and with KBr concentrations of = 115[ and = 65[. By interpolating between the analyte concentrations determined from these two sets of standards, an accurate analyte concentration could be determined in the sample solutions. It is very important in this type of analysis that the solution used to prepare the standards be free of any analyte species. For the lead analysis, the absence of a characteristic ultraviolet absorption band in the crystalline KBr ensured a lead concentration well below the detection limit of atomic absorption. For the calcium analysis, the absence of a calcium emission line from the pure reference solutions ensured a negligible calcium concentration. At the wavelength of the cadmium absorption line the background absorption from KBr is
~g
Col~|umOop=dCryoEaIo*
25
/
/ /
C=d.ium Oop.d Cry.~..l.=
Eo- 0 M _
~_. ~ - 3
v
c ry . k =]
123-82 365-81
fix/
cry.~=l
ale-e!
f
• ...... o ...... o ......
Cry.E=I
,"
.
/
/
I /~
[
5 ,~ . 4 m -
.OL lg--
l"
t-
O t.J
2
±
0 I I
g.O
l.g
:
I I
2.8
PoeiEion in
..
I I
9..g Cr'yeEo!
:
5-
.
~
°
{
4. g
5. g
(om)
Fig. 1. Calcium concentration as a function of distance from top of crystal.
='
E
r 2
',
I 4
PooiEion in
,
I •
CrymEol
,
t e
, 11
(ore)
Fig. 2. C a d m i u m concentration as a function of distance from top of crystal.
742
P . R . Collins, W . J . F r e d e r i c k s / Distribution coefficients o f C a 2 +, C d 2 + a n d P b 2 + in K B r
4~0
LtadDop,~dCr-ye~.al.:
?
/
• --_---_-cr~.~.122o-Bl
a. 13- 311g-
/
/
/
g o ..~ 2811u" L .4J E W lell0 [ tJo
0
,
I
,
2
I
,
4
Poai~ion in
I
,
0
Crye~al
I
,
8
IB
(cm)
Fig. 3. Lead concentration as a function of distance from top of crystal.
Table 2 Distribution coefficients Calcium in KBr
Cadmium in KBr
Lead in KBr
ID
D ID number
D number
ID
D number
37-82 34-82
<0.14 <0.12
123-82 365-81 316-81 116-82
<0.0029 <0.0020 < 0.0028 <0.0022
228-81 127-82
<0.0177 <0.0193
Average
<0.13
Average
<0.0025
Average
<0.0185
negligible. Thus, the absence of cadmium absorption in the pure reference solution could be determined directly. The data in figs. 1 through 3 has been fitted to eq. (3) using a least-squares procedure. From these plots, using values for Cm(0) from table 1, the distribution coefficients were calculated. These are listed in table 2.
3. Discussion
The variation of D for each impurity is relatively small for this type of measurement considering the large variations in Cm(0). This supports the premise that the impurity is lost from the melt as an equimolar complex and KBr is present in
such excess that the concentration Cm(0) remains relatively constant even though the evaporation occurs during reactive gas treatment. From the magnitude of D(Cd) and D(Ca) the solubility of Cd 2÷ is about 200 times less than that of Ca 2÷. This is in agreement with other studies on Cd 2÷ on Hg 2÷ [11,151 which show Group lib ions to be much less soluble than IIa ions even though their ionic radii are nearly the same and their charge identical. Ikeya et al. [1] found the distribution coefficient for Ca2+ in KBr to be 0.35 based on zone refining measurements. This substantially larger D(Ca) may have occurred because their purification method allowed the formation of a calcium-anionic impurity complex in the KBr. In any solution such complexes increase the solubility of slightly soluble ions [13]. Fritz et al. [14] have shown Ca2+ in KCI: O H forms a complex which either binds the charge carrying vacancy or compensates the excess Ca2÷ charge. A mole fraction of 2.1 X 10 -5 of OHinKC1 can increase the amount of dissolved Hg 2+ by a factor of about five by forming a very stable, immobile complex [14]. If the mercury-hydroxide complex is treated with C12 at high temperatures it is converted to an oxygen complex [16]. If an alkali halide contains only OH- effusion at high temperature is greater enhanced in a halogen atmosphere [17]. Ideda [18,19] has shown that in the enhancement process the halogen combines with OH- increasing the apparent effusion of OHby forming a hydrogen halide and 0 2 . Before zone refining Ikeya et al. vacuum dried the KBr-CaBr 2 mixture, then fused it before treating with Br 2. It was reported by Johnson, in 1935 [20], that fusion of nominally dry alkali halides caused hydrolysis to the hydroxide. Rolfe [21] reported that if KBr was heated in a vacuum for 88 h while slowly increasing the temperature, then growing a crystal in a dried, oxygen free nitrogen atmosphere he did not observe the 214 nm OHband. Later Fredericks, Schuerman and Lewis [22] found that treatment by hydrogen halides was required to completely remove OH- from alkali halides and halogens were effective in removing organic residues. Other investigators reported similar results [23,24].
