3064
J. Phys. Chem. 1981, 85,3064-3072
Defect Processes Involving Oxygen-Compensated Sites in CaF, Precipitates Doped with Lanthanides and Actinides M. V. Johnston7 and J. C. Wright’ Department of Chemistry. University of Wisconsin, Madison, Wlsconsln 53706 (Received: October 14, 1980; In Final Form: June 26, 198 1)
Oxygen incorporation into calcium fluoride precipitates doped with lanthanides and actinides is investigated by use of the technique of site-selective spectroscopy. Fluorescence from erbium in specific fluoride- and oxygen-compensated sites is monitored as a function of the ignition temperature of the precipitate to study the conversion from fluoride to oxygen compensation. Another process, thermal annealing of a disordered precipitate to give a well-defined lattice, is also followed. Changes in both oxygen compensation formation and lattice annealing are found to occur upon the addition of other trivalent and monovalent ions. The results provide a better understanding of the solid-state chemistry involved in new methods of chemical analysis using rare-earth-doped CaF2precipitates, and how certain interference effects can arise. Also included is a study of the temperature dependence of fluorescent sites in CaF2:U6+.
Introduction Oxygen readily incorporates into CaF2upon heating in air.l” Reaction with H20 or O2 removes lattice fluorides, leaving oxide ions in normal fluoride positions (OF, using the notation of Kroger and Vink’). These oxide ions both cause additional absorption in the UV and introduce fluoride ion vacancies (Vp) which enhance the low-temperature electrical conductivity of Charge neutrality is preserved by creation of equal numbers of the two oppositely charged defects: H,O(,) + 2FJ 0;+ VF + 2HF(,)
-
-+
+ 2Fb
OF -k V F + F2(g) Oxygen penetration into CaFz single crystals has been studied by opacity changes in pure CaF? and by color changes in Sm2+-dopedCaF’2.4 The process was found to be isotropic and limited only by the intrinsic oxygen diffusion rate (surface effects negligible). Water was found to be much more reactive in providing oxygen ions in CaF2 than pure oxygen: although, under certain conditions, reaction with water has been shown to produce a greater amount of OH-than of 02-in the latticeV5The hydrolysis reaction is presumed to follow a two-step process involving an intermediate OHF speciesa8 However, kinetic studies’$ have been unable to provide a detailed mechanistic description. Trivalent rare earth dopants in CaF, are known to enhance oxygen incorporation. In the oxygen-free lattice, charge compensation of the aliovalent ion is accomplished by incorporation of an additional fluoride interstitial (Fihg Rare earths in a number of crystallographic sites, differing in the relative locations of the defects, have been observed by EPRlOJ1and optica111J2 techniques. Heating in air results in the formation of a new set of sites where charge compensation is accomplished by OF.^^^^^ ENDOR studies16 have identified the two major oxygen-compensated where sites (Figure 1)as (REc,*OF)’ and (REC,~~OF’~VF)~, RE is a trivalent rare earth. These correspond to the G1 and G4 sites observed in our previous optical studies involving site-selective spectroscopy.16 Recent calculations8 have shown the hydrolysis reaction to form the G1 site to be highly exothermic. Therefore, one might expect that f/2oZ(g)
?Eastman Kodak Fellow, 1979-1980. 0022-3654/81/2085-3064$01.25/0
G1 site formation should be limited only by the intrinsic oxygen diffusion process. In this paper, we report on the oxygen compensation formation process in CaFz precipitates doped with rare earth and other ions. In particular, we find that the reaction to form oxygen-compensated sites can be modified depending upon the nature of the dopant ions. Annealing of a disordered precipitate to give well-defined short-range order is similarly affected. We also include a discussion of oxygen-compensated site formation in CaF2:U6+.The motivation for using precipitates in this study arises from their importance in new methods for chemical analysis by laser excitation.151s Ultratrace amounts of the rare earths in water can be detected by coprecipitation into CaF2 followed by selective excitation of fluorescence from specific ions in specific crystallographic sites. The procedure involves the conversion of intrinsic fluoride-compensated sites to oxygen-compensated sites, which are monitored in the analytical determination, by ignition of the precipitate in air. This paper presents a study of the solid-state chemistry that is fundamental to both the methods and the interferences of the methods. Experimental Section The precipitates were made by adding NH4F to a solution of calcium plus the appropriate dopant ions. Previous (1)Bontnick, W. Physica 1968,24,650. (2) Phillips, W. L., Jr.; Hanlon,J. E. J.Am. Ceram. SOC.1963,46,447. (3)Messier, D. R. J. Electrochem. Soc.:Solid State Sci. 1968,115,397. (4)Muto, K.;Awazu, K. J. Phys. Chem. Solids 1968,29,1269. (5)Bollmann, W. Phys. Status Solidi A 1980,57,601. (6)Jacobs, P.W.M.; Ong, S. H. J. Phys. Chem. Solids 1980,4I,437. ( 7 ) Kreger, F. A.; Vink, H. J. Solid State Phys. 1956,3, 307. (8)Catlow, C. R. A. J. Phys. Chem. Solids 1977,38,1131. (9)Fong, F. K.B o g . Solid State Chem. 1966,3,135. (10)Weber, M. J.; Bierig, R. W. Phys. Reu. A 1964,134,1492. (11)Rector, C. W.; Pandey, B. C.; Moos, H. W. J. Chem. Phys. 1966,
45,171. (12) Tallant, D. R.; Wright, J. C. J. Chem. Phys. 1975,63,2074. (13)Bobrovnikov, Y.A,; Zverev, G. M.; Smirnov, A. I. Sou. Phys. Solid State 1967,8,1750. (14)Zverev, G. M.; Smimov,A. I. Sou. Phys. Solid State 1968,9,1686. (16)Reddy, T.R.; Davies, E. R.; Baker, J. M.; Chambers, D. N.; Newman, R. C.; Ozbay, B. jPhys. Lett. A 1971,36,231. (16)Gustafson, F. J.; Wright, J. C. Anal. Chem. 1977,49,1680. (17) Gustafson, F. J.; Wright, J. C. Anal. Chem. 1979,51, 1762. (18)Johnston, M. V.; Wright, eJ. C. Anal. Chem. 1979,51,1774.
