Geochemical Behavior of Disposed Radioactive Waste - American

19 Transformation Characteristics of LaV Nb O Compounds x

1-x

4

M. V. NEVITT and A. T. ALDRED

Downloaded by EAST CAROLINA UNIV on January 13, 2018 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch019

Argonne National Laboratory, Materials Science and Technology Division, Argonne, IL 60439

X-ray diffractometry measurements were made as a function of temperature on a series of polycrystaliine LaV Nb O compounds (0 < x < 0.3) to determine the effect of V substitutions on the temperature of the monoclinic/tetra gonal transformation. The purpose was to provide basic information relating to crystal lattice stability in ABO compounds that are either candidates or are appropriate models for candidate materials for hosting nuclear waste ions. Partial substitution of V on the Nb site significantly lowers the tetragonal scheelite (I4 /a) to monoclinic fergusonite (I2/c) transformation, from 770 Κ in LaNbO to approximately 215 Κ for LaV Nb O (the solubility limit is close to x = 0.35). The trans formation is displacive, of second order, involving two coupled order parameters. Heat capacity measurements on LaV Nb O showed that the specific heat anomaly at the transformation point is extremely small. It is con cluded that the two polymorphic forms of LaV Nb O have very nearly the same free energies over a substantial range of temperature below the transformation. x

1-x

4

4

5+

5+

1

4

0.25

0.75

0.25

0.75

4

4

x

1-x

4

The need f o r high chemical and p h y s i c a l s t a b i l i t y i n the sub stances that w i l l hold nuclear waste ions i n an immobilized s t a t e has stimulated extensive materials research. In support of the candidacy of the c a t i o n - l i n k e d t e t r a h e d r a l compounds having the monazite, s c h e e l i t e or z i r c o n c r y s t a l s t r u c t u r e , studies have sought t o determine the extent t o which various elements o c c u r r i n g i n the waste can occupy s t a b l e s u b s t i t u t i o n a l p o s i t i o n s on the host l a t t i c e s i t e s . These ABO4 compounds, although they have high thermodynamic s t a b i l i t y , have a pronounced tendency toward polymorphism, the r a t i o n a l e f o r which i s incompletely formulated Ç2, 3)· There i s

0097-6156/ 84/ 0246-0315S06.00/ 0 © 1984 American Chemical Society

Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

316

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

only a modest amount of i n f o r m a t i o n , p r i n c i p a l l y on B 1 V O 4 and to a l e s s e r extent on LaNbO^, concerning the t r a n s f o r m a t i o n mechanism and o r i g i n (4^-7)· The present work, the i n i t i a l phase of a b a s i c transformation and l a t t i c e dynamics study of relevant oxides, focuses on compounds i n the system LaV Nbi_ 04 (0 < χ ? 0.3). This choice i s based on the f o l l o w i n g f a c t o r s :


x

x

(1) There i s considerable e x i s t i n g i n f o r m a t i o n on the host compound, LaNbO^; (2) The s u b s t i t u t i o n of V f o r Nb provides a simple (and h o p e f u l l y interprétable) c a t i o n replacement on the t e t r a h e d r a l s i t e ; (3) P r e l i m i n a r y s t u d i e s i n t h i s Laboratory have shown that the V s u b s t i t u t i o n has a s i g n i f i c a n t e f f e c t on transformation temperature. The t r a n s f o r m a t i o n from the high temperature t e t r a g o n a l s c h e e l i t e ( I 4 i / a ) s t r u c t u r e to the low temperature monoclinic f e r g u s o n i t e (I2/c) s t r u c t u r e [770 Κ i n pure LaNb04 (2)] i n v o l v e s two coupled order parameters (a_-x) and ($-90) which describe the deformation of the base of the s c h e e l i t e u n i t c e l l from a square t o a p a r a l l e l o g r a m . Materials Preparation P o l y c r y s t a l l i n e samples having d e n s i t i e s about 80% of t h e i r t h e o r e t i c a l values were prepared from 99.9+% pure powders of 2°3> 2°5 * 2 ° 5 by r e p e t i t i v e s i n t e r i n g , g r i n d i n g , mixing and r e s i n t e r i n g of p e l l e t s at 1700 Κ i n oxygen gas at one atm pressure. The La2U^ was obtained by decomposition of lanthanum oxalate, and care was maintained t o perform a l l the p r e p a r a t i o n steps as f a r as p o s s i b l e under an i n e r t atmosphere. In some cases t h i s procedure was followed by a hot i s o s t a t i c p r e s s i n g (HIP) treatment of the s i n t e r e d and crushed p e l l e t s at the same temperature i n h e r m e t i c a l l y sealed capsules i n argon gas at 2000 atm pressure. The HIP treatment produced samples with d e n s i t i e s w i t h i n 3% of t h e o r e t i c a l values, and with an average g r a i n diameter of approximately 20 pm. As w i l l be discussed, the t r a n s f o r m a t i o n c h a r a c t e r i s t i c s of the compounds were i n f l u e n c e d by the sample p r e p a r a t i o n process. Compositions of the c o n v e n t i o n a l l y - s i n t e r e d samples were checked by scanning e l e c t r o n microscopy; the compositions of the HIP-treated samples are nominal. La

