Laser-Excited Rare Earth Luminescence as a Probe of Mineral

Chapter 7 Laser-Excited Rare E a r t h Luminescence as a P r o b e o f M i n e r a l C h a r a c t e r i s t i c s

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J. C. Wright Department of Chemistry, University of Wisconsin—Madison, Madison, WI 53706 The rare earth elements are different from other elements because the optical transitions between levels of the f configuration are inherently very sharp-lined and have well-resolved structure characteristic of the local crystal fields around the ion. In minerals, this characteristic provides an excellent probe of the local structure at the atomic level. Examples will be shown from our work of how site selective laser spectroscopy can be used to determine the thermal history of a sample, the point defect equilibria that are important, the presence of coupled ion substitution, the determination of multiple phases, and stoichiometry of the phase. The paper will also emphasize the fact that the usefulness and the interpretation of the rare earth luminescence is complicated by the presence of quenching and disorder in mineral samples. One in fact needs to know a great deal about a sample before the wealth of information contained in the site selective luminescence spectrum can be understood. n

Our research group at the university of Wisconsin has developed a new approach to the study of minerals that can provide great detail at the atomic level. Our program is founded on the idea that site selective laser spectroscopy can be used to simplify the spectra of complex materials. Recent reviews of our work are published in reference 1^. The methods can be used to: 1) Identify different phases in a complex mixture even when the phases differ in concentration by many orders of magnitude. 2) Measure the solid state equilibria between point defects to determine the relative number of different defects and the previous temperature history of the mineral that characterized the defect distribution. 3) Identify intergrowths of phases and the development of superstructure within the mineral lattice. 4) Observe the aggregation kinetics as point defects coalesce to form clusters or other phases. 0097-6156/90/0415-0135$06.00/0 © 1990 American Chemical Society

Coyne et al.; Spectroscopic Characterization of Minerals and Their Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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5) Determine the stoichiometry of phases. 6) Identify new s i t e s that are the r e s u l t of coupled ion s u b s t i tution where incorporation of one ion encourages incorporation of other ions that can compensate each other i n the l a t t i c e . Space l i m i t a t i o n s prevent the discussion of a l l the points so t h i s synopsis w i l l be directed toward o u t l i n i n g and giving examples of the fundamental mechanisms that underlie the methodology and i t s limitations. Probe Ion Spectra Crystal F i e l d S p l i t t i n g s . The key idea i s to use fluorescent probe ions that have spectra that are sensitive to the c r y s t a l f i e l d s they experience within a material (2). Such ions are generally ones with u n f i l l e d inner o r b i t a l s such as lanthanides (e.g. P r , E u , E r ) , actinides (e.g. U), or t r a n s i t i o n metals (e.g. C r , R e , 0 s ) . The spectra that are seen depend upon the r e l a t i v e strengths of the c r y s t a l f i e l d and the atomic interactions within the ion such as the Coulombic interactions between electrons and between electrons and the nucleus and the spin o r b i t interactions between the electrons and the nucleus. The f electrons within the 4 f configuration of the lanthanides have only weak interactions with the c r y s t a l f i e l d because they are shielded by outer 5s and 5p electrons. Consequently the spectra are dominated by t r a n s i t i o n s between the atomic states of the lanthanide ion. The l e f t side of Figure 1 shows the i o n i c energy l e v e l s of E u i n the gas phase. When the ion enters a c r y s t a l l a t t i c e , there w i l l be additional c r y s t a l f i e l d i n t e r a c t i o n s . The interactions cause small c r y s t a l f i e l d s p l i t t i n g s on the order of 200 cm" that are superimposed on the atomic t r a n s i t i o n s and are e a s i l y observable. The t r a n s i t i o n s are inherently extremely sharp so a monochromator with a reasonable resolution or a dye laser (ca. 0.1 cm" ) w i l l be able to observe spectra that are completely resolved and are highly c h a r a c t e r i s t i c of the p a r t i c u l a r c r y s t a l f i e l d s encountered by the lanthanide ion. In that way, the ion acts as an excellent probe of the l o c a l l a t t i c e i n a mineral. The r i g h t side of Figure 1 shows an expanded view of the c r y s t a l f i e l d s p l i t t i n g s f o r one state of a E u ion i n two d i f f e r e n t crystallographic states. The other states w i l l have a corresponding s p l i t t i n g . When one records an absorption spectrum of t h i s material, the spectrum w i l l contain absorption l i n e s corresponding t o t r a n s i t i o n s from the ground state t o each of the f i v e l e v e l s i n each of the two s i t e s . The spectrum on the f a r r i g h t i l l u s t r a t e s a t y p i c a l spectrum. The d electrons within the d electron configuration of the t r a n s i t i o n metals have much stronger interactions with the c r y s t a l f i e l d so c r y s t a l f i e l d interactions are comparable to the interactions within the atom. The spectral t r a n s i t i o n s can change more d r a s t i c a l l y to r e f l e c t the changes i n the c r y s t a l f i e l d . The l i n e widths of the t r a n s i t i o n s are also much broader. Not a l l t r a n s i t i o n metals are useful as probe ions because the l i n e s are too broad to allow one to resolve features i n samples where there are multiple l o c a l environments about the ion. In f a c t , only some atomic states of the probe ions are useful because most states i n t e r a c t too strongly with the c r y s t a l f i e l d to give narrow enough l i n e s . 3 +

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Laser-Excited Rare Earth Luminescence as a Probe


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>~465 nm

SITE

SITE 2

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Figure 1.