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 21, 1981 3065
Oxygen-Compensated Sites in CaF, Precipitates
TABLE I: Crystallographic Sites in CaF,:ErS+ a)
site
symmetry
A
cubic tetragonal
B
trigonal
LL
C
D1 (11similar sites) D2 (4similar sites) G1 G2
trigonal
G3 0
G4 ErLa-1
trigonal
ErY-1 ErSc-1
0
= 0;
=
v;
0 = FF 0
= REca
Figure 1. (a) GI site, (RE,*OF)'.
(b) G4 site, (RE,"KIF*3VF)'.
work" has shown that essentially all of the rare earth present in the solution will coprecipitate. Ignitions were performed in a Lindberg box furnace equipped with an external controller providing a rise time to temperature of 10 min, an overshoot of less than 5 "C, and a temperature stability of f0.5 "C. After ignition, the samples were pressed into a sample holder, cooled to 13 K, and fluorescence from specific fluoride- and oxygen-compensated sites was individually monitored with a nitrogen laser pumped dye laser system. These measurements give information on bulk site distribution changes in a precipitate sample, but cannot give the absolute site concentrations. Each site for a rare earth will provide a unique crystal field and hence a unique splitting of the rare-earth energy levels. Since the crystal field splittings are in general greater than the rare-earth line widths at low temperatures, fluorescence from a specific ion in a specific site can be selectively excited with a narrow-band dye laser and selectively monitored with a high-resolution monochromator. Spectra containing excitation lines from all sites for a given ion can be obtained by monitoring fluorescence with a low-resolution monochromator having a wide enough bandwidth to pass transitions from all sites for a given electronic transition of that ion. Complete descriptions of our experimental apparatuslS and its application to site-selective spectroscopy of precipitates20 appear elsewhere, as do specific discussions of our sample preparation pro~edure."J~~~~ The use of precipitates may cause some special problems in a study of this type due to variations in particle size and morphology. In fact, we do observe spectral differences (19) Miller, M. P.; Tallant, D. R.;Gustafson, F. J.; Wright, J. C. Anal. Chem. 1977,49, 1474. (20) Gustafson, F. J., Ph.D. Thesis, University of Wisconsin, 1978.
formula Erba (Fidistant) (ErCaFi) (Fi in (100) position) ( ErCaFi)X (Fi in (111)Dosition) (2Erca:iFi) (proposed in ref ( rnErCa.nFi) (proposed in ref (rnErcinFi), rn > (proposed in ref (ErCaOF1" (Erca.2 0 ~ * v ~ ) ' (proposed, but not confirmed) ( E r c a - 3 0 ~2vF ' )" (proposed, but not confirmed) (Erca.40~.3V~)' ( ErCa.LaCa-20F)x (proposed) (ErCa'YCa'20F 1" (proposed) ( ErCa-ScCa*2 0 ~ ) ' (proposed)
among unignited precipitates of the same nominal dopant composition, but they are removed upon ignition. Each of the site distribution vs. temperature curves we present in this paper were obtained by igniting different portions of the same precipitate to each temperature. We have found that the relative site distribution obtained at a given temperature can be quantitatively reproduced by igniting another portion of the same bulk precipitate to the same temperature. Different precipitates having identical dopant concentrationsgave the same general site distribution vs. temperature curves. Quantitative differences were observed, but they were found to be much smaller than the effects due to the addition of different dopant ions. Site distribution data were obtained by monitoring the absolute fluorescent intensity produced when the dye laser was tuned to a specific absorption line of each site. Measurement errors were limited to 10% relative standard deviation or less.
Results and Discussion Ignition Characteristics of CaF2:EFPrecipitates. In this study, erbium was employed as a probe of oxygen incorporation since the optical transitions of both fluoride-compensatedl2and oxygen-compensatedmsites and the defect chemistry of the intrinsic fluoride-compensated sitesz1have been extensively studied. Unignited precipitates of CaFz:Er3+exhibit very poor short-range order. If enough thermal energy is present to allow the lattice ions to migrate, annealing will occur. The excitation spectrum in Figure 2a (unignited CaFgEP (0.02 mol %I) was obtained by monitoring the total fluorescence from all sites of the 4S3/2 4115/ztransition of erbium. The only sharp features observed are due to the LL site (distant FI charge compensation, see Table I). Other broadened peaks appear in the 447-nm region. If different precipitate samples containing the same erbium concentration are made and their spectra recorded, one finds that, in all cases, the sharp LL lines are the dominant feature. However, broadened peaks in the regions around 447 and 450 mm may appear with varying intensities or even be totally absent. Unfortunately, these lines are too broad
-
(21) Catlow, C. R. A.; Norgett, M. J.; Ross,T. A. J. Phys. C 1977,10, 1627.
