Mat. Res. Bull. Vol. 4, pp. 323-328, 1969. in the United States.
Pergamon P r e s s , Inc.
SOLID SOLUBILITY IN THE SYSTEM ZIRCON - HAFNON* S. S. Ramakrishnan, K. V. G. K. Gokhale and E. C. Subbarao Indian Institute of Technology I~nlmr (U. P. ), India
(Received March 13, 1969; Communicated by R. C. DeVries)
ABSTRACT The silicates, zircon and hafnon were synthesised from the constituent oxides. In both cases, synthesis was essentially complete after heating for 24 hours at 1450°C. X-ray diffraction studies confirmed complete solid solubility in the system 2~SiO4-HfSiO4. Introduction Hafnium silicate (HfSiO4) , the hafnium analog of zircon (ZrSiO4) , has been svnthesised in the solid state from I-IfO2 and SlO2 (1, 2). thesis of zircon has recently been studied extensively (3-5).
Also the synThe striking
s i m i l a r i ~ in the chenical properties, crystal s y m m e t r y (tetragonal) and lattice p a r a m e t e r s of hafnium silicate to those of zircon prompted Curtis et al (1) to name it "Ha/non".
The lattice p a r a m e t e r s of zircon and ha/non are indeed
very close (2), and a study of the solid solutions in the binary, system zirconha/non would t h e r e f o r e be interesting.
Even though the presence of hafnium
in most zircon m i n e r a l s has been reported (8, 7), the form in which it may be p r e s e n t (as a separate phase or in solid solution) has not been thoroughly investigated.
Khalezova and Chernitsova (8) have predicted isomorphous substi-
tution of Zr 4+ ions in zircon by Hf4+ ions up to 10~.
Their findings a r e based
on the observation of a gradual change in the x-ray diffraction intensity of the (301) line in natural zircons.
A study of this system could be of great value
*Work supported by the Department of Atomic Energy, Government of India. 323
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because these silicates are useful as space-age r e f r a c t o r i e s and as single crystals (9) for solid state applications. Experimental The raw m a t e r i a l s used were r e a c t o r grade monoclinic ZrO 2 and HfO2 (obtained from Wah Chang Corporation, Albany, Oregon, U . S . A . ) and quartz of 99.996 purity (from Pennsylvania Glass Sand Corporation, U.S.A. ).
quired amounts of ZrO2, HfO2, and quartz were taken to yield compositions in the HfSiO4-ZrSiO 4 system at intervals of 20 mole percent.
The mixtures of
these oxides were ground well in an agate m o r t a r to a size of about -20 microns. Using 596 dextrin as a binder, pellets of 0. 5 inches d i a m e t e r were p r e s s e d at 32,000 p. s . i .
These were heated in platinum crucibles from room t e m p e r a t u r e
to 1450 + 10°C in a "Globar" furnace with a i r atmosphere.
The peak t e m p e r a -
ture was maintained for 24 hours, using an " E l e c t r o m a x " on-off controller with a Pt - Pt 10 Rh thermocouple.
The samples were then quenched in air.
Room temperature x-ray patterns of the crushed samples were obtained with a General Electric XRD-6 dtffractometer employing filtered copper K alpha radiation.
The 2 8 values were d e t e r m i n e d accurately, with a scanning speed
of 0. 2 ° per minute, and were confirmed b~ step scanning at 0. 02 ° intervals. The x-ray data were indexed using published data on zircon-(10) and hafnon (2). The tetragonal lattice p a r a m e t e r s (a and c) were d e t e r m i n e d from the interplanar spacings of (400), (420), (404), (600), (116), (316) and (552) sets of planes.
Least square analysis was c a r r i e d out, using an IBM 7044 computer;
the results are given in Table 1 and Figure 1. Results and Discussion In all the specimens e~amined, the strongest reflections of ZrO2, HfO2 and cristobalite were present only as very weak lines compared to the intensity of these lines in the pure oxides (less than 596) indicating that the s~nthests of ZrSiO 4 and HfStO 4 were essentially complete even at 1450°C.