Eu

energy levels for two different sites.


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The 5 f electrons of the actinides represent an intermediate case where there i s s t i l l shielding of the c r y s t a l f i e l d s but i t i s not as e f f e c t i v e as i n the lanthanides. The c r y s t a l f i e l d interactions are larger than the lanthanides but not as large as i n the t r a n s i t i o n metals. The l i n e s of most t r a n s i t i o n s are sharp and a l l the a c t i n i a e ions could be used p o t e n t i a l l y as probes of the l o c a l environments of minerals. Energy Relaxation. The c r y s t a l f i e l d s p l i t energy l e v e l s of probe ions can be excited by tuning a laser to t h e i r absorption wavelength. The energy relaxation mechanisms are reviewed i n more d e t a i l i n reference 3. The energy that i s deposited i n the ion can either be re-emitted as fluorescence, l o s t to the l a t t i c e , or transferred to another ion. The l a t t e r two processes w i l l determine whether f l u o r escence w i l l be seen from a mineral sample. Each c r y s t a l f i e l d l e v e l w i l l have a t o t a l r a d i a t i v e t r a n s i t i o n rate that characterizes the probability/second that energy i n that l e v e l can be emitted as l i g h t i n a r a d i a t i v e relaxation to a lower energy l e v e l . There w i l l also be rates for loss of energy to the l a t t i c e and energy transfer. The fluorescence quantum e f f i c i e n c y w i l l be the r a t i o of the r a d i a t i v e t r a n s i t i o n rate to the t o t a l rate for a l l three processes. I t i s therefore important to know the factors that control the two nonradiative relaxation processes. The rate at which energy i s l o s t to the l a t t i c e depends upon how many l a t t i c e phonons are required to soak up the energy. A given e l e c t r o n i c state w i l l be able to lose energy by relaxing to some lower state. The energy difference between the two states i s the energy that must be given to the l a t t i c e i n the form of phonons or quantized l a t t i c e v i b r a t i o n s . The number of phonons required depends on the energy of an i n d i v i d u a l phonon. I f there are energetic phonons i n the l a t t i c e , fewer phonons are required and the multiphonon relaxation can be e f f i c i e n t . Conversely, i f the l a t t i c e i s soft and the phonons are not energetic, many phonons are required and the multiphonon relaxation i s not e f f i c i e n t . Thus, hard l a t t i c e s with energetic phonons and energy l e v e l s with small energy gaps to the next lower l e v e l are unfavorable for fluorescence t r a n s i t i o n s . Levels that are separated by energy gaps that are small enough f o r a single phonon to accept the energy are so e f f i c i e n t l y relaxed that one cannot observe fluorescence. In f a c t , the relaxation can occur so quickly that the uncertainty p r i n c i p l e demands that the l e v e l i s broadened by the short l i f e t i m e . The energy transfer rate i s dependent upon the energy gap as well. The two ions that are exchanging energy w i l l have an i n i t i a l t o t a l energy before transfer occurs and a f i n a l t o t a l energy a f t e r transfer occurs. The difference must be accommodated by l a t t i c e phonons and again the e f f i c i e n c y of the transfer i s enhanced when the fewest l a t t i c e phonons are required. The e f f i c i e n c y also depends on the distance between the two ions exchanging the energy. Energy transfer i s usually subdivided into d i f f e r e n t classes that depend on the nature of the two ions. I f the ions are i d e n t i c a l and the energy transfer does not involve any net loss of e x c i t a t i o n energy ( i . e . the l a t t i c e does not receive any energy), the transfer i s c a l l e d migration or exciton propagation. I t can be very rapid when the ions are close. I f the ions are i d e n t i c a l and the transfer