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The Journal of Physical Chemistry, Vol. 85, No. 2 1, 198 1
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I
I
b)
300
500
700
Temperature ("C)
GI
446
I
I
448
450
- - -
x (nrn.)
-
Flgure 2. (a) Excitation spectrum, Z
H (41,,,, 4F,,,), of erbium Z in CaF,:Er(O.OP%) unignited, monitoring fluorescence, E 'I,,,,), from all sites wlth a low-resolution monochromator at 550 nm. (b) Excitation spectrum, same as (a), for CaF,:Er(O.O2%) ignited to 550 OC under vacuum. (c) Excitation spectrum, same as (a), for CaF,:Er(O.O2%) ignited to 500 O C in air.
and too low in intensity to give an unambiguous site classification. These results are interpreted as a sign of sample heterogeneity, either within individual CaFz particles or because of different phases. If enough thermal energy is present to allow lattice ions to migrate, annealing will occur, yielding a number of sharp, strong excitation lines. Ignited precipitates do not exhibit gross heterogeneities. Unignited samples having dissimilar spectra yield essentially identical spectra once ignited to a temperature where lattice annealing may begin. Ignition to a high temperature where lattice annealing is completed in an evacuated cell containing PbFz to prevent oxygen incorporation results in the appearance of several sharp, strong features due to several F-compensated sites (Figure 2b). If the ignition is performed in air, oxygen is incorporated and only the G1 site is formed (Figure 2c). Ignition in air results in a convolution of two processes, oxygen compensation formation and lattice annealing.
Flgure 3. Site distribution vs. ignition temperature plot for CaF2:Er(0.02 % ) ~ a ( 1 0 - ~1.%
A plot of fluorescence intensity vs. ignition temperature (3-h ignition time) for various sites in CaFz:Er(0.02%)L a ( W % ) is given in Figure 3. As will be explained later, the small amount of lanthanum was doped into the solid to study the ignition temperature dependence of oxygencompensated rare earth clusters. Four separate temperature regions can be identified. Below 300 "C, little change from the unignited site distribution is observed. Between 300 and 400 "C enough thermal energy is available to begin the annealing and oxygen compensation formation processes. The site distributions plotted in this temperature range reflect "quasi-equilibrium" states which are reached after about 3 h of heat treatment at a given temperature. That is, ignition for a longer time does not affect the G1, A, B, and LL site intensities appreciably. The C and D2 sites (Table I), however, are observed to increase by a factor of 2 with longer ignition periods due to the cation migration processes which are necessary for these clusters to form. The temperature range 450-500 "C is a transition region where the fluoride-compensatedsites are falling off rapidly. Ignition for periods of time longer than 3 h results in a steady decrease of all fluoride-compensated sites. A t or above 500 "C, total oxygen compensation is achieved in less than 15 min of ignition, with higher concentrations of G2, G3, and G4 sites being formed at higher temperatures. Ignition for long periods of time also increases the G2, G3, and G4 site intensities relative to G1, but to a much lesser extent. More important is the problem of temperature cooling rate after ignition. If the precipitate is quenched from a high temperature, enhanced G2, G3, and G4 site intensities, and broad features in the region of G1 excitation, are obtained (Figure 4a). If the precipitate cools over a longer time period of 1h or more, the G1 site is enhanced relative to G2, G3, and G4 (Figure 4b). In contrast, site distributions of precipitates ignited below 500 "C are independent of the cooling process.
The Journal of Pliysical Chemistry, Vol. 85, No. 21, 1981 3087
Oxygen-Compensated Sites in CaF, Precipitates ( x 50)
a)
GI
I
G4
-
+
Although step 1i s endothermic, the reaction is driven by the large exothermicity of step 3. At higher temperatures, where OF becomes sufficiently mobi!e, other pathways may become accessiblle which involve OF formation from OH;. One possibility iincludes a reaction with free fluoride interstitials which are present due to the intrinsic antiFrenkel defect clistribution and to rare earth-fluoride defect pair dissociation: (2a') reaction to form OF: IF; + OH) 0; HFf
(XI01
b)
(2) migration of OHFto the trivalent dopant; (3) reaction at the trivalent ion site: (REca.Fj)' OH6 (REc,.OF)' + HFf
I
-
455
450 X
- -
(om.)
- -
Figure 4. (a) Excltation spectrum, 2 H (41,5/p 4F5/2),of erbium In CaF,:Er(O.O2%) ignited to 750 OC and quenched to room temper2 (4S3/2 41,5,2), from all sites ature, monitoring fluorescence, E wRh a low-resolution monochromator at 547 nm. (b) Excitation spectrum, same as (a), for CaF,:Er(O.O2%) ignited to 750 OC and cooled slowly to room temperature.
