Preparation and X-ray characterization of pollucite (CsAlSi2O6)

.L inorg, nucL Chem. Vol. 43, pp. 1773-1777, 1981 Printed in Great Britain.

0022-1902/81/081773--05502.00/0 Pergamon Press Ltd.

PREPARATION AND X-RAY CHARACTERIZATION OF

POLLUCITE (CsA1Si206) SARAH ANN GALLAGHER*I"and GREGORYJ. McCARTHY:~ Materials Research Laboratory, University Park, PA 16802, U.S.A.

(Received 22 August 1980; receivedfor publication 16 October 1980) Abstraet--A systematic study of the preparation of synthetic pollucite (CsA1Si206)by high temperature solid state reaction was performed. Eighteen different combinations of cesium oxide, alumina and silica sources and three firing temperatures were evaluated. The most effective methods of preparing phase-pure pollucite were identified and the chemical compositions of large batches of pollucite prepared by three of these methods were obtained. Reference X-ray powder diffraction data were obtained for the most nearly stoichiometric synthetic pollucite specimen.

INTRODUCTION

Pollucite (CsAISi206) has been receiving a great deal of attention recently as a radiocesium-host phase for nuclear materials applications. One use of such materials is isotope-sources for such applications as spectrometric calibration, endocurie therapy, teletherapy, sterilization, or isotopic generation[l]. At Sandia Laboratory, pollucite and other aluminosilicate phases containing t37Cs are being studies as a means of sterilizing sewage sludge before it is fed to livestock[2, 3]. Another important use of pollucite is for radioactive waste management applications. Pollucite has been proposed as the cesium-fixation phase in glass-ceramic and in all current ceramic nuclear waste forms ("supercalcine-ceramic", titanate ceramic and cermet) except "SYNROC" [4]. Pollucite may be important even if glass is chosen as the nuclear waste form. Here, the cesium which volatilizes from the glass melt could be converted to pollucite by reaction with "aluminosilicate clay" filters maintained at 600°C[5-8]. Pollucite is also" being evaluated as a material for the immobilization of megacurieamounts of 137Cs separated from nuclear wastes at the Hanford site in Richland, Washington. A cesium-host phase for nuclear materials applications should be refractory to at least 1000°C, have a low leachability in water and other aqueous solutions and have a high cesium content. In order to evaluate the relevant properties of pollucite for these applications, it

is necessary to have available phase-pure and in some cases, chemically-pure specimens. Impure and hydrated natural pollucite would generally be unsatisfactory. Therefore, a systematic study of the preparation Of pollucite by the routine solid state reaction technique was performed using various cesium, aluminum and silicon oxide sources and a typical set of firing conditions. Single crystals of pollucite may also be required for some applications or measurements. Thus, the ease of preparing pollucite single crystals by hydrothermal synthesis was evaluated. Because there are significant differences between the relative intensities of the X-ray reflections of naturally-occurring pollucite and synthetic pollucite, X-ray powder diffraction data for a chemicallyanalyzed synthetic pollucite were obtained. EXPERIMENTAL