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involves loss of energy to the l a t t i c e , t h i s process i s c a l l e d i o n pair decay. The excited ion decays to a lower l e v e l while the neighboring ion i s excited to a higher l e v e l . Energy differences i n the i n i t i a l and f i n a l t o t a l energies are released as phonons. I f the ion accepting the energy w i l l subsequently lose the energy to the l a t t i c e through multiphonon relaxation, one c a l l s the accepting ion a quenching center. Iron often provides a quenching center i n minerals and w i l l prevent the s i t e s e l e c t i v e methods described i n t h i s paper from being applied to many mineral systems. I f migration can occur i n the l a t t i c e because the mineral has a high concentration of the excited ion, i t i s common that an e x c i t a t i o n w i l l r a p i d l y migrate to a quenching center or sink where the fluorescence i s quenched. With t h i s overview of why d i f f e r e n t probe ions have t h e i r chara c t e r i s t i c spectra and what happens to an e l e c t r o n i c e x c i t a t i o n , we are ready to see how the probe ions can be used to study minerals. Probe Ions i n Minerals Point defects are an important part of the work i n t h i s paper. There are many reasons for the formation of point defects i n minerals and t h e i r presence can exert important perturbations on the properties of the material (4). Point defects are formed because of the thermally driven i n t r i n s i c disorder i n a l a t t i c e , the addition of a l i o v a l e n t impurities or dopants, the presence of metal-nonmetal nonstoichiometry, and the creation of nonideal cation r a t i o s . The f i r s t three source of defects are well-known from binary compounds but the l a s t i s unique to ternary compounds. Ternary compounds are much more complex than the binary compounds but they also have gained a great deal of attention because of the variety of important behavior they exhibit including now the presence of superconductivity at high temperatures. The point defects can be measured by introducing probe ions i n t o the l a t t i c e . There are two ways to use probe ions. One can introduce probe ions i n t e n t i o n a l l y i n synthetic samples during the process of sample preparation. One can also take advantage of the natural abundance of probe ions i n natural minerals because laser techniques provide excellent signal l e v e l s that allow one to work at very low concentrat i o n l e v e l s . In general, a probe ion w i l l replace a l a t t i c e ion that has a d i f f e r e n t valence. In order to maintain charge n e u t r a l i t y , the l a t t i c e must have a charge compensation that can be either an i n t e r s t i t i a l ion, a vacancy, or an electron or hole. Each of these point defects change the l o c a l environment or the probe ion and the absorpt i o n and fluorescence spectra r e f l e c t the r e s u l t i n g changes i n c r y s t a l f i e l d s p l i t t i n g s . Any i n t e r e s t i n g mineral w i l l i n general have many types of charge compensation a v a i l a b l e and the charge compensations can e x i s t i n d i f f e r e n t positions r e l a t i v e to the probe ion. Thus, one commonly has a number of d i f f e r e n t centers or s i t e s involving the probe ions and s i t e s e l e c t i v e spectroscopy i s e s s e n t i a l f o r i s o l a t i n g the contribution from the i n d i v i d u a l defect structures. The r e l a t i v e numbers of the d i f f e r e n t environments depends on the concentrations of a l l the possible ions that could be i n a mineral, the temperature at which the d i s t r i b u t i o n e q u i l i b r a t e d and the atmosphere that was i n equilibrium with the mineral. More d e t a i l e d descriptions require reference to s p e c i f i c c r y s t a l systems.


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These ideas are i l l u s t r a t e d i n Figure 2 f o r CaO. The i n t r i n s i c defects are Ca and 0 vacancies. When a E u ion enters the l a t t i c e , i t substitutes f o r a Ca and a charge compensation i s required. The charge compensation i s provided by a C a vacancy which serves to compensate two E u ions. There are d i f f e r e n t arrangements f o r the Eu and the C a vacancy as i l l u s t r a t e d i n the f i g u r e . The cubic s i t e has the C a vacancy compensation so distant that the l o c a l symmetry of the E u i s not perturbed. The tetragonal s i t e has the vacancy i n the nearest neighbor p o s i t i o n . The dimer s i t e s have the vacancy i n the nearest neighbor positions f o r two symmetrically located E u ions. Each of these d i f f e r e n t E u s i t e s has d i f f e r e n t c r y s t a l f i e l d s p l i t t i n g s and spectra. 3 +