This t e m -
perature, 1450°C, is 100°C less than the value reported e a r l i e r for hafnon synthesis (1, 2).
In the case of zircon synthesis, Curtis et al (3) found about
10 - 2096 of the starting m a t e r i a l (ZrO2) as free ZrO 2 after heating for 8 hours at 1537°C.
Ramani et al (5) reported the formation of about 4096 zircon after
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ZIRCON- HA FNON SYSTEM
TABLE 1. Lattice p a r a m e t e r s and i n t e n s i ~ ratios in the system ZrSiO4-HfSiO 4. Integrated Intensitv Ratios
Lattice P a r a m e t e r s (A) Mole % HfSiO4
5. 981 (5.979) (a) 5.980 5.978 5.974 5. 791 5.9.67 (5. 964) (b)
(6. 604) (a) 6. 596 6.593 6. 585 6. 578 6. 569 (6. 573) (b)
20 40 60 80 100
I211/I200 I101/I200 12
16 23 26 32 34
38 42 48 52 56
(a) From Reference (10) (b) From Reference (2)
5'7 I 5.96 I 0
Mole % HfSiO 4
FIG. 1 Variation of lattice p a r a m e t e r s with composition in the system ZrSiO4-HfSiO 4. heating the (ZrO 2 + SiO 2) mixture at 1400°C for 8 hours.
The nearly complete
syntheses of ZrStO 4 and HfSiO4 from the constituent oxides in the present study could be due to the extremely fine (-20 microns) particle size of the raw mat e r i a l s , the thorough mixing, and the longer time of sintering.
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Absence of an eutectic in the binary system ZrSiO 4-HfSIO 4 up to 1450°C was indicated by the fact that the degree of slngering was not visibly affected by. varying the composition from I00~ (ZrO 2 + SiO2) to 100~ (HfO 2 +
sio2). Complete mutual solid solubility in the system ZrSiO4-HfSIO 4 was inferred from the following observations: (1) A gradual decrease in the tetragonal lattice parameters (a_and c) was observed with increasing additions of (HfO 2 + StO 2) to (ZrO 2 + SIO2) (Table 1 and Figure 1). This decrease could be attributed to the substitutional replacement of the Zr tons of ionic radius 0.81~ (2) in the zircon lattice by. the slightly smaller Hf tons (of ionic radius 0. 80A). (It) Only one set of peaks were observed at all the (h k I) reflections scanned, and these could not be split or resolved further even under the best conditions of slit width, scanning speed, chart speed, etc. (iil)The ratios of the integrated intensities of selected reflections, to the intensity,of the strongest reflection (200), showed a gradual, almost linear, variation with composition (Table 1). This gradual variation could be due to the larger scattering factor of the hafnium ions which have substitutionaUv replaced the zirconium ions in the zircon lattice. Acknowledgements The authors wish to thank Dr. R. N. Pattl for helpful discussions. References
I. C. E. Curtis, L. M. Doney and J. R. Johnson, J. Am. Ceram. Soc. 3__7, 458 (1954). 2. D. J. Salt and G. Hornung, J. Am. Ceram. Soc. 50, 549 (1967). 3. C. E. Curtis and J. G. Sowman, J. Am. Ceram. Soc. 36, 190 (1953). 4. K. V. G. K. Gokhale, S. V. Ramant and E. C. Subbarao, J. Materials Science (in press). 5. S. V. Ramant, E. C. Subbarao and K. V. G. K. Gokhale (to be published). 6. O. Van Knorring and G. Hornung, Nature 190, 1098 (1961). 7. V. V. LyakhovRch and I. D. Shevalevvskit, Geochemistry 5, 508 (1962).
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8. E. B. Khalezova and N. M. Chernitsova, Dokl Akad. Nauk S. S. S. R. 180, 195 (1968). 9. A. A. Ballman and R. A. Laudise, J. Am. Ceram. Soc. 48, 130 (1965). 10. H. E. Swanson, I~ K. Fuyat and G. M. Ugrenic, Standard X-ray Powder Diffraction Patterns, VoL 4, Nat. Bur. Std. (U. S. ), Circ. No. 539, 68