For each of the six preparation methods, a different alumina and silica source was mixed with each of the cesium oxide sources (reagent grade CsNO3, Cs2CO3and CsOH) in the correct proportions to form CsAISi206. The first method approximated standard "ceramic" practice. The alumina and silica sources were a-alumina§ and a-quartz If. For the "amorphous" preparation, amorphous aluminas and amorphous silicatf were used. In the "partial gel" preparation, the alumina-silica source was a desiccated gel with a molar ratio of IAI203:4SIO2necessary for forming CsAISi206.The gel was prepared as described in Ref. [9] from a 1.0 M AI(NO3)3'9H20solution and a 2.0 M Ludox-AS:~: solution. A standard gel preparation again described in Ref. [9] was used for the "complete gel" method. For this preparation, the cesium o~ide sources were 1.0M solutions of CsNO3, CsOH *Author to whom correspondence should be addressed. and Cs2CO3.The alumina and silica sources were the same as for tPresent address: Rockwell Hanford Operations, Energy Sys- the "partial gel" method. The set gel was dried before receiving tems Group, MO-037 Bldg., 200 West Area, Richland, WA 99352, further heat treatment. For the "decomposition" method, the U.S.A. various cesium oxide, alumina and silica solutions previously :~Present address: Department of Chemistry and Geology, described were mixed together and dried on a stirrer-hotplate North Dakota State University, Fargo, ND 58105, U.S.A. before receiving heat treatment. The alumina-silica source for §The c~-aluminawas prepared by heating AI(NO3h'9H20 at the "clay" preparation was a mixed sodium and calcium mont1050°C overnight. morilloniteclay "Gelwhite L".§§ nThea-quartz was suppliedby General Electric Co., Willoughby, The mixed powders, desiccated gels and dried solutions were Ohio. prepared from the various cesium oxide, alumina and silica $1"he non-crystalline alumina was prepared by decomposing sources, ground, and pressed into 0.635-cm pellets of approx. AI(NO3)3'9H20at 800°C overnight. 0.25 geach at a pressure of 103,000psi. The pellets were fired at ftThe non-crystallinesilica was IMSIL A-2 supplied by Illinois 600"C overnight to decompose the cesium oxide source material Minerals Co., Cairo, Illinois. and to begin the reactions. Additional heat treatments of 850, :~:~Ludox-ASis an ammonia stabilized colloidal suspension of 1050, ll00 and 120&C for 18 hr each were used to complete the silica supplied by E. I. du Pont de Nemours and Co., Inc., reaction and to further crystallize the samples. The 600 and Wilmington,Delaware. 850°C firings were performed in a nichrome wound resistance §§"Gelwhite L" was supplied by Georgia Kaolin, Elizabeth, furnace, and the 1050, 1100and 1200°Cfirings were performed in New Jersey. a SiC-elementmuffle-typefurnace. After each heat treatment, the 1773