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Experimental The key development f o r using probe ions to study mineral systems i s s i t e selective laser spectroscopy. A tunable dye laser i s tuned t o match the absorption l i n e of a p a r t i c u l a r ion with a p a r t i c u l a r environment or s i t e within the sample. Only that ion w i l l be excited and only that ion w i l l fluoresce so the r e s u l t i n g fluorescence spectrum i s much simpler than the conventionally obtained spectrum. Single s i t e e x c i t a t i o n spectra can likewise be obtained by monitoring the fluorescence at a s p e c i f i c wavelength of a s p e c i f i c s i t e and scanning the laser over the possible absorption l i n e s . One can systematically dissect an absorption spectrum into the component s i t e s that make i t up. Figure 3 shows the e x c i t a t i o n spectrum of CaO doped with E u when fluorescence from a l l s i t e s i n the sample was monitored. The dye laser was scanned over the possible absorption l i n e s and each time the wavelength matched a t r a n s i t i o n on any s i t e , the fluorescence i n t e n s i t y increased and gave a l i n e . Figure 4 shows the same procedure on the same c r y s t a l except now a high resolution monochromator was used to monitor the fluorescence that occurred at a wavelength c h a r a c t e r i s t i c of a s p e c i f i c s i t e . Now, one sees increases i n the fluorescence only when the dye laser matches an absorption l i n e of the same s i t e that has the fluorescence l i n e being monitored. The r e s u l t i s a single s i t e e x c i t a t i o n spectrum. By systematically choosing d i f f e r e n t fluorescence l i n e s f o r monitoring, one can correl a t e a l l of the l i n e s i n Figure 3 with the single s i t e spectra of Figure 4. These basic techniques can be used i n a number of ways to get d e t a i l e d information about point defect e q u i l i b r i a and dynamics. A succinct summary of the c a p a b i l i t i e s of the s i t e s e l e c t i v e laser methods i s given below. 1) One can follow changes i n s i t e concentration over more than four orders of magnitude. 2) One can measure the s i t e concentrations i n absolute units to ±25% by measuring the absorption c o e f f i c i e n t and r a d i a t i v e t r a n s i t i o n p r o b a b i l i t y (which i n turn comes from the l e v e l l i f e t i m e , r a d i a t i v e quantum e f f i c i e n c y and r a d i a t i v e branching r a t i o s ) or to ±15% by nonlinear regression f i t t i n g of r e l a t i v e i n t e n s i t i e s to t o t a l dopant concentration over a range of s i t e d i s t r i b u t i o n s . 3) One can assign the s i t e symmetry by determining the number of c r y s t a l f i e l d l e v e l s of d i f f e r e n t e l e c t r o n i c states, the p o l a r i z a 3 +


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Structure


Site Cubic Tetragonal

Dimer ο Figure 2.

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Ca 2+ Different Eu

Eu3+ 3+

s i t e s i n CaO

528

530

Wavelength (nm) Figure 3. Excitation spectrum of Eu:CaO obtained by monitoring a l l fluorescence wavelengths.


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Figure 4. Excitation spectra of Eu:CaO obtained by monitoring a l l d i f f e r e n t fluorescence wavelengths.



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t i o n of the t r a n s i t i o n s , and the anisotropy of the Zeeman s p l i t t i n g as the c r y s t a l s are rotated about l o c a l symmetry axes. 4) One can determine the number of cation dopants associated with a given defect structure by measuring the two body energy transfer rates between probe ions, the e f f i c i e n c y of three body upconversion, and the e f f e c t of adding a second dopant ion i n higher concentration on the s p l i t t i n g s of the f i r s t dopant ion. 5) One can measure the equilibrium constant f o r defect associat i o n between a dopant ion and i t s charge compensation by measuring the r e l a t i v e concentrations of the paired and dissociated probe ion concentrations over a range of dopant concentrations. 6) One can measure the concentration of the i n t r i n s i c l a t t i c e defects by measuring the r a t i o of the paired and dissociated p a i r s and having a previous value f o r the equilibrium constant. 7) One can measure the a c t i v i t y c o e f f i c i e n t s f o r the nonideality corrections i n defect e q u i l i b r i a by comparing the r a t i o s of the r e l a t i v e s i t e concentrations i n the high dopant concentration range with the low concentration range. 8) One can determine the association rate constants and thermodynamic a c t i v a t i o n energies for migration by measuring the changes i n s i t e d i s t r i b u t i o n as a sample i s annealed over a range of times and temperatures. These a b i l i t i e s provide the s i t e s e l e c t i v e laser techniques with unique c a p a b i l i t i e s to determine the changes i n defect e q u i l i b r i a at a microscopic l e v e l . They are complementary to conductivity, d i f f u sion, d i e l e c t r i c and other bulk measurements which measure nonlocal properties that must be modeled to extract microscopic d e t a i l s . They have two advantages over magnetic resonance methods: they are sensitive over a much wider dynamic range of defect concentrations and t h e i r s e l e c t i v i t y enables one to study even complex materials where there may be large differences i n the r e l a t i v e concentrations of d i f f e r e n t s i t e s . Our work has applied these techniques to the study of the binary i n s u l a t i n g materials including the f l u o r i t e s , a l k a l i halides, alkal i n e earth oxides, and perovskites. Many of these are simple materials that are commonly used as models f o r a l l s o l i d state defect e q u i l i b r i a . Our work has had the goal of determining at the microscopic l e v e l the defect e q u i l i b r i a and dynamics that are important i n understanding s o l i d state chemistry as well as developing new t o o l s for the studies of s o l i d materials. Example Studies of Mineral Systems A l k a l i n e Earth Oxides. I f a t r i v a l e n t lanthanide i s substituted f o r Ca i n CaO (5) or a C r i s substituted f o r a M g i n MgO (6), the extra p o s i t i v e charge i s compensated by a cation vacancy as described e a r l i e r . The vacancy can be i n a near neighbor position or i t can be very distant from the probe ion (7-10). I f another ion l i k e Na i s i n the cation s i t e , i t can serve as a charge compensation f o r the probe ion either as a nearby ion or d i s t a n t l y . This case i s coupled ion substitution that commonly a f f e c t s the p a r t i t i o n i n g of ions into d i f f e r e n t mineral phases (11)· One can envision many other ions that could also a f f e c t the l o c a l environment or the probe ion. A l l of the p o s s i b i l i t i e s lead to d i f f e r e n t s i t e s with d i f f e r e n t crystal f i e l d splittings. 2 +