These observations can be explained by the following mechanism. Formation of the higher order oxygen compensated sites requires additional oxygen incorporation:
(2b') migration of' 0; to the trivalent dopant (requires the presence of VF defects); (3') recombination at the trivalent ion site: (REc,.Fi)" + 0; VF (REc,-O,)' + F$
+
+
Reaction of FI with OH: (step 2a') is highly exothermic8 as is Fi - VF recombinationgin step 3, which should provide enough of a driving force to overcome step 1. Another pathway could involve dehydration of hydroxyl groups to form OF: (2a") diehydration
followed by stepci 2b' and 3'. An oxidative mechanism, which might include steps 2b' and 3', would have a different initial step:
-
Solid solutions of CaO-CaF2 exist above 550 OCS5As the ignition temperature is increased, more and more oxygen can be accommodated in the lattice, driving the successive equilibria toward G4 formation. If the temperature is decreased, microprecipitates of CaO begin to form, reducing the total oxygen content in the lattice and pushing the equilibria back toward G1. If cooling is too rapid, the system may not have enough time to respond, leaving a G2 to G4 enhanced site distribution. It should be noted that not all rare earths form higher-order oxygen-compensated sites. As the ionic radius of the dopant ion increases, its tendency to cluster with (OyVF)' defect pairs decreases. Thus, G4 transitions for Pr, Nd, and Sm have not been found.20 Oxygen Compensation Formation i n CaF2:Er(0.02)Lu(IO-~). Both calculations8 and experimental work4 suggest that water rather than oxygen is the more reactive species for oxygen incorporation upon ignition. Kinetic factors, particularly at low ignition temperatures, favor hydrolysis since OH; (the proposed intermediate in a hydrolysis mechanism) should have a mobility similar to that of intrinsic fluoFides. The migrating species in an oxidation mechanism, OF, would be expected to have a lower mobility, especially in doped crystals where an interstitial anion migration mechanism should predominate.8 Bollmann5 has shown,that hydroxyl groups in CaF2 will dehydrate, leaving OF and V; species, if the solid is annealed in a dry atmosphere at 925 OC. However, at the ignition temperatures used in this study, one would expect hydroxyl groups to be stable in the lattice. Calcium hydroxide, for example, does not dehydrate until 550 "C. Based upon the work of Catlow: we suggest a three-step mechanism for the hydrolysis process at low temperatures: (1)surface proton exchange:
+
f/zO2(@:) + 2F6 0; + vi + F2(g) Here, two lattice fluorides must be displaced at the surface for oxygen incorporation, whereas only one lattice fluoride is removed in step 1of the hydrolysis mechanism, resulting in a larger endotlhermicity for the initial oxidation step relative to the iniitial hydrolysis step.8 This barrier difference suggests that a hydrolysis mechanism should be preferred even at high temperatures where steps 2b' and 3' become important. Lattice annealing, the other process which occurs upon sample ignition, can be followed by increases in A and B site intensities at low temperatures. Its progress is determined by both the cation and anion mobilities. The annealing process involves more than just the removal of microstrains in the lattice structure, since the small changes in line width of the A and B site transitions in going from a 300 to a 400 "C ignition temperature cannot account for the huge increases in peak intensity. (Minor imperfections in the lattice should serve only to broaden transition line widths.) More likely, the annealing process involves the removal of inhomogeneous distributions or even separate phases of the rare earth in the solid which were formed during the precipitation step. Heating to an elevated temperature provides enough cation mobility to create a homogeneous rare-earth-doped CaF2 phase and a suitable anion mobility to yield a normal distribution of fluoride-compensated sites. In Figure 3, both the oxygen compensation formation and annealing processes affect the site distribution below 400 OC. The two processes are related in the sense that discrete G1 site formation can occur only if lattice annealing is complete. Since the A, B, and G1 sites all increase in intensity with temperature to the same extent below 400 "C, one may conclude that intrinsic lattice annealing, rather than processes unique to the hydrolysis reaction, limit G1 site formation. No similar relationship
3068
The Journal of Physical Chemistry, Vol. 85, No. 2 1, 198 1
Johnston and Wrlght
6
4
a)
5
-.-
3
c C
E
3c 4
e
.-c
+
-
e 2
??
.-ce
c %
-
e 3
C
-
.I-
.-c
81
C
-E
-1
2
m 1
0
I
0
5 Ignition Time (hr.)
b)
0 300
500 Temperature
GI
I
7 00
(OC)
Flgure 5. Site dlstrlbution vs. ignition temperature plot for CaF2:Er-
IO
?
In c .-
5
-
(0.02%)La(10-3%)precipitated in the presence of 0.1 M Ll' and K'.