1774

S. A. GALLAGHERand G. J. McCARTHY

pellets were ground, and X-ray diffractograms of the products taken to mean that all reflections in the X-ray diffracwere obtained. The powders were then repelletized and fired at tograms, obtained with the instrument set for very high the next temperature. sensitivity, could be attributed to pollucite and that there Large batches (50-100g) of pollucite were prepared from were no superimposed broad maxima in the background "complete gel", "decomposition"and "clay" preparations. For attributable to diffuse scattering from one or more the "complete gel" preparation the firing temperatures were 600, amorphous phases. However, additional phases could 780, 975, 1050and 1200°Cfor approx. 23 hr at each temperature. The firing temperatures for the "decomposition" preparation have been present in quantities which were below the were 600, 850, 940, 1050 and 1200°Cfor approx. 23 hr at each level of detection (typically 1-3 wt%). Small amounts of temperature. The "clay" preparation was fired at 600°Cfor 2 hr, non-isotropic phases were observed by optical micros800°C for 6 hr and 1200°Cfor 2 hr. After the final firing, these copy in several of the successful pollucite preparations. pollucite specimens were washed with distilled water to remove Many of the products of the 850°C firing contained any excess cesium or other soluble impurities. pollucite, but none were phase-pure. Lack of phase The pollucite phases were identified by comparison to the purity as well as the presence of some of the starting JCPDS X-ray powder data for natural pollucite, PDF #15-317 material indicated incomplete reaction. Phase-pure poland #25-194.The CsAISiO4phase was identified using the X-ray lucite was first observed after the 1050°C firing. The diffraction data of Gallagher et a/.[10] and CsAISisOj2from data of Ito [11]. Wet chemicalanalyses were performed in the Mineral phase or phases observed in the X-ray diffractograms of Constitution Laboratory of the Pennsylvania State University. the products of the 18 mixtures fired at 1050, I100 and X-Ray diffraction data were collected on a Siemens diffrac- 1200°C are listed in Table 1. After the 1050°C firing, all of the preparations except tometert usingCuK~radiation,a diffractedbeammonochro-mator, a scintillation detector, and solid state electronics at a one contained some pollucite. A number of preparations scan rate of 1/2° 20/min. The data were corrected by the use of also contained other phases such as CsAISisO~2, CsA1the NBS SRM-640 silicon internal standard (ao = 5.,13088,~). A Si04 and/or amorphous material. Four of the preparacomputer program described by Evans et a/.[12] was employed tions yielded phase-pure pollucite--the "complete gel" for indexingand least squares unit cell refinement. The computer and the "clay" preparations using CsNO3, the "decomprogram of Clark et a/.[13] was used to calculate an X-ray position" preparations using CsOH, and the "complete powder diffractionpattern for poUucite. gel" preparations using Cs2C03. Because of the presence of unreacted (amorphous) RESULTS AND DISCUSSION material and/or extraneous cesium aluminosilicates, all Pollucite preparation the preparations were reground and retired at 1100 and For the pollucite preparation study, the cesium, alu- 1200°C. After the ll00°C firing, only one preparation still minum and silicon oxide sources and the firing tem- contained an amorphous phase evident in the X-ray peratures were chosen for specific reasons. The three diffractograms. In many preparations, the intensity of the readily available cesium oxide sources were chosen in pollucite reflections increased while the CsAISiO4 order to determine whether or not possible differences in reflections decreased in intensity or disappeared. Four of the decomposition rates of these sources could effect the the preparations yielded phase-pure pollucite and four stabilization of cesium in the products of the prefirings. others contained only a trace of additional CsAISiO4 or The aluminum and silicon sources were selected so that a CsAISisO~2. After the additional 1200°C firing, seven wide range of homogeneity and reactivity of the starting preparations yielded phase-pure pollucite and three materials could be investigated. Homogeneities ranged others had only a trace of an impurity phase. from the colloidal-scale of 100-200 A for the "complete There are several reasons for the appearance of adgel" preparation to the micron-scale of 10-50/xm for the ditional phases in the diffractograms. The most apparent "ceramic", the "clay" and the "amorphous" prepara- reason is the volatilization of a cesium-rich species early tions. It would be expected that the least reactivity in the firing sequence. This volatilization resulted in the would be shown by the "ceramic" preparation, and the formation of the cesium-poor phase CsA1SisOI2 or the most reactivity by the "complete gel" preparation. cristobalite form of Si02. Volatilization occurred most The mixture of powders, dried gels or dried solutions frequently when the alumina-silica sources were the required a heat treatment which would decompose the simple mixtures of crystalline or amorphous A1203 and nitrates, hydroxides and carbonates of cesium and in- SIP2. Gallagher [16] has shown that, over the temperature itiate reaction without causing excess cesium volatiliza- range 1200-1400°C, the subtraction of Cs20 from the tion. Therefore, a temperature of 600°C which is close to CsAISi206 bulk composition moves the composition into the decomposition temperature of both CsNO3 and the three phase assemblage CsAISi206+CsA1SisO~2+ A16Si20~3 (mullite). The frequent appearance and perCs2CO3 was chosen. It had also been previously established that an 850°C sistence of CsAISi50,2 in the products is consistent with prefiring will promote reaction and is sufficient to fix this mechanism. Mullite was not observed in any of the cesium in a particular phase[14,15]. For this reason, product diffractograms, because it would only be present each of the 18 samples received this 850°C firing. A in relatively small amounts and because its strong temperature of 1050°C was originally chosen to be high reflections are masked by cesium aluminosilicate enough to yield highly crystalline phase-pure specimens reflections. The formation of the CsAISisOI2+SiO2 and low enough to minimize any cesium volitalization. assemblage indicates loss of aluminum also, perhaps However, two additional firing temperatures of 1100 and combined with cesium as a CsAIO2 vapor species. 1200°C were included when it was found that a majority Some CsAISiO4 reflections were observed in the of the preparations still contained some amorphous diffractograms of many of the batches fired at lower material and/or other cesium aluminosilicates. temperatures, but the reflections became less intense or For the purposes of this study, "phase-pure" was disappeared in subsequent higher temperature firings. This is evidence for early formation of a metastable CsAISiO4 structure phase that slowly converts to more tSiemens Corp, Iselin, New Jersey. stable CsA1Si206 with further firing. Note that this phase,

1775

Preparation and characterizationof pollucite Table 1. Summaryof the X-ray diffraction analyses for the study of methodsfor preparingpollucite Preparation llethod/Alumina, Silica Source Firing Temperature