v

a n (

N b

Experimental Methods X-ray d i f f r a c t o m e t r y was performed, by methods described i n a companion paper ( 8 ) , between 80 and 600 Κ at ambient pressure on t h i n s o l i d wafers cut from the prepared samples. Mounted with high temperature cement on r e l a t i v e l y massive metal backing p l a t e s , these wafers could be maintained at tempera tures constant t o w i t h i n ± 3 K, as measured by e i t h e r a platinum thermometer or a copper-constantan thermocouple. The


19.

NEVITT AND A L D R E D

LaV Nb 0 x

lx

Compounds

4

317

widths of the (004) and (008) l i n e s , u n s p l i t by the t r a n s f o r mation, provided assurance of the chemical homogeneity of the samples; the f u l l width a t h a l f maximum (FWHM) of these r e f l e c t i o n s d i d not s i g n i f i c a n t l y exceed the FWHM of a standard m a t e r i a l and the i n s t r u m e n t a l r e s o l u t i o n , thus i n d i c a t i n g that the experimental observations could be a s s o c i a t e d with d i s c r e t e , uniform V-ion s u b s t i t u t i o n s [See a l s o ( 8 ) ] . L a t t i c e parameters were determined by a least-squares procedure based on d-spacings d e r i v e d from ten t o twenty r e f l e c t i o n s . The s p e c i f i c heat at constant pressure, c , of the HIPt r e a t e d sample with nominal composition L ^ V Q ^ 25^0^7504 was measured over the temperature range 4-400 Κ by the"heat pulse method i n a c a l o r i m e t e r that i n c o r p o r a t e s a feedback system t o regulate the temperature of c o n c e n t r i c r a d i a t i o n s h i e l d s surrounding the sample ( 9 ) . The Cp values are accurate t o w i t h i n 1%, as determined by c a l i b r a t i o n runs u s i n g a p o l y c r y s t a l l i n e copper sample and a sapphire s i n g l e c r y s t a l sample.


p

Results The upper part of Figure 1 shows the c o l l a p s e of the m o n o c l i n i c a_ and jç_ l a t t i c e parameters t o become the t e t r a g o n a l a_ parameter of LaVQ25NbQ7504 as temperature i s i n c r e a s e d . These data are t y p i c a l * o f HIP-treated samples, and the t r a n s f o r m a t i o n , which occurs at 215 ± 1 Κ f o r t h i s composition (see below), i s d i s p l a c i v e and of higher order than f i r s t . S i m i l a r r e s u l t s have been reported f o r LaNb04 (10). I f the t r a n s f o r m a t i o n can be described i n a mean-field model, we would a n t i c i p a t e that the order parameters (aj-x) and (3-90) would vary as ( T - T ) with a = 0.50 over some temperature range near T » Thus a p l o t of the square of the order parameters versus temperature would y i e l d s t r a i g h t l i n e s e x t r a p l a t i n g t o zero at T . F i g u r e 2, showing t y p i c a l p l o t s of t h i s type, i n d i c a t e s that the t r a n s formation point i s s t r o n g l y dependent on the l e v e l of V s u b s t i t u t i o n . C a r e f u l , r e p e t i t i v e measurements made with r i s i n g and f a l l i n g temperature, and a f t e r v a r y i n g times at constant temperature, show that the t r a n s f o r m a t i o n has no observable h y s t e r e s i s and no measurable i n d u c t i o n p e r i o d . The upper part of F i g u r e 3 shows the t r a n s f o r m a t i o n temperature as a f u n c t i o n of composition. By analogy with the upper part of F i g u r e 1, the lower p o r t i o n of F i g u r e 3 shows the c o l l a p s e of the roomtemperature monoclinic l a t t i c e parameters a^ and ^c as a f u n c t i o n of composition i n the LaV Nbi_ 04 system. The t r a n s f o r m a t i o n should occur at ambient temperature and pressure a t the approximate composition, LaVo 22 0.78°4 The samples prepared w i t h o u t the HIP treatment showed s u b s t a n t i a l l y the same t r a n s f o r m a t i o n behavior as the HIPt r e a t e d m a t e r i a l , w i t h one notable e x c e p t i o n : The m o n o c l i n i c and t e t r a g o n a l forms were seen t o c o e x i s t over at l e a s t a 100 Κ temperature range i n a l l vanadium-substituted samples that d i d e