A
1
c
between the fluoride- and oxygen-compensatedsites above 400 "C is observed, suggesting that the additional barriers due to hydrolysis, steps 1 and 2, have become important. Figure 5 is a site distribution vs. temperature plot for CaF2:Er(0.02)La(10-3)precipitated in the presence of 0.1 M lithium and potassium. These ions are selective interferences for measurements if they are present in uncontrolled amounts in an actual analysis. They coprecipitate unfavorably into CaF2, so only a relatively small amount goes into the lattice.z0 In this precipitate, annealing occurs to a much greater extent between 300 and 400 "C and is not the limiting factor in G1 site formation. The addition of lithium and potassium does not appear to affect the hydrolysis reaction appreciably since, in both Figures 3 and 5, complete conversion to oxygen compensation does not occur until 500 "C. When doped into CaF2,monovalent ions require fluoride vacancies, which can be either associated or dissociated, as charge compensation. Therefore, the addition of lithium and potassium will increase the number of free fluoride vacancies in the lattice. Both cation and anion mobilities in trivalent-doped CaF2 could be enhanced by increased VF concentration. Anion migration normally proceeds through fluoride interstitial^.^ If a suitable concentration of fluoride vacancies could be injected into the lattice, a second avenue of migration, involving a vacancy mechanism, could become important. Cation migration, which normally proceeds by an isolated cation vacancy mechanism, in trivalent-ion-doped CaF2,could also occur via a divalency mechanismz1 if the V; concentration is increased.z2 The enhanced annealing "rates" in Figure 5 suggest that fluoride vacancies do facilitate cation and anion mobility at low temperatures, opening up new avenues of migration. (22) The divalency mechanism proposed in ref 21 involves cation "hopping" to a nearest-neighbor cation vacancy which is adjacent to a fluoride vacancy.
\
0
0
300
300,400
400
Flgure 6. (a) Ignition time dependence of sites in CaF2:Er(0.0 %.> La(10-3%) with LI+ and K+ ignited to 400 "C. (b) Single ion slte dlstrlbutlons for the precipitate In (a) Ignited to 300 "C for 3 h, 400 "C for 3 h, and 300 "C for 3 h followed by 400 OC for 3 h.
The ignition time profiles of the site distributions obtained below 450 "C deserve special attention. If simple activated rate processes were responsible for these distributions, one would expect to see a steady increase in the extent of oxygen compensation with ignition time. However, it is observed that a "large" conversion (the extent of which is dependent upon the ignition temperature) to G1 site occurs very quickly followed by at best a minimal amount of additional oxygen incorporation at longer times. An ignition time vs. site distribution plot of CaF2:Er(0.02%)La(10-3%)with lithium and potassium at 400 "C is shown in Figure 6a. Within experimental error, only slight changes are observed between 1 and 3 h of ignition and virtually none after that (except for the C and D clusters as explained in an earlier section). Moreover, the "steady-state" site distribution is unique for a given temperature, independent of the pathway used to get there. In Figure 6b, site distributions are plotted for three ignition profiles: ignition to 300 "C for 3 h, 400 "C for 3 h, and 300 "C for 3 h followed by 400 "C for 3 h. The similarity of the results for the last two ignitions shows that the site distributions reflect a unique partition of erbium ions between fluoride- and oxygen-compensated sites at a given
The Journal of Physical Chemlstty, Vol. 85, No. 2 1, 198 1 3060
Oxygen-Compensated Sites in CaF, Precipitates
3
I I
I
I
350
1
450
Temperature ("C)
Flgure 7. Concentration dependence of Gl/A site ratio vs. ignition temperature: ( 0 )= 0.1% Er and (A)= 0.02% Er in CaF,Er,La(lo4%) with Li' and K+.
ignition temperature and are probably not artifacts of ancillary processes such as oxygen incorporation into a disordered lattice followed by annealing. There are insignificant changes in the relative fluoride conversion amounts with erbium concentration (Figure 7). The existence of these "quasi-equilibrium" states (one would expect thermodynamic equilibrium to require complete conversion to G1 site) cannot be easily explained, although it is probably connected to limited ion mobilities and/or oxygen solubility at the low temperatures. Unfortunately, we have no way of determining whether the bulk fluorescence intensities we observe from given sites reflect a homogeneous or inhomogeneous distribution of erbium in the crystallites or phases. A homogeneous distribution throughout the lattice would suggest that at a given temperature equilibrium is reached and only a small amount of oxygen may be incorporated. Alternatively, an inhomogeneous distribution of erbium might suggest that different portions of the precipitate have different tendencies to react because of different surface morphologies or other inhomogeneities among particles. It is interesting to note that the characteristics of the quasi-equilibrium states considered above apply to site distributions showing incomplete lattice annealing as well as incomplete conversion to oxygen compensation (such as those in the 300-400 OC range of Figure 3). It appears that lattice annealing, too,will proceed to an extent unique to the final ignition temperature. In each of the CaF2:Er(0.02%)La(10-3%)precipitates, the small amount of lanthanum was doped into the solids to study the effect of ignition temperature upon oxygen compensated rare earth dimers. The fluorescence from erbium-erbium oxygen-compensated dimers is partially quenched by energy transfer but mixed dimers containing a closed or half-filled shell configuration rare earth ion do not have energy levels that can quench erbium fluorescence.l8 In Figure 3, the La-1 site, an erbium-lanthanum oxygen-compensated dimer, has a similar temperature dependence to the G1 site, suggesting that it also contains only one oxygen per rare earth. Unlike G2, G3, and G4, the La-1 site does not require an excess oxygen content in the precipitate to form. Similar behavior is obtained in Figure 5 although La-1 intensity below 500 "C is not
300
400
500
Temperature ("C) Figure 8. Log (GllA) site ratios vs. ignition temperature for (Er) = CaF,:Er(O.O2%); (La) = CaF2:La(0.02%)Er(1O4%); (Y) = CaFt:Y(0.o2%)Er(1O4%); (Sc) = CaF,:Sc(O.l %)Er(lO"%), ail with Li+ and K+.