Cesium Oxide Source

Ceramic a-A12% a-Si%

Amorphous amorphous AI203 amorphous SiO2

Partial Gel Complete Gel Decomposition Clay 1:4 1:1:4 Al(NO3)3"9H20 Gelwhite L A12%:Si02 Cs~O:AI203:SiO; Ludox AI(N03)3'91;20 gel gel

1050°C

m-CsAlSisO]2 m-CsAlSi2% amorphous

m-CsAISisOl2 s-CsAISi2% m-CsAISiO~ m-CsAISiO,+ amorphous

s-CsAISi206 s-CsAISi20s w-CsAISiO~

m-CsAISi20~ m-CsAISi5012 amorphous

m-CsAISi206 s-CsAISi2O6 m-CsAISis012 w-CsAISiO4 amorphous

s-CsAISi20s s-CsAISi206 s-CsAISi206 amorphous tr-CsAISiO,.

Cs2C03

w-CsAISi20s amorphous

m-CsAISi2O~ s-CsAISi20s w-CsAISisOl2 m-CsAISiO~ w-CsAISiO~ amorphous

s-CsAISi206 s-CsAISi20s tr-CsAISi04

s-CsAISi20G tr-CsAISi04

CsNO3

s-CsAISis%2 w-CsAISisOl2 s-CsAISiz06 m-Cristobalite w-Cristobalite w-CsAISiO~

s-CsAISi2% s-CsAISi20s w-CsAISisOl2 tr-CsAlSiO~

s-CsAISi206

CsOH

s-CsAISisO]2 s-CsAlSi206 s-CsAISi206 w-CsAISi206 m-CsAISIsO~2 tr-CsAISiO, tr-CsAISiO.

s-CsAISi206 s-CsAISi2% s-CsAlSi20G m-CsAISisOl2 tr-CsAISi5012 tr-CsAlSiO,,

Cs2C03

m-CsAISisO]2 s-CsAISisOI2 s-CsAISi206 w-Cristobalite m-CsAISi2OG w-CsAISiO. amorphous

s-CsAISi206 s-CsAISi206 s-CsAISi2O6

CsNO3

s-CsAISis012 s-CsAISisOt2 s-CsAISiz% m-Cristobalite w-Cristobalit( tr-CsAISiO~

s-CsAISi206 s-CsAISi20s w-CsAISisOl2

CsOH

s-CsAlSis012 m-CsAISi~06 :r-Cristobalite

s-CsAISi20G s-CsAISi20s m-CsAISisOl2

s-CsAISi20G m-CsAISisOl2

s-CsAlSi2% s-CsAISi20~ tr-CsAISisOt:

Cs2C03

s-CsAISisOl2 amorphous

s-CsAISisO~2 s-CsAISi2% m-CsAISiz% tr-CsAISiOw

s-CsAISi2O s

s-CsAlSi.%

CsN03

CsOH

s-CsAISi20G

llO0°C

1200°C s-CsAISi2%

s-CsAISi~%

Relative Intensity of the Reflections for the Phase in the X-ray s = strong; m = moderate; w = weak; tr = trace

while isostructural with CsAISiO4, may have a composition closer to CsAISi206. As evident from the data in Table 1, any of the cesium sources can be used in pollucite preparation if certain alumina-silica sources are also used. However, CsOH and Cs2CO3 are generally better, probably because they decompose at lower temperatures than CsNO3. Cesium would, therefore, be available for reaction at temperatures at which its species have lower vapor pressures. Another important factor in the preparation of pollucite was the provision of a well-mixed alumina-silica source. The alumina-silica sources that gave the best results would have been mixed on at least the colloidal scale. Phase-pure pollucite was produced most consistently from the "decomposition" and the "clay" preparations. The uniform success of the "clay" preparation method at 1200°C may have resulted from a topochemical reaction in which the alumina-silica source present as a sheet

silicate reacted with the cesium oxide source to produce the framework silicate pollucite.

