J O U R N A L OF M A T E R I A L S SCIENCE LETTERS 10 (1991) 352-000354

Discrepancies in the crystal structures assigned to precipitated zirconia

RAM SRINIVASAN*, S. F. SIMPSON:I:, J. M. HARRIS:I:, B. H. DAVIS* *Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, KY 40511-8433, USA tDepartment of Chemistry, University of Utah, Salt Lake City, UT 84112, USA

Zirconia exists in three well-known polymorphic forms: monoclinic, tetragonal and cubic structures [1, 2], besides the one recently reported as a high-pressure orthorhombic phase [3]. Whereas the monoclinic phase is quite stable at low temperatures, the tetragonal and cubic phases are high tem- perature (> 1000 °C) stable phases. However, these latter polymorphs can be made stable or metastable even at room temperature by manipulating the preparation procedures or by the addition of rare- earth oxides in appropriate proportions. Apart from the usage of additives as stabilizers, it was reported by Garvie [4, 5] that the tetragonal phase can be stabilized at temperatures well below 500 °C if the crystallite size was < 30 nm. Other data were not consistent with size as the stabilizing factor [6-10]. The stabilization of the tetragonal phase in zirconia at low temperatures has been attributed to chemical factors involved in the precipitation methods [11-13]. It was reported that the zirconia precipit- ated at a pH of 13.5 exhibited tetragonal symmetry. However, recent papers by Benedetti et al. [14, 15] have evoked interesting arguments about the assign- ment of crystal symmetries to the zirconia precipit- ated at a pH of 13.5. These authors [14, 15] indicate that 3 wt % Na incorporated in the precipitation step stabilizes the cubic form. In this letter the questions raised by Benedetti et al. [14, 15] are addressed.

A zirconia solution (0.3 M) was prepared using anhydrous ZrCI4. Precipitation was effected at a pH of 13.5 by rapidly adding excess 4 M NaOH. The gels were thoroughly stirred. Sample A was washed three times and sample B was washed exhaustively (about eight times). Sample A contained 3 wt % Na, whereas sample B had < 0 . 5 w t % Na, as deter- mined by atomic absorption analysis. The two samples were calcined at 500 °C for 4 h.

The X-ray diffractometer (XRD), the Raman spectrometer and the experimental conditions used to collect Raman spectra are described in detail elsewhere [16].

Clearfield [6] found that by refluxing zirconyl chloride in NHeOH, monoclinic hydrous zirconia could be obtained; refluxing in a 20% NaOH solution resulted in the formation of tetragonal hydrous zirconia. Katz [17], however, reported that the precipitate formed from a zirconyl chloride solution with NaOH addition led to a metastable cubic phase when the hydrous oxide was heated at 285 °C; at 300 °C the splitting of some of the cubic lines indicated the onset of a c ~ t transformation

352

and at 320 °C the monoclinic form began to appear together with the tetragonal form. These types of transformations in such a small temperature region could be easily detected by thermal analysis, such as differential thermal analysis or differential scanning calorimetry; Katz [17], however, did not provide supporting evidence for his suggestions. Similarly, Clearfield [6] also provided only XRD evidence for the existence of the metastable cubic phase at low temperatures. Hence, the verification of crystal phase by Clearfield [6] and Katz [17] is open to question.

First of all, the particles produced by chemical precipitation and calcined at < 500 °C are extremely small, and fall in the sub-nanometre range. Hence, X-ray line broadening makes determinations of peak position a demanding task. Slight variations in the lattice constants (a = b ~ c) of tetragonal zirconia give rise to characteristic tetragonal line splittings; this feature is absent in the cubic structure. The distinct feature between cubic and tetragonal struc- tures is that the tetragonal phase exhibits a limited number of additional higher-order, low-intensity XRD reflections; these are due to its lower degree of symmetry. Typical tetragonal line splittings are (0 02) and (2 0 0), (0 0 4) and (4 0 0), and (0 0 6) and (6 0 0). For ultrafine sub-nanometre particles, the splitting of (0 0 2) and (2 0 0) peaks is not clearly discernible; therefore, the assignment of the struc- ture based on this splitting must be viewed with caution. The (00 4) and (40 0) line splittings, however, occur at higher 20 angles (72-76 ° ) and this splitting may be identified. Frequently, however, these peaks overlap to such an extent that the result is a single peak which exhibits asymmetry. This is exactly the case with the investigations of Benedetti et al. [14, 15], wherein the asymmetry of the (400)c profile can be clearly seen (see fig. 4 in [15]).

The XRD traces of sample A prepared in this study and containing 3 wt % Na are shown in Fig. 1. In this figure the asymmetry of the (004) and (400) tetragonal lines can be seen due to the overlap of these two lines. Caution must be exercised in assigning a crystal phase to the material represented by this trace. The (006) and (600) peaks are separated by 2 °, whereas the (004) and (400) peaks are separated by about 0.7°; hence, separate peaks are sometimes difficult to observe. Based on our XRD data for the (004)t and (400) t peaks, we may have assigned our material a cubic phase just as Benedetti et al. did in their studies [14, 15].

0261-8028/91 $03.00 + .12 ©1991 Chapman and Hall Ltd.

e~

(400) t (006) t

(O04)t ,l~lr~il~ (600) t

70 72 74 76 78 120 123 126 129 132 135 2e (deg} 2e (deg)

Figure 1 X-ray diffraction patterns for sample A. Although the (0 0 4)t and (4 0 0)t are not well resolved, the (0 0 6)t and (6 0 0)t profiles can be seen clearly resolved.

However, the XRD data for the (0 0 6) and (6 0 0) peaks clearly indicate that this would be an incorrect assignment in our case, and we can assume that it is equally incorrect for the assignment in [14, 15].