P.R. Collins, I'E.J. Fredericks / Distribution coefficients of Ca 2 +,
Based o n this b e h a v i o r of alkali h a l i d e - d i v a l e n t h a l i d e m i x t u r e s it a p p e a r s that d u r i n g the i n i t i a l f u s i o n K B r : C a ( O H ) 2 : Ca2 ÷ was f o r m e d . D u r i n g the s u b s e q u e n t h a l o g e n a t i o n K B r : C a O : C a 2÷ was p r o d u c e d . T h e C a O w o u l d b e h a v e as a c a l c i u m c o m p l e x i n c r e a s i n g the a n a l y t i c a l c a l c i u m c o n centration above that in samples containing only C a 2÷, w h i c h c a u s e s D ( C a ) to b e larger t h a n its t r u e value.
References [1] M. Ikeya, N. Itoh and T. Suita, Japan. J. Appl. Phys. 7 (1968) 837. [2] J.W. Hastie, Thermodynamic Studies, by Mass Spectrometry, of Molten Mixed Halide Systems, in: Advances in Molten Salt Chemistry, Vol. 1, Eds. J. Brannstein, G. Mamantov and G.P. Smith (Plenum, New York, 1971) p. 225. [3] M.A. Bredig, J. Chem. Phys. 37 (1962) 451. [4] H. Bloom and J.W. Hastie, Australian J. Chem. 21 (1968) 583. [5] H. Bloom and R.G. Anthony, Australian J. Chem. 24 (1971) 2001. [6] E.E. Schrier and H.M. Clark, J. Phys. Chem. 6 (1963) 1259. [7] G.I. Novikov and F.G. Gavryuchenkov, Russ. Chem. Rev. 36 (1967) 156. [8] W.J. Fredericks, Purification of Potassium Bromide and
Cd 2 +
and Pb 2 + in KBr
743
Alkaline Earth Fluorides for Laser Components, in: Laser Induced Damage in Optical Materials, Eds. A.V. Glass and A.H. Guenther, NBS Special Publication 509 (National Bureau of Standards, Washington, DC, 1977). [9] W.J. Fredericks, J. Sci. Instr. 44 (1967) 561. [10] J. Rolfe, Can. J. Phys. 41 (1963) 1525. [11] L.W. Barr and A.B. Lidiard, Defects in Ionic Crystals, in: Physical Chemistry: An Advanced Treatise, Vol. X (Academic Press, New York, 1970) p. 188. [12] W.J. Fredericks, P.R. Collins and D.F. Edwards, J. Phys. Chem. Solids 45 (1974) 471. [13] J.N. Butler, Ionic Equilibrium (Addison-Wesley, Reading, MA, 1964) p. 214. [14] B. Fritz, F. Luty and J. Anger, Z. Physik 174 (1963) 240. [15] C.A. Allen and W.J. Fredericks, Phys. Status Solidi (b) 55 (1973) 615. [16] C.A. Allen and W.J. Fredericks, Phys. Status Solidi (a)3 (1970) 143. [17] C.A. Allen and W.J. Fre,dericks, J. Solid State Chem. 1 (1970) 205. [18] T. Ikeda, Japan. J. Appi. Phys. 12 (1973) 1810. [19] T. Ikeda, J. Phys. SOc. Japan 41 (1976) 1968. [20] C.R. Johnson, J. Phys. Chem. 39 (1935) 791. [21] J. Rolfe, Phys. Rev. Letters 1 (1958) 56. [22] W.J. Fredericks, L.W. Schuerman and L.C. Lewis, An Investigation of Crystal Growth Processes, Final Report, AF-AFORS-217-63 (1966) p. 23. [23] J.M. Peach, D.A. Bower and P.O. Pohl, J. Appl. Phys. 38 (1967) 2166. [24] R. CapeUetti, V. Fano and M. Scalvini, Science Fische 38 (1968) 886.