Chemical analyses The results of the chemical analyses of the large batches of pollucite prepared by the "complete gel", the "decomposition" and the "clay" preparations using CsNO3 as the cesium source and a final firing temperature of 1200°C, along with the theoretical composition of pollucite, are given in Table 2. Only traces of impurities were found to be present in the pollucite prepared from the "complete gel" and the "decomposition" preparations. After normalization of the composition to 100% (Cs20 + A1203 + SiO2), the formulae for these pollucites based on six oxygen atoms per formula unit were Cso.90Alo.9sSi2.0406 and Cso.96Alo.96Si2.0406, respectively. As expected, the pollucite synthesized from the "clay" preparation contained a greater percentage of impurities, assuming that these impurities substitute in the pollucite structure and are not present as undetected

1776

S.A. GALLAGHERand G. J. McCARTHY Table 2. Chemicalanalyses of pollucite specimens i

Pollucite *

I

("complete gel")

Pollucite *

Pollucite *

Theoretical

("clay")

(CsAlSi206

I ("decomposition") Weight Percent

42.11

44.11

41.21

Al203

16.72

15.99

12.75

16.3

5i02

41.12

39.82

39.34

38.5

Fe203

<0.02

<0.02

0.49

~gO

<0.02

<0.02

2.21

CaO

<0.02

<0.02

1.64

Na20

<0.02

<0.02

1.19

Cs20

45.2

* K20 and Rb20 were each less than O.l wt% and are included in the Cs20 value.

Table 3. X-Raypowder diffractiondata for pollucite [Cubic, Space Group la3d, Z=16] ao

13.677(I)X Peak

Integrated hkl

dobs

dcalc

lobs 6

calc

lobs

Icalc

9

13 2

211

5.58

5.58

220

4.84

4.84


2


321

3.66

3.66

46

55

49

54

400

3.42

3.42

lO0

lO0

lO0

I00

420

3.06

3.06

3

2

4

2

332

2.914

2.916

44

49

45

47

431

2.684

2.682

3

4

4

4

521

2.499

2.497

9

II

9

I0

440 611,532 631

12

2.419

2.418

29

33

29

29

2.220

2.219

16

20

18

18

2.018

2.017

7

lO

7

8

5

4

8

444

1.975

1.974

3

940

1.897

1.897


2


1

721,552

1.862

1.861

17

22

16

18

651,732

1.737

1.737

18

24

16

19

800

1.709

1.710

5

7

5

5

741

1.684

1.684


2


1

653

1.635

1.635

4

2

3

3

752

1.549

1.549

4

6

4

4

840

1.529

1.529

6

lO

5

7

(by X-ray diffraction analysis) minor phases, the complete formula, normalized as above and calculated on the basis of six oxygen atoms per formula unit would be (Cso.9oNao.12Cao.ogMgo.17)(Alo.77Feo.o2)Si2.o206. Cerny[17, 18] has found that sodium, calcium, magnesium and iron often occur as impurities in natural

pollucite. He has also reported an extensive solid solution range for analcime (NaA1SizO6.H20) in pollucite[17, 18]. Preparation of an iron analog of pollucite has also been reported[19, 20]. However, it is still possible that both calcium and magnesium are present as minor phases which could not be detected.

1777

Preparation and characterization of pollucite All three pollucite specimens were rich in silicon and poor in cesium and aluminum as compared to theoretical composition. These specimens apparently lost some cesium oxide and/or some sort of cesium-aluminum vapor species, possible CsA102, during the 36 hr of the 1100 and 1200°C preparation firings. Deviation from the theoretical composition is, however, consistent with the stoichiometry of naturallyoccurring pollucite which for each six oxygen atoms typically has 0.90 univalent cations and an aluminum and silicon content of A1o.9oSi2.o2[17]. X-Ray powder diffraction