TABLE 11. Ionic Radiiz8 ion
ionic radius, A
ion
La Er
1.16 1.00 1.02
A1
Y
sc
ionic radius, A 0.87 0.54
obtainable due to overlap with D2 excitation lines. Effects of Other Dopant Ions. Different trivalent ions induce different reactivities of erbium toward oxygen compensation formation at low temperatures. In Figure 8, the Gl/A site ratio is plotted vs. ignition temperature for precipitates containing 0.02% lanthanum, yttrium, or erbium, or 0.1% scandium. In each case (except for the erbium precipitate), erbium was added as a probe of oxygen compensation formation. The data suggest that, as the ionic radius (Table 11) of the impurity ion becomes smaller, the amount of conversion of the erbium sites to oxygen compensation is reduced. An explanation for this effect may involve differences in fluoride association energies of the various dopant ions. It is well that the larger rare earths at the beginning of the series have larger association energies with fluoride interstitiah, forming only the A site single ion pair unless heated to a high temperature. Smaller rare earths at the end of the series, however, form both single ion pairs (A,B sites) and the dissociated LL or cubic site at all temperatures. Therefore, the free Fi concentrations in the (23) Osiko, V. V.; Shcherbakov, I. A. Sou. Phys. Solid State 1971,13, 820. (24) Zakharchenya, B. P.; Rusenov, I. B. Sou. Phys. Solid State 1966, 8, 31. (25) Gil'fanov, F. Z.; Stolov, A. L.; Yakovleva, Zh. S. Opt. Spectrosc. 1968, 24, 576. (26) Andeen, C.; Link, D.; Fontanella, J. Phys. Rev. B 1977,16,3762. (27) Fontanella,J.; Treacy, D. J.; Andeen, C. J.Chem. Phys. 1980, 72, 2235.
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The Journal of Physical Chemistty, Vol. 85, No, 2 1, 198 1
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Johnston and Wright
The defect equilibrium between single pair and cluster sites (shown below) can be perturbed at higher temperatures either by overcoming the clyster association energy or by producing more intrinsic OF + VF species, allowing the formation of higher-order single-ion oxygen-compensated sites. ( Er c o *RE ca*2
11
0I ~
+ = (Erc,*20~'V~)~ OF'
OF'
VF'
(EIc~'OF)~
+ (RECp.OF)*
"
400
500
600
700
Temperature ("3
Flgure 9. Log (RE-1IEr GI) site ratio vs. ignitlon temperature for the precipitates in Figure 7.
precipitates considered in Figure 8 should decrease in the order Sc > Y, Er > La. From the intrinsic anti-Frenkel defect equilibrium one would then expect the VF concentration to decrease in the order La > Y, Er > Sc. In the previous section, the presence of VF was found to be important in determining low-temperature intrinsic ion mobility. A similar effect may be occurring here, as the energy required to allow oxygen migration (step 2) may be highly dependent upon the fluoride vacancy concentration. This explanation is not unique, however. There may be a competition between the larger and smaller rare earths for the available oxide ions. The observation that the smaller rare earths tend to form higher order clusters more readily20might be due to greater rare earth-oxygen association energies. The interconversion (Erca*OF)'+ (RE-Fi)"+ (ErCa-Fi)'+ (RECa-OF)' would then lie more to the left when large rare earths such as lanthanum are present resulting in an increase in the erbium Gl/A site ratio with increasing dopant ionic radius. Although the explanation is not unique, the trend in Figure 8 is clear-decreasing erbium Gl/A site ratios with decreasing ionic radius of the dopant ion. Differences in site distribution among these precipitates also exist at higher temperatures. At moderately high concentrations, clustering of several dopant ions into a single site may Dimer sites involving oxygen compensation have been identified on the basis of concentration dependencies.ls Scandium, yttrium, and lanthanum form dimer sites (labeled Sc-1, Y-1, and La-1, respectively) with erbium whose fluorescence can then be monitored. The similarity of the crystal field splittings of the dimer energy levels observed in fluorescence to those of the erbium G1 site suggesta that the former contain only one oxygen per dopant ion providing an approximately trigonal point symmetry at the dopant ion. The temperature dependencies of these sites are shown in Figure 9. It is seen that as the ionic radius of the ion complexing with erbium decreases, so does the dimer site's high-temperature stability. This effect is even more pronounced for higher order clusters involving one erbium and two or more of the other ion. Shannon, R. D. Acta Crystallogr., Part A 1976,32,751. Tallant, D. R.; Moore, D. S.;Wright, J. C. J. Chem. Phys. 1977, 67,2897. (30) Moore, D. S.; Wright, J. C. Chem. Phys. Lett. 1979, 66, 173. (31) Moore, D.S.,Ph.D. Thesis, University of Wisconsin, 1980.