e

a

t

t

t

x

x

ND

e

e



318

GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE

150

200

250

300

TOO

F i g u r e 1. L a t t i c e parameters and s p e c i f i c heat of LaV^ 25^0.75^4 as a f u n c t i o n of temperature. 0.10.

1

1

1

1

wx = 0.09

0.08 Lo Nb,_ V 0 x

x

4

0.06 _x = 0.20 θΧ=0.Ι5

x=0.25

\ .

\

—

0.185 —

"

100

X

200

lX\l

300

400

500

Τ (Κ)

F i g u r e 2. V a r i a t i o n of the square of order parameter, a-c^, w i t h temperature f o r L a V N b j _ 0 A compounds. S o l i d c i r c l e s represent data from HIP-treated samples. x

x



NEVITT A N D A L D R E D

Figure 3.

LaV Nb 0 Compounds x

lx

4

Transformation temperature as a f u n c t i o n of composition f o r L a V N b i - 0 4 compounds (upper) and v a r i a t i o n of room temperature l a t t i c e parameters w i t h composition f o r LaV Nb^_ U4 compounds. x

x

x

x



320

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

not receive a p o s t - s i n t e r HIP treatment and a l s o i n HIP-treated samples that were subsequently reground and examined i n the powdered c o n d i t i o n ( p a r t i c l e s i z e < 64 pm). Faint tetragonal r e f l e c t i o n s could be observed at low temperatures, and these r e f l e c t i o n s increased i n i n t e n s i t y with i n c r e a s i n g temperature, merging i n d i s t i n g u i s h a b l y with the c o a l e s c i n g monoclinic r e f l e c t i o n s at the transformation p o i n t . Both the i n t e n s i t i e s and the 2Θ p o s i t i o n s of these t e t r a g o n a l r e f l e c t i o n s v a r i e d smoothly and reproducibly on c y c l i n g through the transformation with no evidence of h y s t e r e s i s . A d d i t i o n a l l y , no timedependent e f f e c t s on the r e l a t i v e i n t e n s i t i e s or 2Θ p o s i t i o n s of the two phases were observed. As there i s no evidence, Xray or otherwise, of chemical inhomogeneity i n any of the samples, i t i s tempting to a s s o c i a t e t h i s anomaly with a constrained phase transformation, which has been connected to the metastable r e t e n t i o n of t e t r a g o n a l Zr02 at room temperature (11 12). It i s p o s t u l a t e d that when Zr02 grains are s u f f i c i e n t l y s m a l l , the combined e f f e c t s of surface energy and s p e c i f i c volume f a v o r the r e t e n t i o n of the high temperature t e t r a g o n a l s t r u c t u r e over the monoclinic form. The a p p l i c a b i l i t y of t h i s concept to the LaV Nbi_ 04 transformation i s under study i n t h i s Laboratory. x

x

Figure 1 shows the temperature dependence of cp f o r 0.25 0.75 4 temperature range bracketing the t r a n s formation p o i n t . There i s no d i s c e r n a b l e l a t e n t heat at the transformation p o i n t , thus confirming the X-ray observations that the order of the transformation i s higher than f i r s t . It i s a l s o evident that the s p e c i f i c heat anomaly, which accompanies a higher order transformation, i s exceedingly small, not g r e a t e r than 0.05 J/g a t . Κ at 215 K. The absence of a d i s c e r n a b l e s p e c i f i c heat anomaly at the transformation p o i n t , taken together with the observed coexistence of the two phases, i n d i c a t e s that there i s l i t t l e d i f f e r e n c e between the f r e e energies of the two polymorphic forms over a s u b s t a n t i a l range of temperature below the transformation. L a V