To reduce this confusion, supporting evidence from other techniques is needed. Raman spectro- scopy provides another method for the identification of crystal symmetry in zirconia polymorphs. The number of Raman-active bands of a zirconia crystal phase can be obtained from a factor group analysis based on the space groups to which the polymorphs belong. The space groups of the three ZrO2 poly- morphs and the number of Raman-active bands are shown in TableI [18]. Researchers [18-23] have been able to distinguish consistently the tetragonal structure from the cubic symmetry using a Raman scattering technique. Mercera et al. [24] have recently indicated the need to utilize additional characterization data to augment the XRD crystal phase assignment; their results with zirconia preci- pitated at pH 10 support our earlier work [11-13].

Raman spectra for samples A and B are shown in Fig. 2. The Raman bands at 262, 330 and 473 cm -i , and their relative band positions, are similar for both of the samples. The Raman intensity for sample B is somewhat lower. Even so, the characteristic Raman bands corresponding to the tetragonal symmetry were obtained whether the zirconia precipitated at a pH of 13.5 was subjected to limited or thorough washing. The conspicuous absence of the 625 cm -1 (not shown in Fig. 2) band, which is characteristic of the cubic symmetry in sample A, also demonstrates that this sample consists of tetragonal symmetry and not cubic structure.

In conclusion, the prevailing discrepancies in the assignment of crystal structures in zirconia precipit- ated at pH of 13.5 have been addressed. Higher-an- gle XRD line splittings of (0 0 6) and (6 0 0) indicate that the pH 13.5 material is tetragonal. Additional evidence was obtained from Raman spectroscopy to

T A B L E I Space groups and Raman frequencies of zirconia polymorphs

No. of Space group Phase Raman-active bands

CSzh (P2i/c) Monoclinic 18 D415 (P42/nmc) Tetragonal 6 On (Fm3m) Cubic 1

300 400 500 Rarnan shift {crn -1)

Figure2 Raman spectra for samples A and B, showing the characteristic Raman bands at 262, 330 and 473 cm -1 that correspond to the tetragonal structure. The relative band posi- tions for A and B indicate that both samples A and B have tetragonal structures.

indicate that this material possesses tetragonal sym- metry, not the cubic structure. The data clearly demonstrate that the precipitation of zirconyl salts using NaOH at a pH of 13.5 results in the formation of tetragonal phase whether the washing is limited or extensive. It does not appear that conclusive proof is available to demonstrate that the presence of about 3 wt % Na can stabilize the metastable cubic phase at low temperatures. The conclusions of [14, 15] need to be supported by data from other character- ization techniques.

Acknowledgements This work was supported by the Commonwealth of Kentucky, the Department of Energy, Contract No. DE-FG05-85ER45186 at the University of Kentucky and by a grant from the Office of Naval Research at the University of Utah.

References 1. A . H . HEUER and M. ROHLE, in "Advances in ceram-

ics", Vol. 12, edited by N. Claussen, M. Rtihle and A. H. Heuer (American Ceramics Society, Columbus, Ohio, 1984) p. 1.

2. E. C. SUBBA RAO, in "Advances in ceramics", Vol. 3, edited by A. H. Heuer and L. W. Hobbs (American Ceramics Society, Columbus, Ohio, 1981) p. 1.

3. R. SUYAMA, T. ASHIDA and S. KUME, J. Amer. Ceram. Soc. 68 (1985) C314.

4. R.C. GARVIE, J. Phys. Chem. 69 (1965) 1238. 5. Idem, ibid. 82 (1985) 218. 6. A. CLEARFIELD, Inorg. Chem. 3 (1964) 146. 7. Y. MURASE and E. KATO, J. Amer. Ceram. Soc. 62

(1979) 527. Idem, ibid. 66 (1983) 196. P. E. D. MORGAN, ibid. 67 (1984) C204. J. LIVAGE, K. DOI and C. MAZIERES, ibid. 51(1968) 349. B. H. DAVIS, ibid. 67 (1984) C168. R. SRINIVASAN, R. J. DE ANGELIS and B. H. DAVIS, J. Mater. Res. 1 (1986) 583. R. SRINIVASAN, R. J. DE ANGELIS, M.B. HARRIS, S. F. SIMPSON and B. H. DAVIS, ibid. 3 (1988) 787. A. BENEDETTI, G. FAGHERAZZI and F. PINNA, J. Amer. Ceram. Soc. 72 (1989) 467. A. BENEDETTI, G. FAGHERAZZI, F. PINNA and S. POLIZZI, J. Mater. Sci. 25 (1990) 1473.

8 ,

9. 10.

11. 12.

13.

14.

15.

353

16.

17. 18. 19. 20. 21.

22.

R. SRINIVASAN, R. J. DE ANGELIS, G. ICE, S. F. SIMPSON, J. M. HARRIS and B. H. DAVIS, J. Mater. Res. submitted. G. KATZ, J. Amer. Ceram. Soc. 54 (1971) 531. v . G. KERMIDAS and W. B. WHITE, ibid. 57 (1974) 22. C. H. PERRY and D. W. LIU, ibid. 68 (1985) C184. M. ISHIGAME and T. SAKURAI, ibid. 60 (1977) 367. J. C. HAMILTON and A. S. NAGELBERG, ibid. 67 (1984) 686. R. E. BENNER and A. S. NAGELBERG, Thin Solid Films 84 (1981) 89.

23. C. M. PHILLIPPI and K.S. MAZDIYASMI, J. Amer. Ceram. Soc. 54 (1971) 254.

24. P. D. L. MERCERA, J. A. VAN OMMEN, E. B. M. DOESBURG, A. J. B U R G G R A F F and J. R. H. ROSS, Appl. Catal. 57 (1990) 127.

Received 27 June and accepted 21 August 1990

354