OF'
+ VF'
7 (REca'20F*VF)
x
+ VF'
etC
OF'+VFL
etC
As indicated earlier, higher-order oxygen-compensated sites of small dopant ions are more favorably produced than those of large dopant ions. The La-1 site could fall off more slowly than Y-1 or Sc-1 at high temperatures because G2 to G4 sites of lanthanum do not form. Differences in binding energy among the dimer sites could also exist. One might interpret the data as an indication of higher stability of complexes involving larger ions than of those involving smaller ions relative to the individual G1 sites. Other differences in site distribution, which do not directly relate to the oxygen compensation formation process, are observed in these precipitates. Although each of the ions studied here and in a previous workla appear to form the same types of oxygen-compensated clusters, differences are observed in fluoride compensation. In CaFz:ErS+(0.02%) three types of cluster sites are observed: C, D1, and D2. Moore31has identified the C site and D1 family as dimers and the D2 family as clusters involving more than two erbium ions. For the other rare earths, formation of these sites is dependent upon the cation size. Dielectric relaxation s t ~ d i e s have ~ ~ , shown ~~ the D2 sites to be preferentially formed with small rare earths whereas only the C site is formed with large rare earths. In our precipitates, the overall cluster equilibrium is essentially governed by the higher concentration (0.02%) ion (La, Y, or Sc). Erbium may then substitute into the sites that are present forming mixed clusters. Crystal field splittings of the erbium levels for these sites will be similar to those for erbium-erbium clusters found in CaFz:Er(0.02%). Therefore, the presence of C, D1, or D2 type excitation lines in the erbium spectrum of CaFz:RE(0.02%)Er(10-3%) will reflect the clustering preferences of the higher concentration ion. We find that, in CaFz:La(0.02%)Er(10-3%), only the C site is formed. In the scandium precipitate, only D2 (and possibly some D1) is formed. Yttrium, which has an ionic radius similar to erbium, forms all the clusters that erbium does: C, D1, and D2. These results confirm the results of Fontanella et al.26,27which show a tendency toward higher order clustering with decreasing dopant ion size. Another interesting trend across the lanthanide series involves single-ion fluoride-compensatedsites. The larger rare earths, which exhibit larger fluoride binding energies, form only tetragonal (A site) single defect pairs. Smaller rare earths form increasing numbers of trigonal (B) and cubic (LL) sites. Simple equilibrium models have, in general, been unable to explain the relative concentrations of these single-ion sites as a function of temperature or c~ncentration.~~ In our precipitates, we find the erbium A to B site ratio to be highly dependent upon the identity of the other dopant ion. Figure 10 shows that the A/B ratio for erbium in general decreases as the ionic radius of the second dopant ion decreases. This phenomenon
The Journal of Physical Chemistry, Vol. 85, No. 21, 1981 3071
Oxygen-Compensated Sites in CaF, Precipitates
-I;-
6
5
I
I
c
'E
4
e
+
g
- 3
.-c21 u)
0 c
8 2
J
I "
300
400
500
Temperature ("C)
0
Flgure 10. Log (A/B) site ratio vs. ignltion temperature for the precipitates in Figure 7.
cannot be explained at present, although differences in cluster formation or in the extent of oxygen compensation formation may be important. Interferences to Rare Earth Analysis in CaFP The previous discussion can yield valuable insight into the nature of certain interference processes in rare earth analysis in CaFz.17J"Precipitates formed in the presence of 10 ppm (in solution) or greater aluminum concentrations are found to have different ignition characteristics. In Figure 11,the site distribution vs. ignifson temperature plot for CaFz:Er(0.02%)La(10-3%)precipitated from a solution containing 50 ppm A13+is shown. In comparison to Figure 5, aluminum is found to inhibit both the annealing and oxygen compensation formation processes, although the latter is affected to a much greater extent. One may extrapolate the trends established in the previous section based upon the small ionic radius of aluminum (Table 11) to explain these effects. Aluminum should have a small fluoride interstitial association energy which will reduce the intrinsic ion mobility resulting in slower annealing rates. However, an argument to explain G1 site inhibition based upon lower oxygen mobility due to a lower VF concentration does not appear to be totally valid. The mobility argument asserts that the presence of aluminum serves only to negate the effect of lithium and potassium. A comparison of Figures 3 and 11reveals that the addition of aluminum does not affect the annealing process as much as if lithium and potassium were not present, whereas the G1 site formation is much slower with aluminum present than without the monovalent ions. Even if mobility considerations were more important to the hydrolysis reaction than to annealing, one would not expect G1 site formation to be slower than it is in Figure 3 without the annealing process being at least as slow as it is in Figure 3. Therefore, it seems likely that other effects are important in Figure 11. By exhibiting a great tendency to cluster with lattice oxygen, aluminum could be an efficient oxygen scavenger with the interconversion (Erca-OF)' + (Alca.Fi)' * (Erca.Fi)' + (Alca.OF)I
Figure 11. Site distribution vs. ignition temperature for CaF,:Er(O.02%)La(lO3%) with Li+ and K+, precipitatedin the presence of 50 ppm of AI^+.