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There i s an equilibrium between the d i f f e r e n t s i t e s that determines the observed d i s t r i b u t i o n . For example, the r a t i o of the s i t e s with the cation vacancy nearby to those with the vacancy distant w i l l depend upon the concentration of vacancies by the law of mass action. One can write the conventional equilibrium relationships given by mass action considerations f o r e q u i l i b r i a between vacancies and probe ion s i t e s with l o c a l and distant vacancy compensation. The e q u i l i b rium constant w i l l depend on the temperature under which the equilibrium was established. Since a l l of the s i t e s can be observed by s i t e s e l e c t i v e laser spectroscopy, one can measure the equilibrium d i s t r i b u t i o n s d i r e c t l y . We f i n d that the s i t e s and t h e i r d i s t r i b u tions are described e x c e l l e n t l y by the mass action relationships of conventional e q u i l i b r i a . This work i s described i n more d e t a i l elsewhere (5,6). F l u o r i t e s . As the second example, we choose CaF doped also with t r i v a l e n t rare earth ions. The replacement of divalent Ca by a t r i valent lanthanide i s accompanied either by an extra f l u o r i d e i n t e r s t i t i a l ion or an external ion such as oxygen or hydrogen. I f one excludes other ions, the compensating f l u o r i d e i n t e r s t i t i a l ion can be located i n either a nearest neighbor p o s i t i o n , a next nearest neighbor position or i t can be very distant. The lanthanide's c r y s t a l f i e l d w i l l r e f l e c t either the tetragonal, t r i g o n a l or cubic symmet r i e s of the three respective s i t u a t i o n s . S i t e s e l e c t i v e spectroscopy also reveals many additional lanthanide s i t e s that are caused by c l u s t e r s of several lanthanides and t h e i r f l u o r i d e i n t e r s t i t i a l charge compensations (12). Again, there w i l l be an equilibrium between the d i f f e r e n t defect s i t e s that w i l l depend on the number of fluoride i n t e r s t i t i a l s . Using the same arguments as we used i n CaO, one would expect that an increased concentration of lanthanide dopant should introduce a l i k e number of f l u o r i d e i n t e r s t i t i a l s into the l a t t i c e . I f the pairing of the i n t e r s t i t i a l s with the lanthanide i s not complete, the number of free i n t e r s t i t i a l s would increase. The larger number of i n t e r s t i t i a l s should lead to an increase i n the number of lanthanides that have the f l u o r i d e i n t e r s t i t i a l charge compensation i n a nearby position r e l a t i v e to the ones that have the i n t e r s t i t i a l distant according to the law of mass action. Laser spectroscopy shows the opposite e f f e c t though. There has been a long standing question about the nature of the defect e q u i l i b r i a i n the f l u o r i t e s (13-22) . Measurements of the conductivity and d i f f u s i o n i n f l u o r i t e s i s commonly interpreted with simple mass action relationships but the s i t e s e l e c t i v e laser spectroscopy (as well as other techniques) has shown that the s i t u a t i o n i s more complex and that simple mass action relationships don't even describe the observed e q u i l i b r i a q u a l i t a t i v e l y (23-28). The p r i n c i p a l reasons f o r the f a i l u r e of standard relationships i s either other e q u i l i b r i a (in p a r t i c u l a r , the FJ: scavenging e q u i l i b r i a ) compete f o r F that F~ i s an unusual ion that i s known f o r forming associates with i t s e l f that could change the defect e q u i l i b r i a , or there are abnormally large nonideality e f f e c t s . These e f f e c t s were studied i n a l l of the a l k a l i n e earth f l u o r i d e s and we found the same anomalies were always observed. 2