The lattice parameters for the pollucite specimens prepared from the "complete gel", the "decomposition" and the "clay" preparations were ao = 13.683(1)A, ao = 13.677(1)/~ and ao= 13.663(2)~, respectively. The cell parameters increased as the aluminum to silicon ratio increased as is typical when a larger ion such as AP ÷ is substituted for a smaller ion such as SP +. ,The complete observed and calculated X-ray powder data for the pollucite specimen prepared by the "decomposition" method are given in Table 3. The composition of this specimen was closest to the ideal stoichiometry for pollucite. The R-factor for the observed and calculated structure factors was 0.16 which compares favorably with Newnham's R-factor of 0.13 for the complete structure analysis of natural pollucite[21]. CONCLUSIONS Pollucite was formed in a majority of the solid state preparations studied. The most effective methods of synthesizing phase-pure pollucite were found to be the "complete gel", the "decomposition" and the "clay" preparations. When the appropriate firing temperatures are employed, all of the cesium oxide sources (CsOH, Cs2CO 3 and CsNO3) will produce phase-pure pollucite by these methods. The most straightforward method is the "clay" preparation. The resulting pollucite phase, however, will be somewhat impure depending on the impurity content of the clay. Montmorillonite-type clays with the correct atomic ratio of 1AI:2Si necessary to form pollucite are preferred. In both the "decomposition" and the "complete gel" preparations, the impurity content of the pollucite can be regulated by using reactants of known purity. Acknowledgements--Support was provided by the Department

of Energy through the Battelle Pacific Northwest Laboratories. N. H. Shur and J. B. Bodkin performed the chemical analyses. D.

E. Pfoertsch provided advice on experimental procedures, and D. K. Smith participated in the calculation of the X-ray powder data. REFERENCES

1. G. Langlet, Synthesis and crystallographic study of compounds of the system Cs20-AI203-SiO2.Ph.D. Thesis, University of Paris, Paris, France (1969). 2. J. K. Johnstone and B. T. Kenna, Am. Ceram. Soc. Bull. 56, 342 (1977). 3. R. L. Schwoebel and J. K. Johnstone, In Ceramic and Glass Radioactive Waste Forms (Edited by D. W. Ready and R. C. Cooley), p. 101. ERDA-CONF-770102(1977). 4. G. J. McCarthy, Crystalline and coated high-level forms. In Proc. Conf. High-Level Radioactive Solid Waste Forms

(Edited by L. A. Lasey), p. 630. 19-21 December, 1978 at Denver, CO, NUREG/CP-0005, (1978). 5. J. Mukerji and P. B. Kayal, J. Am. Ceram. Soc. 57, 229 (1974). 6. J. Mukerji and P. B. Kayal, J. Sci. Ind. Res. 34, 457 (1975). 7. J. Mukerji and P. B. Kayal, Mater. Res. Ball. 10, 1067(1975). 8. P. B. Kayal, Reaction of alkalimetal nitrates (RbNO3, CsNO3) with silica and aluminosilicates. Ph.D. Thesis, University of Calcutta, Calcutta, India (1976). 9. G. J. McCarthy and R. Roy, J. Am. Ceram. Soc. 54, 639 (1971). 10. S. A. Gallagher, G. J. McCarthy and D. K. Smith, Mater. Res. Bull. 12, 1183 (1977). 11. J. Ito, Am. Miner. 61, 170 (1976). 12. H. T. Evans, D. E. Applemen and D. S. Hanwerker, The least squares refinement of crystal unit cell with powder diffraction data by an automatic computer indexing method (Abs.). Program, Ann. Mtg. Am. Cryst. Assoc., Cambridge, Mass. 42--43,28 March (1963). 13. C. M. Clark, D. K. Smith and G. G. Johnson, A Fortran IV program for calculating X-ray powder diffraction patterns-Version 5. College of Earth and Mineral Sciences. The Pennsylvania State University (Sept. 1973). 14. G. J. McCarthy and D. Pfoertsch, Unpublished. 15. G. J. McCarthy, Advanced waste forms research and development, Ann. Rep. COO-2510-5, Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania (1975). 16. S. A. Gallagher, Cesium aluminosilicates tor nuclear materials applications. Ph.D. Thesis, The Pennsylvania State University, University Park, Pennsylvania (1979). 17. P. Cerny, Can. Miner. 12, 334 (1974). 18. P. Cerny, Pollucite and its alteration in geological occurrences and in deep burial radioactive waste disposal. In Scientific Basis for Nuclear Waste Management (Edited by G. J. McCarthy), p. 231. Plenum Press, New York (1979). 19. C. Kopp, L. A. Harris, G. W. Clark and H. L. Yakel, Am. Miner. 48, 100 (1973). 20. S. Kume and M. Koizumi, Am. Miner. 50, 589 (1965). 21. R. E. Newnham, Am. Miner. 52, 1575 (1%7).