Zircon (ZrSiO4)

Zircon (ZrSi04) EDWARD D, PALIK Institute for Physical Science and Technology and R. KHANNA Department of Chemistry and Biochemistry University of Maryland College Park, Maryland

The interest in zircon is primarily as a gemstone. Many varieties and colors are found all over the world. The colors are due to the impurities, which give line absorption in the visible. We will not discuss these extrinsic effects in any detail. Many of the properties of zircon are summarized by Deer et al. [1]. ZrSiO4 is a tetragonal crystal with space group symmetry I4/amd (D4h). The clear crystal is termed high zircon, but containing impurities, it may have various colors. A somewhat amorphous phase made up of some SiO2 and ZrO2 is called low zircon and bears a green color [2]. The crystal form should have four E, (EIc) and three A2u (E lie) vibration modes in the infrared spectral region. The IR from 200 to 1100 cm -- 1 has been studied by Dawson et al. [3]. Gem-quality crystals from Ceylon were oriented, cut, and mechanically polished. An RIIC Michelson interferometer FS-720 was used at low frequencies, while single-beam and double-beam grating spectrometers (both built in the King's College physics department) were used at higher frequencies. The samples were either at 300 K or 100 K. Wire-grid polarizers were made of gold lines on AgC1 or polyethylene sheets. The reflectivity spectrum was Kramers-Kronig (K-K) analyzed, but no mention was made of including the high-frequency reflectivity above the band gap. No Lorentz-oscillator analysis of reflectivity was made (which is often useful for comparison). The absorption spectrum was also measured for samples of thickness 0.3 to 3 mm with the c-axis in the plane of the samples. The data were presented in small graphs of e' (real dielectric function) and g' (imaginary dielectric function) for polarization II (e) and _L(o) to the c-axis. These graphs were expanded by photocopying and read, and the 300 K data are listed in Table I. There is a significant 987 HANDBOOK OF OPTICAL CONSTANTS OF SOLIDS III

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988

Edward D. Palik and R. Khanna

uncertainty in n and k, especially where these parameters are changing rapidly. Although we include n to three figures and k to two figures, the reading uncertainty is surely +20%. There is also an uncertainty in the wave number of probably _+3 cm-~. Four Eu modes are observed. Three A2u modes are seen with a hint of a fourth one. The A2u modes (Ell c) have been measured by Gervais et al. [4]. The sample was a natural crystal, mechanically polished and then annealed at 1300 K. Both a K-K analysis and an oscillator model have been used to fit the reflectivity. The oscillator model gave the complex dielectric function as

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The parameters at 295 K for four oscillators are given in Table II. Note that group theory predicts only three A2u modes, and a fourth mode has been added at high frequency for the model fit. The dc dielectric constant is calculated to be e'o(e) = 10.71. We have calculated n(e) and k(e) to extend and fill in the gaps of the data of Dawson et al. [3] and also give them in Table I. The magnitudes of n and k are similar for the two sets of data, but do differ by +20% in places if account is taken of the mode frequencies being slightly different. This gives slightly shifted spectra. Dawson et aL [3] measured the transmittance of thin plates at 100 and 300 K with polarized radiation. They displayed absorption spectra with arbitrary but linear ordinate. They also gave the peak absorption coefficient = 4rrk/:t at three points in the spectrum. (We assume these are lowtemperature data; if this is the wrong choice, there are up to 20% differences.) These are (_1_: 887 cm -~, 85000 cm-1; 1900 cm -~, 70 cm-~; 2750 cm -a, ---0 cm-a); (11:994 cm -1, 93000 cm-1; 1900 cm -~, 100 cm-1; 2750 cm-~, 1.4 cm-1). Assuming we can make the comparison of absorption coefficient with arbitrary absorption, we have calculated k in the two- and threephonon absorption region near 1900 and 2800 cm-~, respectively. We did not use a grid fine enough to show all the details of the observed spectral lines. The results are listed in Table I. We could find no near-IR, visible data for n(o) and n(e) obtained by prism minimum-deviation methods. Many books on gems give the indices of refraction at the Na D line (0.5890 /zm) but usually do not quote the source. Clear high zircon has n(o) = 1.925 and n ( e ) = 1.984 in one gem book [5] and is given a range of values n(o) = 1.924-1.934 and n(e) = 1.970-1.977 in another book [1]. However, because of the ubiquitous presence of impurities, which change the color of the gem, these constants can vary drastically; therefore, we should focus on clear high zircon. Vance [6] has measured the visible and near-IR transmittance of various

Zircon (ZrSi04)

989

gem stones of low zircon at 15 and 290 K. The spectra contain many weak, sharp lines attributed to impurity atoms of U and Th. It is believed that high zircon containing U and Th degenerates into low zircon over the years because of the radioactive decay of the impurity atoms. Only the lowtemperature spectra are given in graphs, and they are labeled as o-polarized. The atomic impurities often appear only in o- or rr polarization. We assume this refers to and perpendicular and parallel to the c-axis of the crystal. The c-axis was presumably located in the plane of these gem samples. One ospectrum shows the onset of band-edge absorption. Even though the spectrum was obtained at 15 K, we have smoothed out the impurity structure and show k(o) in Table II, since we can find no detailed study of the absorption edge at room temperature. This is not quite the correct thing to do, since low zircon is an "amorphous" mixture of S i O 2 and ZrO2 (but showing a c-axis). But the band gap of crystalline S i O 2 is ~ 9 eV, while that of crystalline ZrO2 (zirconia) is --~ 3 eV. We would expect a mixture to have a band gap somewhere in between these limits. Deer et al. [1] gives several references to optical absorption studies, but none of these were quantitative regarding k. We could find no data above the band gap into the UV that could yield the optical constants and, therefore, cannot obtain an accurate value of the band gap. The data listed in Table I are plotted in Fig. 1. The ordinary n(o) and k(o), primarily from Dawson et al. [3], are shown in Fig. la; the extraordinary n(e) and k(e), primarily from Dawson et al. [3] are shown in Fig. lb; the extraordinary n(e) and k(e) from Gervais et al. [4] are shown in Fig. l c for comparison. ACKNOWLEDGMENT

We thank James E. Shigley, Director of Research, Gemological Institute of America, 1630 Stewart St., Santa Monica, CA 90404-4088 for kindly providing us with a copy of the zircon section of Deer et al. [1]. REFERENCES

1. W. A. Deer, R. A. Howie, and J. Zussman, "Rock-Forming Minerals," Vol. 1A, 2nd Ed., Orthosilicates, p. 431. Longmans, London, 1982. 2. R. Webster, "Gems," p. 131. Newnes-Butterworth, London, 1975. 3. E Dawson, M. M. Hargreave, and G. R. Wilkinson, The vibrational spectrum of zircon (ZrSiO4). J. Phys. C: Solid State Phys. 4, 240 (1971). 4. E Gervais, B. Piriou, and E Cabannes, Anharmonicity in silicate crystals: temperature dependence of Au type vibrational modes in ZrSiO 4 and LiA1Si206. J. Phys. Chem. Solids 34, 1785 (1973). 5. R. T. Liddicoat, Jr., "Handbook of Gem Identification," p. 272. Gemological Institute of America, Santa Monica, CA, 1969. 6. E. R. Vance, The anomolous optical absorption spectrum of low zircon. Minerol. Mag. 39, 709 (1974).

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999 TABLE II Oscillator Parameters for A2u Vibrational Modes at 295 K; ~ = 3.8

rj 5.75 0.36 0.78 0.022

(cm - j )

(cm -1)

339 605.7 977 1020

9.5 9.5 12.2 65