N b

0

o

v

e

r

a

Discussion The work reported here shows that the transformation tempera ture i n the LaV Nbi_ 04 system i s s i g n i f i c a n t l y a f f e c t e d by the simple, p a r t i a l s u b s t i t u t i o n of one 5+ i o n f o r another at the center of the tetrahedron. This e f f e c t i s connected c l o s e l y to the dynamical c h a r a c t e r i s t i c s of the c r y s t a l l a t t i c e . A study i n v o l v i n g i n e l a s t i c neutron s c a t t e r i n g , heat capacity and Raman s c a t t e r i n g i s under way i n t h i s Laboratory to c l a r i f y these aspects of the problem. With reference to the candidacy of g e n e r i c a l l y - r e l a t e d c r y s t a l l i n e m a t e r i a l s to host and immobilize nuclear waste i o n s , these r e s u l t s prompt s e v e r a l comments regarding s t a b i lity. D i s p l a c i v e transformations, as observed here, represent x

x



19.

NEVITT A N D A L D R E D

LaV Nb x

0

lx

4

Compounds

321

r e l a t i v e l y small d i s t o r t i o n a l changes i n s t r u c t u r e that are not l i k e l y t o produce p h y s i c a l l y d i s r u p t i v e e f f e c t s such as would accompany f i r s t order, r e c o n s t r u c t i v e transformations i n v o l v i n g n u c l e a t i o n and growth of another phase. This i s a p o s i t i v e c o n s i d e r a t i o n as regards long term p h y s i c a l i n t e g r i t y of the m a t e r i a l * On the other hand, the occurrence of a compositions e n s i t i v e , and p o s s i b l y s t r a i n - s e n s i t i v e , d i s p l a c i v e t r a n s formation, l i n k i n g polymorphic forms with very s m a l l d i f ferences i n s t r u c t u r e and f r e e energy, may r e f l e c t conditions favorable t o the occurrence of the metamict s t a t e * Since these conditions w i l l probably occur i n the waste storage e n v i r o n ment, where there w i l l be d i v e r s e and time-variant i o n s u b s t i t u t i o n s , the connection t o a tendency toward metamict behavior should be c l a r i f i e d . Acknowledgements The authors wish t o thank t h e i r colleagues, p a r t i c u l a r l y Drs. S.-K. Chan, J . Faber, G. S. Knapp, and D. J . Lam, and Mr. J . W. Downey f o r valuable advice, s c i e n t i f i c help and t e c h n i c a l support. Literature Cited 1.

2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12.

Schutz, Β. E.; Freeborn, W. P.; Pepin, J.; White, W. Β., in "The Scientific Basis for Nuclear Waste Management"; Topp, S.V., Ed.; North-Holland: New York, 1982; Vol. 6, pp. 155-61. Gingerich, Κ. Α.; Bair, Η. E. Advanc. X-ray Anal. 1964, 7, 22-30. Stubican, V. S. J. Am. Ceram. Soc. 1964, 47, 55-8. Pinczuk, Α.; Welber, B.; Dacol, F. H. Solid State Comm. 1979, 29, 515-18. David, W. I. F.; Glazer, A. M.; Hewat, A. W. in "Phase Transitions"; Gordon and Breach: London, 1979; Vol. 1, pp. 155-69. Wood, I. G.; Welber, B.; David, W. I. F.; Glazer, A. M. J. Appl. Cryst. 1980, 13,224-9. Wada, M.; Nakayama, Y.; Sawada, Α.; Tsunekawa, S.; Ishibashi, Y. J. Phys. Soc. Japan 1979, 47, 1575-80. Aldred, A. T., 1983, this Symposium. Trainor, R. J.; Knapp, G. S.; Brodsky, M. B.; Pokorny, G. J.; Snyder, R. B. Rev. Sci. Instrum. 1975, 46, 95-98. Brixner, L. H.; Whitney, J. F.; Zumsteg, F. C.; Jones, G. A. Mat. Res. Bul. 1977, 12, 17-24. Garvie, R. C.; Hannink, R. H.; Pascoe, R. T. Nature 1975, 258, 703-4. Lange, F. F. J. Mat. Sci. 1982, 17 225-34 (Part 1) and 23539 (Part 2).

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October 20, 1983


Geochemical Behavior of Disposed Radioactive Waste - American

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