i #


7. WRIGHT

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We then studied S r C l as a model system ( 2 9 - 3 1 ) . This material had the f l u o r i t e structure but i t d i d not have the F~. Furthermore, the c r y s t a l could be quenched from high temperatures and a l l possible competing e q u i l i b r i a could be eliminated. The only remaining cause for departures from mass action relationships i s nonideality e f f e c t s . We studied the d i f f e r e n t s i t e s i n S r C l and measured the s i t e d i s t r i butions as a function of annealing temperature and dopant concentrat i o n ( 2 9 ) . At low dopant concentrations, the s i t e d i s t r i b u t i o n s do follow the expected mass action r e l a t i o n s h i p s . At high concentrations there are the same strong deviations that are observed i n a l k a l i n e earth f l u o r i d e s with the f l u o r i t e structure. We then measured the absolute s i t e concentrations and from comparisons between the low and high concentration regions, we obtained the f i r s t measurements of a c t i v i t y c o e f f i c i e n t s i n a s o l i d . These c o e f f i c i e n t s are required t o describe the n o n i d e a l i t i e s . The values are much larger than one would expect from Coulombic contributions t o the a c t i v i t y c o e f f i c i e n t and the only remaining forces that would be large enough to explain the nonrandom d i s t r i b u t i o n of defects are s t r a i n f i e l d s . We also obtained values f o r the association constant and found them s i g n i f i cantly smaller than t h e o r e t i c a l estimates would i n d i c a t e . These values suggest that the t h e o r e t i c a l description does not t r e a t the i n t e r i o n interactions c o r r e c t l y . A d d i t i o n a l l y , we d i d not observe that c l u s t e r i n g had the strong dependence on i o n i c radius that was predicted from t h e o r e t i c a l estimates. These factors point t o inadequacies i n the potential models that are assumed i n the theoretical calculations. 2


2

The strong nonideality e f f e c t s have important consequences f o r the presence of superionic conductivity i n the f l u o r i t e s . A superionic conductor i s a material where one of the ion s u b l a t t i c e s undergoes p a r t i a l melting or disordering a t r e l a t i v e l y low temperatures so the material acquires a large i o n i c conductivity. PbF i s the best of the f l u o r i t e s f o r superionic conductivity and we studied t h i s material to determine how strong the nonideality e f f e c t s were ( 3 2 - 3 3 ) . We found that the e f f e c t s were much stronger i n PbF and that they also correlated with the t r a n s i t i o n t o superionic conductivity. The c o r r e l a t i o n was interpreted as a thermodynamic i n s t a b i l i t y that was caused by the large nonideality e f f e c t s . Increases i n temperature caused more d i s s o c i a t i o n of defects which led t o an increase i n the i o n i c strength of the material. The higher i o n i c strength caused more shielding and that encouraged more d i s s o c i a t i o n . There i s therefore p o s i t i v e feedback between the i o n i c strength and the degree of d i s s o c i a t i o n that can lead t o the i n s t a b i l i t y . 2

2

The other factor that could cause deviations from the convent i o n a l defect models was competing e q u i l i b r i a from F | scavenging e q u i l i b r i a . This behavior was observed i n CdF , a material that can undergo a t r a n s i t i o n from i n s u l a t i n g t o semiconducting behavior (34-35). We found that c l u s t e r i n g was f a r more extensive i n CdF than i n any of the other materials we have studied and that some of the c l u s t e r s could scavenge F[ t o form charged defect centers. The semiconducting behavior i s the r e s u l t of c r y s t a l reduction «rtiere F* are replaced by e~. This process occurs more r e a d i l y f o r the more loosely bound F[ that have been scavenged and makes the replacement possible. The importance of the scavenging i s a strong function of the dopant radius and those ions that are too large to have strong scavenging e q u i l i b r i a cannot i n fact be converted to semiconductors. 2

2



146


The r e l a t i v e s i t e importance i n a l l of the f l u o r i t e s i s a strong function of the temperature that established the equilibrium d i s t r i bution. At high temperatures, the association between the f l u o r i d e i n t e r s t i t i a l and the lanthanide would be expected to decrease as the s i t e s dissociated. S i m i l a r l y , the association of several lanthanides and t h e i r i n t e r s t i t i a l compensators into clusters would also be expected t o decrease. Experimentally, we observe that higher temperatures cause d i s s o c i a t i o n of the clusters and the larger numbers of free i n t e r s t i t i a l s that r e s u l t from the d i s s o c i a t i o n can associate with i n d i v i d u a l lanthanide ions. This observation has been used as the key to measuring the k i n e t i c s of the defect aggregation. I f high temperatures are used to d i s s o c i a t e the clusters and create a nonequilibrium d i s t r i b u t i o n , the c r y s t a l can be held at a low temperature to allow the reaggregation of the ions to form the c l u s t e r s . The rate at which the cluster formation occurs can be measured as a function of dopant concentration and temperature to obtain k i n e t i c information about the defect aggregation mechanism. KC1. There has been a long standing controversy about the nature of the defect aggregation i n a l k a l i halides. I n i t i a l experiments showed that aggregation occurred by d i r e c t formation of trimers from three monomers (36-42). Others quickly objected stating that trimers require improbable three body c o l l i s i o n s and that i t i s more l i k e l y that dimers form i n i t i a l l y (43-49). There was also a b i t t e r controversy about the nature of the defects i n KCl:Sm . One group of researchers found that the compensation could be located i n a number of d i f f e r e n t l a t t i c e positions r e l a t i v e to the Sm (50-58) while another group found that i t was only located i n a nearest neighbor position (59-61). Both of these questions were studied by s i t e s e l e c t i v e laser spectroscopy and we found that the dominant defect was a simple pair of Sm with a nearest neighbor charge compensation (62,63). Other positions of the charge compensation were not observed but there were a number of a d d i t i o n a l s i t e s formed by c l u s t e r i n g of the simple p a i r s . We also found that these clusters could be dissociated by quenching from high temperatures and that we could then watch the reformation of the c l u s t e r s by watching the s i t e d i s t r i b u t i o n evolve i n time as the c r y s t a l was annealed at low temperatures. The k i n e t i c s could be followed f o r each s i t e i n d i v i d u a l l y and the rate constant that we obtained could be determined as a function of the dopant concentration. These studies showed d e f i n i t i v e l y that the aggregation dynamics was controlled by the formation of dimers and that only at long times did the dimers aggregate t o form larger clusters and other phases (64). 2+