displaced well to the right. Additionally, aluminum could form separate phases of its own which might impede oxygen incorporation onto CaFz surface. Inspection of Figure 11reveals the cause of aluminum interference to rare earth analysis in CaFP If the ignition step in the analytical procedure is set between 500 and 600 "C where total oxygen compensation is normally obtained, incomplete conversion will result. If the ignition is performed above 600 "C, total conversion occurs but changes in site distribution are still present. The La-1/Er G1 intensity ratio (used for lanthanum analysis in CaF2) is smaller when aluminum is present (Figure 11)than when not present (Figure 5). Additionally, the G2 to G4 sites are completely absent and a much greater LL site intensity is obtained. These observations suggest that aluminum, a small ion, acts as an efficient scavenger of excess oxygen at high temperatures by preferentially forming higherorder oxygen-compensated sites. Successful competition by the aluminum for excess oxygen would inhibit erbium G2 to G4 site formation, and would increase the LL site concentration by shifting the G1 site dissociation equilibrium to the right: (Erca-OF)"+ Eri;ra + OF G1 LL This latter process would affect dimer dissociation in a similar way:
The net result would be smaller La-1/Er G1 and Er Gl/LL site ratios in accordance with the experimental results. Several ions are found to inhibit oxygen compensation f o r m a t i ~ n . ~For ' ~ ~some ~ (Sc3+,Ti4+, Zr4+),arguments similar to those for aluminum are possible. For others
3072
Johnston and Wright
The Journal of Physical Chemistry, Vol. 85, No. 21, 1981
n
z
,u-20
C
-
ln c 'E 4
i?
e
+
g
- 3
=a C
c
g 2
-I
I
1
400 0
I
500
500
700
Temperature ("C) Flgure 12. Site distribution vs. ignition temperature for CaF,:Er(O.02%)La(1O3%) with U+ and K+ precipitated in the presence of 0.15 M acid.
(Mg2+,SO?-, Pod3-),simple mobility and equilibrium arguments cannot be made. It is possible that magnesium scavenges oxygen through the reaction MgF2 + H2Oo MgO + 2HF(,,
-
forming either a separate phase or a solid solution. For the anions, surface effects or secondary phase formation could be important. Hydrogen presents an interesting situation. Precipitation from an acidic solution (0.15 M) yields the site distribution plot shown in Figure 12. The features here are very similar to those obtained with aluminum: slower annealing and G1 formation processes, less G2-G4 at higher temperatures, and smaller La-1/Er G1 and Er Gl/LL ratios. It is most likely that hydrogen enters the lattice as HFf, although substantial hydrogen bonding to lattice fluorides is possible. How such a species affects intrinsic ion mobility or oxygen incorporation is not understood, although steps 1 and 3 in the hydrolysis mechanism could be impeded by high HFf concentrations. From these results, it is obvious that the oxygen compensation formation process can be affected in nontrivial ways by a variety of species. Ignition Properties of CaFz:V+. Hexavalent uranium in the form UOzz+coprecipitates readily into CaF2 Upon ignition in air, three major types of sites containing sharp line features are observed to The first, the U-1 site, a single site equivalent to the C1 center observed by Lupei and LupeiS8and the type I spectra of Nicholas,% consists of a single sharp 0-0 line at 521.25 nm and vibrational sidebands in both excitation and fluorescence. Manson et al.= have determined the symmetry of this site (32) Johnston, M. V.; Wright, J. C. Anal. Chem. 1981,53,1050. (33) Lupei, A.; Lupei, V. J. Phys. C 1979, 12, 1123. (34) Nicholas, J. V. Phys. Rev. 1967,155, 151.
600 800 Temperature ("'2)
1000
Figure 13. Site distribution vs. ignition temperature for CaF,:U6+ (8
x
to be trigonal, suggesting either a (UO,)~,or (UO)62+= (u~;60~.2V~)'species. Studies of y irradiation of CaF2:U6+83 and of luminescence spectra of known U062en ti tie^^^%^ suggest that the second is the correct choice. The U-2a site, the most intense of a group of at least three similar sites, corresponds to the Cz center observed in ref 33 and is believed to be a U06unit associated with an extra fluoride vacancy. A third type, the U-3 site, which corresponds to the type I1 spectra of Nicholas,%appears only at high concentrations and is believed to be a uraniumuranium dimer. These sites appear only when the precipitate is ignited in air. Fluorescence is observed from either unignited precipitates or precipitates ignited under vacuum, but only broad bands are obtained. The ignition temperature dependencies of the U-l, U-2a, and U-3 sites in CaFz:U6+(8 X are shown in Figure 13. None of the sites are formed below 400 OC. The U-1 site rises and levels off at high temperatures while the U-2a and U-3 sites peak and then fall off with increasing temperature. The similarity of the low-temperature profiles of these sites to the G1 site (Figure 3) suggests that extra oxygen must be incorporated to form them, which is consistent with a U06 lattice species. The high-temperature instability of the U-2a site suggests that, once certain association energies are overcome, dissociation into smaller defect entities is possible, which is consistent with the interpretations of Lupei and Lupei.= Dissociation into monomer species, as is observed for rare earth clusters, is probably the cause for the U-3 site decrease in intensity at high temperatures. Acknowledgment. This research was supported by the National Science Foundation under grant no. CHE7825306. (35) Manson, N. B.; Shah, G. A.; Runciman, W. A. Solid State Commun. 1975, 16, 645. (36) De Hair, J. Th. W.; Blasse, G. J. Lumin. 1976, 14, 307.