2+

2+

Perovskite Materials. Our studies of these materials are s t i l l i n t h e i r infancy. An important feature of the perovskites i s the a b i l i t y to form many phases through shear planes where layers of a metal oxide can be inserted between multiple perovskite structure layers. Our work has shown that probe ions can be used to watch t h i s process and we have been able to show that s i t e selective laser spectroscopy i s sensitive t o a l l of the phases with a high s e n s i t i v i t y t o even small concentrations.



7. WRIGHT Laser-Excited Rare Earth Luminescence as a Probe147 Conclusions Site selective laser spectroscopy is a very powerful tool for studying the local environments that are present in samples. We have shown that defect structures in solids are determined by its previous history and that these structures can be measured with site selective spectroscopy. There has been no application of such techniques to geologically important questions since our work has concentrated on understanding fundamental questions about solid state defect chemistry. Our work suggests though that site selective laser spec troscopy could have important application in geological studies if it were used in the hands of people with that background. Acknowledgments The work reported in this paper was supported by the Solid State Chemistry Program of the Materials Science Division of the National Science Foundation under grant DMR-8645405. Literature Cited 1. Wright, J. C. Latt. Def. and Amorph. Mat. 1985, 12, 505. 2. Hufner, S. Optical Spectra of Transparent Rare Earth Compounds; Academic Press: 1978. 3. Wright, J.C. In Modern Fluorescence; Wehry, E. L., Ed.; Plenum Press: New York, 1981. 4. Kroger, F. A. The Chemistry of Imperfect Crystals; NorthHolland: Amsterdam, 1964. 5. Porter, L. C.; Wright, J. C. J. Chem. Phys., 1982, 77, 2322. 6. Poliak, J. R.; Noon, K. R.; Wright, J. C. J. Sol. State Chem. 1989 (accepted for publication). 7. Schawlow, A. L. J. Appl. Phys. 1962, 33, 395. 8. Imbusch, G. F.; Schawlow, A. L.; May, A. D.; Sugano, S. Phys. Rev. B, 1976, 13, 1893. 9. Henry, M. O.; Larkin, J. P.; Imbusch, G. F. Phys. Rev. B, 1976, 13, 1893. 10. Boyrivent, Α.; Duval, E.; Mantagna, M.; Villani, A. G.; Pilla, Ο. J. J. Phys. C: Sol. State Phys. 1979, 12, L803. 11. Nassau, K. J. Phys. Chem. Solids, 1963, 24, 1511. 12. Tallant, D. R.; Wright, J. C. J. Chem. Phys. 1975, 63, 2075. 13. Franklin, A. D.; Marzullo, S. Proc. Brit. Ceram. Soc. 1971, 19, 135. 14. Vlasov, M. V. Sov. Phys. Crystallogr. 1975, 20, 100. 15. Voron'ko, Yu. K.; Osiko, V. V.; Schcherbakov, I. A. Sov. Phys. JETP, 1969, 29, 86. 16. Gil'fanov, F. Z.; Livanova, L. D., Orlov, M. S.; Stolov, A. L. Sov. Phys. Solid State 1970, 11, 1779. 17. Brown, M. R.; Roots, K. G., Williams, J. M.; Shand, W. Α.; Groter, C.; Kay, H. F. J. Chem. Phys. 1969, 50, 891. 18. Stott, J. P.; Crawford, J. Η., Jr. Phys. Rev. Β 1971, 4, 668. 19. O'Hare, J. M. J. Chem. Phys. 1972, 57, 3838. 20. Crawford, J. H., Jr.; Matthews, G. Ε., Jr. Semicond. Insul. 1977, 2, 213.



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21. Franklin, A. D. J. Chem. Phys. 1976, 63, 1509. 22. Franklin, A. D. Mater. Sci. Res. 1973, 6, 19. 23. Tallant, D. R.; Miller, M. P.; Wright, J. C. J. Chem. Phys. 1976, 65, 51. 24. Tallant, D. R.; Moore, D. S.; Wright, J. C. J. Chem. Phys. 1977, 67, 289. 25. Moore, D. S.; Wright, J. C. J. Chem. Phys. 1971, 74, 1626. 26. Seelbinder, M. B.; Wright, J. C. J. Chem. Phys. 1981, 75, 5070. 27. Hamers, R. J.; Wietfeldt, J. R.; Wright, J. C. J. Chem. Phys. 1982, 77, 683. 28. Moore, D. S.; Wright, J. C. Chem. Phys. Lett. 1979, 66, 173. 29. Wietfeldt, J. R.; Wright, J. C. J. Chem. Phys. 1987, 86, 400. 30. Wietfeldt, J. R.; Wright, J. C. J. Luminesc. 1984, 31/32, 263. 31. Wietfeldt, J. R.; Wright, J. C. J. Chem. Phys. 1985, 83, 4210. 32. Mho, S. I.; Wright, J. C. J. Chem. Phys. 1985, 83, 4210. 33. Weesner, F. J.; Wright, J. C. Phys. Rev. Β 1986, 33, 1372. 34. Mho, S. I.; Wright, J. C. J. Chem. Phys. 1982, 77, 1183. 35. Mho, S. I.; Wright, J. C. J. Chem. Phys. 1984, 81, 1421. 36. Cook, J. S.; Dryden, J. S. Austr. J. Phys. 1960, 13, 260. 37. Cook, J. S.; Dryden, J. S. Proc. Phys. Soc. 1962, 80, 479. 38. Dryden, J. S.; Heydon, R. G. J. Phys. C: Sol. State Phys. 1977, 10, 2333. 39. Dryden, J. S.; Heydon, R. G. J. Phys. C: Sol. State Phys. 1978, 11, 393. 40. Cook, J. S.; Dryden, J. S. J. Phys. C: Sol. State Phys. 1979, 12, 4207. 41. Cook, J. S.; Dryden, J. S. J. Phys. C: Sol. State Phys. 1981, 14, 1133. 42. Cook, J. S.; Dryden, J. S. J. de Phys. 1980, 41, C6-425. 43. Unger, S.; Perlman, M. M. Phys. Rev. Β 1972, 6, 3973. 44. Unger, S.; Perlman, M. M. Phys. Rev. Β 1973, 10, 3692. 45. Unger, S.; Perlman, M. M. Phys. Rev. Β 1975, 12, 809. 46. Crawford, J. H. J. Phys. Chem. Solids 1970, 31, 399. 47. McKeever, S. W. W.; Lilley, E. J. Phys. Chem. Solids 1982, 43, 885. 48. Lilley, E. J. de Phys. 1980, 41, C6-429. 49. Rubio, J.; Murrieta, H.; Powell, R. C.; Sibley, W. A. Phys. Rev. Β 1985, 31, 59. 50. Fong, F. K.; Wong, Ε. Y. In Optical Properties of Ions in Crystals; Crosswhite, Η. M.; Moos, H. W., Eds.; John Wiley: New York, 1967. 51. Fong, F. K.; Wong, Ε. Y. Phys. Rev. 1967, 162, 348 52. Fong, F. K.; Sundberg, M. N.; Heist, R. H.; Chilver, C. R. Phys. Rev. Β 1971, 3, 50. 53. Fong, F. K.; Heist, R. H.; Chilver, C. R., Bellows, J. C.; Ford, R. L. J. Luminesc. 1970, 2, 823. 54. Naberhuis, S. L.; Fong, F. K. J. Chem. Phys. 1972, 56, 1174. 55. Fong, F. K. Phys. Rev. 1969, 187, 1099. 56. Fong, F. K. Phys. Rev. 1970, 94, 4157. 57. Fong, F. K.; Bellows, J. C. Phys. Rev. Β 1970, 1, 4240. 58. Fong, F. K.; Bellows, J. C. Phys. Rev. B, 1970, 2, 2636. 59. Bradbury- R. E.; Wong, Ε. Y. Phys. Rev. Β 1971, 4, 690. Coyne et al.; Spectroscopic Characterization of Minerals and Their Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

7. WRIGHT Laser-Excited Rare Earth Luminescence as a Probe


60. Bradbury, R. E.; Wong, E. Y. Phys. Rev. B 1971, 4, 694. 61. Bradbury, R. E., Wong, E. Y. Phys. Rev. B 1971, 4, 702. 62. Ramponi, A. J.; Wright, J. C. J . Luminesc. 1984, 31/32, 151. 63. Ramponi, A. J.; Wright, J. C. Phys. Rev. B 1985, 31, 3965. 64. Ramponi, A. J.; Wright, J. C. Phys. Rev. B 1987, 35, 2413. RECEIVED February 22, 1989


Laser-Excited Rare Earth Luminescence as a Probe of Mineral

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